Towards ecologically based baiting strategies for rodents in agricultural systems

International Biodeterioration & Biodegradation 45 (2000) 183±197

www.elsevier.com/locate/ibiod

Towards ecologically based baiting strategies for rodents in agricultural systems David S.L. Ramsey*, John C. Wilson School of Natural Resource Sciences, Queensland University of Technology, GPO Box 2434, Brisbane 4001, Qld, Australia

Abstract Rodents are a major pest in Australian agricultural systems where they periodically irrupt (outbreak) and cause serious damage to crops. These irruptions still occur despite extensive baiting campaigns. Most of the economic losses caused by rodents occur in cereal and oilseed crops, although signi®cant damage occurs annually in both sugar cane and macadamia orchards. Currently, attempts to control rodent outbreaks rely heavily upon baiting with rodenticides. As these are normally applied in an emergency situation and without adequate scienti®c insight or planning, they are usually not cost e€ective. We present strategies aimed at reducing agricultural losses in the event of an outbreak that take into consideration important population processes such as: (a) the stage of development of the rodent population; (b) the likely course of population trends given prevailing environmental conditions; (c) the spatial relationship between the available habitat types; (d) the role of refuge areas in seeding adjacent areas and sustaining a high population in the absence of a crop habitat type; (e) the likely dispersal strategy of the population given the landscape of the area; (f) the consequences of dispersal into areas that do not yet sustain a high population, and for crops that will be harvested late in the season. A process, presented in the form of a ¯ow diagram, has been developed to tailor rodent control programs to speci®c outbreak conditions. Initially, the extent of a reported rodent problem and the scope of a control program are determined. Where a control program is required, this is developed for each region individually and is based on results of extensive population and crop surveys. Bait application rates are speci®cally determined for each rodent outbreak, reducing the probability of under- or over-baiting. Two baiting strategies have been developed for inclusion into the control program, a protection strategy and a control strategy. The former is designed to frustrate responsive dispersal into crops at harvest, whilst the latter will reduce numbers and maintain them at a low level until harvest. The procedure is reiterated for as long as the problem persists allowing control procedures to be modi®ed as population and crop changes occur. Using Australian examples to illustrate the ecological concepts underpinning rodent management in an emergency situation, the control processes outlined will have universal application in the management of rodent outbreaks in agricultural systems. 7 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction Rodents are a major pest in Australian agricultural systems where they periodically irrupt (outbreak) and cause serious damage to crops despite extensive baiting campaigns. Infestations of rodents in agricultural crops in Australia can be divided into three groups: (a) infestation of sugarcane by Rattus sordidus (the cane * Corresponding author. Landcare Research, Private Bag 11052, Palmerston North, New Zealand.

®eld rat); (b) infestation of macadamia orchards by R. rattus (the roof rat); and (c) infestation of cereal and oilseed crops by Mus musculus (the house mouse). Mouse plagues occur in the grain belts of eastern and southern Australia on average, once every 4 years and economic losses can run into the tens of millions of dollars (Caughley et al., 1998). Plagues of R. sordidus have been recorded in the Mackay and Herbert River districts in Queensland and annual damage estimates range between $2 and $4 million (Whisson, 1996; Wilson and Whisson, 1987). Damage to macadamia crops by R. rattus can be as high as 30% in some

0964-8305/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 4 - 8 3 0 5 ( 0 0 ) 0 0 0 5 9 - 7

184

D.S.L. Ramsey, J.C. Wilson / International Biodeterioration & Biodegradation 45 (2000) 183±197

orchards and losses average approximately $3 million (White et al., 1997). Currently, attempts to control rodent outbreaks in agricultural crops rely heavily upon baiting with rodenticides. This is despite there being no rodenticide currently registered for use in cereal crops (Caughley et al., 1998). Typically, the response to rodent problems consists of rapid, temporary registration of a poison, often without due regard for its e€ect on the rodent species. As most of these control operations are normally applied in an emergency situation without adequate scienti®c insight or planning, they are usually not cost e€ective. Recent reviews of integrated pest management strategies for rodents in agricultural systems have stressed the need for rodenticide based control strategies to have a ®rm biological underpinning (Singleton et al., 1999). A thorough understanding of the ecology and population dynamics of the pest species will provide better information for the optimum use of rodenticides. For example, studies of R. sordidus population dynamics in sugarcane crops of northern Queensland have resulted in a reduction of the previous reliance on poison baits for control. Emphasis has now been placed on the management of crop and non-crop habitats during the early stages of the breeding season. This e€ectively reduces the breeding potential, and hence maximum density, of the population with the result that baiting is only necessary in emergency situations (Whisson, 1996). Chemical use and habitat management represent the opposite ends of an integrated management continuum. Integrated management is a merger of all appropriate management techniques in such a way that each is used at the time when it will give the greatest contribution to the overall management strategy. While integrated or ecologically based rodent management strategies (sensu Singleton et al., 1999) should provide the basis for a more complete, long-term solution to rodent problems, it is essential to have procedures in place aimed at reducing losses in the event of an outbreak. The design of e€ective baiting strategies cannot be divorced from an understanding of the ecology of the pest rodent species. Important aspects of rodent ecology that must be taken into account in the design and implementation of a baiting strategy should include: (a) (b) (c) (d)

The stage of development of the population in its annual cycle of abundance and breeding. The likely course of population trends given prevailing environmental conditions. The spatial relationship between the available habitat types. The role of refuge areas both in seeding adjacent areas and sustaining a high population in the

(e)

absence of a crop habitat type. The consequences of dispersal into areas that do not yet sustain a high population, and for crops that will be harvested late in the season.

1.1. Rodent ecology and population dynamics Typically, rodents in agricultural areas display an annual periodicity of density and breeding e€ort (Brown et al., 1999; Caughley et al., 1998; Whisson, 1996). However, occasionally very high densities occur and are maintained for considerable time. In Australian house mouse populations breeding usually starts in spring (Cantrill, 1992; Newsome, 1969a) reaching its maximum in late summer and autumn, and declining to an overwinter minimum. As a result, maximum densities typically occur in autumn each year (Newsome 1969a, 1969b). There is considerable evidence that the mouse populations in southern Queensland reach their maximum density somewhat later than their more southerly relatives (Cantrill, 1992; Wilson, 1978). The diversity of models that have been proposed to account for temporal ¯uctuations and outbreaks of house mice throughout Australia serve to illustrate a number of important points (Newsome, 1969a, 1969b; Redhead, 1982; Saunders and Giles, 1977; Hone, 1980; Mutze et al., 1990, Singleton, 1989; Cantrill, 1992; Krebs et al., 1995; Pech et al., 1999). (i) (ii)

Refugia may provide important survival sites. Rainfall may a€ect the population in a number of ways including: (a) decreased juvenile survival; (b) increased ability to burrow and breed; (c) increased food supply and hence extension of the breeding season.

(iii) (iv) (v) (vi)

Landscape heterogeneity may determine the course of an outbreak. Outbreaks may form over a period as long as 3 years or as short as 6 months. Food quantity and quality are important determinants of density. Changes in social behaviour may play a role in triggering outbreaks.

Above all, the diversity of factors a€ecting mouse numbers and the occurrence of outbreaks suggests that models for one agricultural area may not be fully applicable to other agricultural regions. This is exempli®ed by comparing the southern Australian grain belt, with its single crop system, predominantly winter rainfall, and extensive refuge areas, with the northern

D.S.L. Ramsey, J.C. Wilson / International Biodeterioration & Biodegradation 45 (2000) 183±197

(southern Queensland) regions. The latter area has no signi®cant refugia, a dual cropping system and a predominantly summer rainfall. In response to di€ering landscapes, house mouse populations have adopted di€erent strategies. Similar adaption was found in the rice ®eld rat R. argentiventer which modi®ed breeding intensity dependent on the cropping system (Leung et al., 1999). Movements of rodents in response to crop harvest was noted by Newsome (1969a, 1969b), Cantrill (1992) and Whisson (1996). This responsive dispersal is of importance in the management of rodents in agricultural ecosystems as rodents will track available habitat types (Stickel, 1979). Inheritance of rodent problems from one crop by adjacent crops at harvest is thus an important aspect for consideration in control programs. Any baiting strategy implemented during an outbreak must cater for the current status of the population and the most likely future population trend. In any emergency situation a monitoring system should be set in place to track the outbreak. Population density and breeding cycles of rodents can be divided into a number of stages (after Krebs and Myers, 1974): The increase phase The peak phase The decline phase

The low phase

a period of large increase in numbers. a period of little change in numbers. numbers fall gradually over a period of time or crash. If this phase extends over more than 1 year, there may be some recovery during the breeding season. a period of low population density with little or no change. This phase may be absent from some population cycles.

During each of these stages the population possesses particular demographic characteristics. Consideration of these characteristics such as population density, age structure, sex ratios, pregnancy rates and distribution can be used to delineate the present state of the population, and therefore, its potential to cause future damage. In general, population density will not be suf®cient to indicate the stage of development of the population so other population statistics will be required. However, once the stage of development has been ascertained, an index of density will assist in determining the potential for future damage. Distribution of animals will be dependent upon the species and the agricultural system inhabited. In areas of continuous cropping where no signi®cant refuges occur, populations are likely to be highly mobile with their distribution being dependent upon the availability

185

of suitable habitat types (Cantrill, 1992; Wilson, 1978). No single distribution criteria will be available to determine population stage in this situation and interpretation of the population cycle will be dependent on other demographic characteristics. Where stable refuge areas form a moderate or high proportion of the agricultural landscape, movement of animals may depend on density. As refugia become crowded, animals may move into adjacent crops (Mutze, 1991). In such cases, a comparison of the status of populations in both temporally stable and temporally unstable habitat-types will indicate the stage of development of the population. Care must be taken in this situation that the reason for the present distribution is not the harvest of temporary areas forcing animals into refugia as perturbations in the crop system are catastrophic and have dramatic e€ects on the demography of rodents inhabiting these areas (Whisson, 1996). In non-equatorial regions the majority of small mammal populations undergo seasonal ¯uctuations in breeding rates. The breeding cycle of a population can be linked to abiotic, extrinsic and intrinsic factors, and in most cases all three play signi®cant roles. Important abiotic factors include photoperiod (Andrewartha and Birch, 1954; Sadlier, 1969), temperature (Fraps, 1962), rainfall (Rowe-rowe and Meester, 1982; Breed, 1982) and soil moisture (Newsome, 1969a, 1969b). Extrinsic factors include food supply (DeLong, 1967; Fordham, 1971; Singleton, 1985; Bomford, 1987a, 1987b), food components (Olsen, 1981; Alibhai, 1985), and nest sites (Newsome, 1969a, 1969b). Intrinsic factors include crowding (Lee and McDonald, 1985) and associated social stresses (Christian, 1950) and the presence of cuing pheromones (Bronson, 1979). The seasonal pattern of breeding can be utilised to determine the stage of the population cycle in the event of an outbreak (Fig. 1). Restricted winter breeding usually contributes to the annual population decline, and as a result, populations close to their maxima will have low pregnancy rates (<10% of mature females pregnant). When high populations occur with high pregnancy rates, the population is still in its increase phase and signi®cant growth can be expected. Compensatory e€ects within a population as a result of removal of some individuals during a control campaign may result in an increase in breeding e€ort and hence, in pregnancies in subsequent population samples (Adamczyk and Walkowa, 1971). This will in turn, allow the population to recover rapidly if sucient time is available prior to the onset of the decline phase. The combination of density, pregnancies and time to crop harvest will allow some judgement of the potential for future problems to be made. The distribution of pregnancies is also of use in determining the stage of development of the population in areas where signi®cant

186

D.S.L. Ramsey, J.C. Wilson / International Biodeterioration & Biodegradation 45 (2000) 183±197

refugia occur. The occurrence of pregnancies in the refuges but not in the crop areas suggests that the population has not yet had time to establish a breeding population in the latter. The sex and age structure of the rodent population will also provide an indication of the population's potential to increase and cause further damage. Changes in the age structure of the population can also indicate the stage of development of the population. As a general rule, increasing populations have a predominance of young animals whilst stable or declining populations do not (Krebs, 1994; Begon et al., 1986; Newsome, 1969a, 1969b) (Fig. 1). The phase of population development will re¯ect the potential for a major population increase to occur and the approximate time required. Thus, even where populations have been modi®ed through the use of a baiting strategy, assessment of the new stage of development will indicate the necessity for continued management. The four phases of the population cycle will generally have the following demographic characteristics: 1. The phase of low numbers Ð characterised by low population densities, few pregnancies (<10%), an age distribution containing predominantly old animals and the majority of animals present in refuge habitat-types. Populations in this phase will not increase rapidly in the short term (3 months) but may increase and cause appreciable damage in the medium to long-term. 2. The increase phase Ð can be divided into early and

late periods. Demographically these two periods differ in the pregnancy rate and age structure of the population. It is characterised by: (a) (b) (c) (d)

(e)

(f)

Population density is low to moderate. All adult animals are in breeding condition (although not all need to be reproductively active). The pregnancy rate varies from around 70% (early period) to around 20% (late period). In the early stage the age distribution is bimodal with young or immature animals, and old and/or mature animals occurring at approximately the same frequency. As this phase progresses, the age distribution will move to one of predominately young or juvenile animals. Young or juvenile animals are present in temporary and transient habitats with age distributions in refuge areas biased towards older, mature individuals. In the early period, pregnancies occur at a lower frequency in the crop habitats. In the later period, pregnancies in these areas equal or exceed those from refugia. Damage associated with populations at this stage can be high. Populations have the capacity to increase or recover rapidly.

3. The peak phase Ð a period of little change in numbers. It is characterised by: (a) (b)

Moderate to high population densities. Low pregnancy rates (<10%).

Fig. 1. Age distribution in rodent populations at various points in the annual cycles of density (a) and breeding (b).

D.S.L. Ramsey, J.C. Wilson / International Biodeterioration & Biodegradation 45 (2000) 183±197

(c) (d) (e)

The age distribution is biased towards the young and middle aged, mostly non-breeding mature animals. Few old animals occur. The majority of animals are no longer in breeding condition (e.g. many large imperforate females, and large males with abdominal testes). Animals are dispersed throughout both refuges and crop habitats.

4. The decrease phase Ð characterised by moderate to low population densities, nil or very low pregnancy rates (<5%), prevalence of old animals but none or very few in breeding condition. Damage resulting from populations in their decline phase will be temporary. Once in this phase, populations will not generally increase until after the phase of low numbers. 1.2. Timing of control Cost-e€ective control cannot be divorced from a consideration of the population dynamics of the

187

rodent population. Two related concepts are of importance: (a) the timing of control in relation to the density and breeding cycles; and (b) the e€ects of under- or over-baiting. These factors are related through the compensatory behaviour of the population. Consider the simulation of a hypothetical rodent population that has the following characteristics: Initial population size Sex ratio Survival per month Embryo per female Pregnancy rate

10 1:1 0.5 5.6 0.35 in months 1±6, 0 in months 7±12

The population is illustrated graphically (Fig. 2). Consider also that at a population size D, signi®cant damage is evident in the crop. Arbitrarily assume that this population level is equivalent to a population index of 60. Initially, without control procedures (Fig. 2(a)), the population increases and little damage occurs. A threshold is reached when damage is

Fig. 2. Population trajectory of a hypothetical mouse population (a) without control procedures, (b) with 80% of the population removed at peak density, (c) with 80% of the population removed when damage is ®rst observed, and (d) with 80% of the population removed in anticipation of future damage.

188

D.S.L. Ramsey, J.C. Wilson / International Biodeterioration & Biodegradation 45 (2000) 183±197

observed and at this time the population is large enough for damage to be apparent in the ®eld. The population keeps increasing until a peak is attained because no further increase can be supported by the system and population size decreases. Consider that a control procedure is implemented when crop damage is at a maximum and is successful in that 80% of all animals are killed. As Fig. 2(b) shows, the control procedure has achieved little over what was about to happen in the normal course of events without any control being applied, however, there has been an enormous input of time and money and this has mainly been wasted. It is important to note that in this situation it is possible to be fooled into believing that the control procedure has been successful and that the expenditure has been justi®ed when actually, no signi®cant change in the population trajectory has been e€ected. Fig. 2(c) shows what would happen if the same control procedure was implemented at the ®rst sign of damage. Even if 80% of the animals were killed, the population would still reach a peak after the control has been implemented as the habitat is not saturated and breeding will still continue. In this case, damage has been reduced signi®cantly. However, the problem is of course that the control procedure must be implemented immediately when damage is ®rst noticed and logistically, this is dicult. A 1-month delay in implementing the control procedure will quickly cause the situation to revert to that in Fig. 2(b). Fig. 2(d) shows what will happen if a control procedure is applied in anticipation of future damage. Here 80% of the population is removed 1 month before damage is apparent (this would only be possible if the population was being monitored). In this situation, the population is drastically reduced and crop losses are kept at a tolerable level. The conclusion from this qualitative example is that the bene®ts from population management accrue exponentially the earlier in the increase phase they are applied (e.g. Redhead et al., 1985). A similar conclusion was reached by Redhead (1988) when investigating the e€ects of delaying the initial litter of the house mouse breeding season by 1 week. Manipulation at this stage of the population cycle had the potential to reduce maximum densities. Thus, control in anticipation of damage and the population peak will be much more cost e€ective than control after damage reports have been received. In the latter case, the control procedure may not decrease damage signi®cantly despite a huge input of manpower and materials. 1.3. Baiting strategies In an attempt to achieve control of rodents in a agricultural crops numerous bait application methods have been employed or suggested: sustained baiting

(O'Connor, 1948); saturation baiting (Myllimaki, 1984); pulsed baiting (Dubock, 1982); trail baiting (Mutze, 1989); bait stations (Twigg et al., 1991); and perimeter baiting (Kay et al., 1994). Sustained baiting involves setting large bait points and monitoring and replenishing these regularly, ensuring that excess bait is available until rodents cease feeding (Dubock, 1982). The technique has been used widely with slow acting anticoagulants and with other poisons in which repeated feeding is necessary before animals ingest a lethal dose. The method assumes that bait aversion does not occur. Sustained baiting generally requires that excess bait material is available at all times during the treatment, maximising the probability of all individuals in the population ingesting a lethal dose (Richards, 1983). This leads to high bait and labour costs, prolonged environmental exposure and hence high non-target poisoning potential (Dubock, 1982; Richards and Huson, 1982; Richards, 1983). Some of these problems have been solved through the use of specially constructed bait stations in agricultural crops. Bait stations have the advantage of minimising contamination of crops and waterways, and decreasing the risk to non-target species (Twigg et al., 1991). However, their use in-crop is highly labour intensive, and thus impractical on a large scale. It has been suggested that sustained baiting is not a cost e€ective method for use with the faster acting rodenticides, and presents unacceptable risks to nontarget species. Results of ®eld studies indicate that the improved laboratory eciency of fast acting anticoagulants do not necessarily translate into improved ®eld results (Richards and Huson, 1982). Redfern et al. (1976) suggested that the high potency but slow action of brodifacoum resulted in animals that had consumed a lethal dose by day 2 continuing to feed on the bait for several days. These animals were not only consuming excess bait but were behaviourally excluding subordinate members of the population from access to the toxin. Pulsed baiting was suggested as a method to overcome this problem. Richards (1983) detailed the pulsed baiting procedure: many small bait points are placed in the area to be treated and replenished every 7 days. These may be completely eaten by rodents between pulses, and at the time of re-application animals which have eaten from the previous application have had time to become moribund, die or recover. At the second and subsequent pulses, animals not consuming a lethal dose previously, and those previously prevented from eating by dominant individuals, have access to the bait. In high density populations, the use of more rather than larger bait points has been suggested (Dubock, 1982). Dubock (1982) reviewed a number of ®eld trials in agricultural areas in which brodifacoum baits had been used in a pulsed baiting strategy. He found that

D.S.L. Ramsey, J.C. Wilson / International Biodeterioration & Biodegradation 45 (2000) 183±197

almost without exception a 1±3 kg/ha application of 0.005% brodifacoum bait using baits of 5±15 g and replenished every 7 days proved most e€ective. There was also a considerable saving in labour costs (50%) and bait costs (75%). Pulsed baiting cannot be used e€ectively with fast acting acute rodenticide, as sublethally dosed animals will develop bait shyness, and these animals will not take baits at subsequent pulses (Dubock, 1982). Further, the fast action of such baits removes the problem of social inhibition of bait consumption. Fast acting acute rodenticides are most suited to a one-shot widespread (saturation) baiting program. Such baits can therefore be best used when all individuals have access to the ®rst pulse of baits. This situation will occur when the relationship between number and toxicity of bait points and the number and susceptibility of rodents is such that no individual is prevented from ingesting a lethal dose on the ®rst bait application (Dubock, 1982). 1.4. Baiting strategy goals Sustained, saturation and pulsed baiting represent points on a continuum of temporal bait placement, each of which can be applied to a variety of spatial bait placement patterns. Spatial bait placement can be broadly divided into broadacre, trail and perimeter baiting, each of which is applicable to speci®c management programs with speci®c goals. The goal of a rodent control campaign is de®ned by the dynamics of the target population and of the agricultural ecosystem. Three goals have been considered: no action; protection; and control. A protection goal is implemented where crops will remain in place for a short time and/or the population in crops is not expected to increase other than through immigration. Control, on the other hand, is appropriate when crops will remain in the ground for some time and populations are in their increase phase. 1.4.1. The ``control'' goal Broadacre baiting is undertaken over a large continuous area, and is used in rodent control programs where the objective is to maximise mortality rates over as large an area as possible. Broadacre baiting can be used in sustained, saturation or pulsed baiting programs. In cases where broadacre saturation baiting is undertaken, baits are either broadcast by ground application methods into the treated area or aerially applied at the required density. The former method is labour intensive but has the major advantage of being highly speci®c to treatment areas. Aerial application of baits allow large areas to be covered quickly, but drift of poison material may occur. This will cause a signi®cant hazard to non-target species including humans,

189

and may decimate fauna populations in remnant habitats. Broadacre sustained baiting involves the use of bait stations. In their simplest form these are points at which bait is placed throughout the treatment area. As sustained baiting requires each bait station to be visited and bait replenished regularly until the population is reduced, this technique is impractical in other than isolated problem areas. Trail baiting has been suggested for control of mice in cereal crops. This method has been tested using strychnine baits on mice in cereal crops at an application rate of 1 kg/ha; giving 69±99% decreases in population density (Mutze, 1989). The signi®cant population decreases attained suggests that the 30 m spacing between trails was sucient to allow the majority of mice to come into contact with the bait. Bait trails were considered to increase the probability of mice obtaining a lethal dose when using low application rates as animals were found to work along a trail. The use of pre-baiting in this study was found not to signi®cantly improve results (Mutze, 1989). 1.4.2. The ``protection'' goal It has already been established that harvest of agricultural crops results in responsive dispersal into adjacent, unharvested crops. Perimeter baiting is seen as a ®lter to reduce such inheritance problems, and is therefore appropriate for crops that do not yet have populations causing signi®cant damage and when harvest is only a short time away. Both broadacre baiting and trail baiting target the population inhabiting the crop, on the other hand, perimeter baiting targets animals moving into a crop. Twigg et al. (1991) found that grain-growers became aware of mouse problems in their crop once densities reached 200±300 per ha and mice were distributed throughout the area. They, thus concluded that perimeter baiting would be inecient at reducing mouse populations in crops at this stage, although it may be important in limiting crop invasion from adjacent areas. Kay et al. (1994) found that perimeter baiting signi®cantly reduced the rate at which mice inhabiting a refuge habitat, colonised the adjacent crop. However, mouse density was relatively low and the study did not conclude whether perimeter baiting resulted in reduced damage levels to the crop. Mutze (1989) has cautioned that perimeter baiting may present a greater risk to non-target species than in-crop baiting for those animals that forage on the edge of crops. It is suggested that perimeter baiting per se will not be as e€ective as its use in conjunction with a fallow barrier. Studies of movements of rodents across roads (Kozel and Fleharty, 1979; Oxley et al., 1974) have indicated that small mammals are reluctant to venture onto road surfaces where there is more than 20-m

190

D.S.L. Ramsey, J.C. Wilson / International Biodeterioration & Biodegradation 45 (2000) 183±197

between areas of cover. Further, the road surface is not critical. Construction of a 20-m fallow strip of even, road-like surface may go some way to limiting rodent movements into crops. The application of a baited strip around the outside of a fallow strip will help reduce survival of animals that do cross into the crop. A perimeter baiting strategy is not conducive to pulsed baiting techniques as only the mobile portion of the population is targeted. A sustained baiting strategy is most appropriate to this situation. 1.4.3. The ``no action'' goal The ``no action'' goal is appropriate when it has been identi®ed that there is a low potential for future damage. Thus, it will be considered not cost-e€ective to implement a baiting strategy for the a€ected area. Generally this will be dependent on the stage of the population cycle and the time until crop harvest in the a€ected area. Details for determining when a ``no action'' goal is appropriate are outlined in the control process section. 1.5. The role of bait substrates and attractants To date, in emergency situations, baits (usually acute poisons on grain) have been used on an ad hoc basis with little scienti®c justi®cation. Poor success in many baiting campaigns is often due to the failure of the substrate to deliver a lethal dose of the poison to the target animal. This is caused mainly by low palatability of the substrate although the toxicant can be a major source of palatability variation (Miller, 1974). To be successful, a bait must be acceptable to the target species in its natural habitat where alternative food sources are abundant. Further it should be acceptable to the population over all stages of the population cycle. The importance of these conditions has been illustrated in recent work by Whisson (1996) on the baiting of R. sordidus with thallium baits. Acceptance of thallium sulphate baits by R. sordidus is low during the breeding season when their dietary requirements are speci®c. Intake of baits in signi®cant amounts does not occur until after breeding has ceased by which time the population is already causing signi®cant damage. Instinctive preferences and learning experiences of rodents for food items found naturally in their environment (Bullard, 1985) indicate that substrates developed for a particular population may not be as acceptable to other populations of the same species. Thus, an evaluation of bait substrates should be undertaken for ®eld populations in a diversity of situations. Accumulation of this data will provide a rationale for the development of a portable bait base into which a suitable rodenticide can be incorporated.

Attractants have the potential to increase the e€ectiveness of baits by increasing both the sphere of in¯uence and consumption of a bait. Other bene®ts include: masking reactions such as bait shyness (Prakash, 1988; Galef et al., 1988); increased target speci®city (Stoddart, 1988); and reduction of the e€ect on palatability often caused by addition of a rodenticide (Lund, 1988). Attractants can either be biologically based (e.g. pheromones, natural food derivatives) or novel with respect to the target population. Those naturally occurring attractants encountered by the animal in its environment are generally more successful in the long term as the responses to these are more predictable and hence, more useful in integrated management strategies (Stoddart, 1988; Bullard, 1985). 2. The control process 2.1. Rationale In designing the control process, the main objective has been to provide methodologies that allow for rapid implementation of control procedures whilst incorporating the considerations outlined earlier. The process presented in this section has been developed to allow control strategies to be tailored to the prevailing crop and population conditions throughout the a€ected region. The conceptual design of this process is presented in Fig. 3. In designing the control process, it has been assumed that it will be applied separately to each agricultural region a€ected by an outbreak. An initial rapid assessment of the problem is required to determine the scope of future involvement and minimise economic input until the scale of the problem is appreciated. Once the necessity for further input has been determined, an extensive rodent survey should be undertaken. At this time information on the status of crops in the a€ected areas will be collected. Distribution of the target animals, population trends and crop status provide the framework for the implementation of one of the two baiting strategies. These provide for either protection of crops or population reduction. The strategy utilised in each region therefore has a speci®c objective commensurate with the status of the rodent problem. There is provision for the use of alternative strategies in di€erent areas at the same time, the choice depending largely on expected harvest dates and extrapolated population trends. Bait application rates appropriate to one rodent outbreak are not necessarily ideal for another and di€erences in rodent species, population density, and bait type all contribute to this complexity. The application rate appropriate to each outbreak should be determined by ®eld assessment. The process is reiter-

D.S.L. Ramsey, J.C. Wilson / International Biodeterioration & Biodegradation 45 (2000) 183±197

ated periodically since changes due to an initial round of baiting will have rami®cations for subsequent applications. This procedure allows application rates to be tailored to the current needs of each region, reducing

191

the under- or over-baiting prevalent in most rodent control campaigns. The procedure detailed here provides: (1) continuous assessment of the success of the program implemented;

Fig. 3. Flow diagram of the control process. Numbers in parentheses refer to notes in the text.

192

D.S.L. Ramsey, J.C. Wilson / International Biodeterioration & Biodegradation 45 (2000) 183±197

(2) in conjunction with cropping data, allows continued modi®cation of the control procedures to suit the changing situation; and (3) the appropriate control endpoint to be determined. The control procedure (Fig. 3) consists of several sequential phases: 1. Pilot study Ð to determine (a) whether rodents are involved, (b) which species are involved, and (c) the apparent extent of the problem. 2. Regional survey Ð to determine the status of the rodent population and of the crop system in the region. Population trends are predicted and ancillary studies, such as the determination of bait application rates and regular monitoring sites are established. 3. Decision table Ð to determine an appropriate goal for areas of similar status. The decision table is used repeatedly to determine goals for all areas in the region. 4. Implementation of management goals for all areas. 5. Evaluation Ð to determine the success of the control program and the necessity for further control activities. 2.2. Flow diagram notes The ¯ow diagram (Fig. 3) has been indexed by paragraph numbers which refer to the notes that follow. The control process has been developed in order to implement strategies in the shortest possible time. A complete evaluation of the problem leading to implementation of baiting strategies should be completed within 10±15 days of reports of damage being received. 2.3. Initial assessment (1) Reports of damage in themselves constitute a body of information that can be utilised in a preliminary assessment of the problem. The proximity of farms to other areas reporting rodent damage will provide an appraisal of the scale of the problem. The status of the damaged crop should be obtained, this can have a bearing on the diagnosis as: (a) damage reported in the harvest period may be due to high population densities resulting from responsive dispersal; and (b) damage reported early in the growing season may suggest the onset of a signi®cant problem. (2) If the damage reported, warrants further investigation, a pilot study should be undertaken to provide an initial assessment of the problem. The broad objective of the pilot study is to determine whether the problem requires development of a major control program. Speci®c objectives include: (a) assessing whether all damage reports are due to rodent activity;

(b) determining the species of rodent that is responsible for damage; (c) obtaining initial information on the density and breeding status of the population; (d) obtaining a initial estimate of damage in the a€ected areas. (3) No further action will be necessary when: (a) a non-rodent species is involved; (b) only one or two growers experience minor damage and no pregnant females are captured; (c) the damage being reported is old, and density is not high or is decreasing; (d) the population is of low density (not in the phase of low numbers), is not breeding and damage is not extensive. In these cases speci®c recommendations should be made on how to reduce further damage. If further involvement is required, then a full program designed to assess the extent of the problem over a wide area should be implemented. 2.4. Detailed assessment (4) The pilot study concentrated on the areas from which damage was reported and its results are therefore biased. As a result many unlocated areas of potential rodent problems may exist. The assessment of the extent of the problem will target this limitation of the pilot study through a detailed and expanded trapping program. Information on crop status in each region should also be collected at this time. Thus, growers should be consulted to obtain the approximate date of harvest of the crop where each of the traplines were set as well as all adjacent crops. Data obtained will be used when management strategy goals are determined. (5) The information gathered in (2) and (4) will be used to assess population status and trends in density and breeding. The potential for future damage will be determined from the population assessment and time to harvest. (6) In order to assign appropriate management strategies the region should be divided into ``management areas'' suitable to the application of a speci®c management goal. Initially the region should be divided on the basis of natural geographic features and the spatial separation of cropping areas within the region. These areas can then be subdivided into management areas based on uniformity of cropping status, as management criteria in areas of mature and immature crops will di€er. Where isolated outbreaks are occurring, rodent densities may be used to delineate management areas. Once management areas are established, a density estimate can be made for each of the areas based on the trapping results of (4). Each area can then be allocated a management goal based on the potential for damage using the decision table (Fig. 4). (7) The extensive survey suggest that a baiting pro-

D.S.L. Ramsey, J.C. Wilson / International Biodeterioration & Biodegradation 45 (2000) 183±197

gram is necessary, a bait application rate that will prevent under- or over-baiting of the population at its current density should be determined. This can be achieved by monitoring consumption rates using a trial at selected study sites over 1±3 nights. Trials should be undertaken on populations in high, medium and low damage areas to provide ecient tailoring of bait application rates to the current problem. It is suggested that a median consumption value over a selected number of nights be used as the appropriate bait application rate. 2.5. Selection of appropriate strategy (8) All management areas within the region should be allocated a management goal of either: . Protection; . Control; or . No action using the decision table (Fig. 4) and data from (4) and (5). Di€erent goals can be assigned for each management area. The nature of the outbreak will determine which goal is most prevalent which will be dependent on the spatial extent of the problem and the potential

193

for future damage. Thus a ``no action'' goal may dominate the region when outbreaks are isolated. Those management areas with signi®cant damage will immediately be considered in the decision table (Fig. 4) and be assigned a management goal. (9) Management areas with no obvious damage may support populations that will increase to damaging levels before harvest. The trapping results obtained in (2) and (4) can be used to determine an initial average trap success from traplines set in crops with signi®cant damage. This, along with crop status, will be used to determine the potential for future damage within each management area. Management areas with a high potential for future damage will fall into the following categories: (a)

(b)

The management area supports high population densities (arbitrarily de®ned as 70% or greater than the initial average density in damaged areas). The management area supports moderate population densities (arbitrarily de®ned as between 30 and 70% of the initial average density in damaged areas) and populations in the area are increasing and harvest of crops is longer than 1

Fig. 4. Decision table for determining a suitable baiting strategy for a management area based on crop information and rodent population trajectory (see text for details).

194

(c)

D.S.L. Ramsey, J.C. Wilson / International Biodeterioration & Biodegradation 45 (2000) 183±197

month away. he management area supports low population densities (arbitrarily de®ned as 30% or less than initial average density in damaged areas) and populations in the area are increasing and harvest of crops is longer than 3 months away.

Fig. 5 shows hypothetical population densities at 10 management areas prior to baiting. Densities are calculated relative to the initial average density in damaged areas (line A). Given that the time to harvest in all of these 10 areas is 2 months and the populations are increasing, a high potential for future damage is expected for management areas 3, 4, 6 and 7 as the density in these areas is more than 70% of A. Control strategies will also need to be developed for areas 1, 2, 5 and 8 as populations in these areas may cause signi®cant damage prior to crop harvest. Potential for damage in management areas 9 and 10 is low as densities are less than 30% of A and consequently, no baiting strategies should be applied here. Two alternative baiting strategies are considered Ð control and protection. A control strategy is directed to reducing and maintaining population density to below that at which signi®cant damage occurs. As a result, it would only be implemented when harvest is not imminent. When harvest is imminent (<1 month) population densities in crops will only increase through responsive dispersal. A protection strategy is appropriate in these circumstances. A ``control'' strategy will be implemented in a management area when the following conditions apply:

(i) (ii) (iii)

the majority of crops within the management area have greater than 1 month until harvest; and the management area supports damaging population densities; or the management area has a high potential for future damage (from (9)).

A ``protection'' strategy is only appropriate when crops in the management area are undergoing harvest and unharvested crops are therefore, potential sinks for dispersing animals. This strategy is a program using both baiting and farm management techniques in an orderly sequence as outlined below: (i) (ii) (iii) (iv)

perimeter baiting around crops with a high probability of rodent invasion; crop stubble management; crop verge management; refuge management;

(10) Those management areas that do not conform to the criteria in (9) have a low potential for future damage and will be assigned a ``no action'' goal. (11) Once a goal has been established for any of the management areas, baiting should proceed. Signi®cant population reduction by baiting, as required in a control strategy, can only be realised by using either, (a) broadacre or (b) trail baiting within the crop. Broadacre baiting from the ground is preferred, as this will limit the amount of bait that falls outside the crop, reducing access by non-target animals. However, during an extensive outbreak, the only practical

Fig. 5. Hypothetical population densities at 10 management areas prior to baiting. crops with signi®cant damage (line labeled A) (see text for details).

1

Densities are calculated relative to the average densities in

D.S.L. Ramsey, J.C. Wilson / International Biodeterioration & Biodegradation 45 (2000) 183±197

method may be aerial distribution. Consideration should also be given to trail baiting within the crop as this is a rapid technique allowing accurate bait placement (Mutze, 1989). A change of the management goal from a control strategy to a protection strategy will be necessary as harvest approaches. The protection strategy requires the frustration of dispersal from crops at harvest. Refuge or grass verge areas may be used by dispersing mice as highways to invade adjacent areas (Kay et al., 1994). Thus, rodents dispersing from a harvested crop into a refuge area may then invade nearby crops. Perimeter barriers should be established prior to harvest around adjacent crops to act as ®lters reducing colonisation by dispersing rodents. In order to minimise responsive dispersal, the harvested area should not be modi®ed, e.g. through stubble incorporation; until all crops in the management area have been harvested. 2.6. Reassessment of the problem (12) The initial appraisal of the status of the population will need to be regularly reassessed to determine the actual course of the outbreak and hence any necessary modi®cations to the management procedures implemented. Initial assessment was undertaken in (4). Additional assessment of the problem should take place at monthly intervals until the problem has abated. The updated density index for each management area will be used to reassess the potential for future damage. Goals for each management area will then be re evaluated. Thus, repeated application of the ¯ow

195

diagram will allow a continuous assessment of the problem. Fig. 6 illustrates the situation in the 10 management areas of Fig. 5, 1 month after baiting. The potential for future damage in area 3 is still high as the density is greater than 70% of A. Continued development of the baiting strategy will be necessary for this area. Areas 4 and 7 fall into the 30±70% range of A, however, the potential for future damage is low as crops will be harvested within 1 month. These areas would not require further strategy development. The remaining areas do not require further treatment as populations are below 30% of A and will not increase signi®cantly in the month remaining to crop harvest. (13) As rodent densities will have declined due to baiting, application rates should be periodically reevaluated to prevent either under- or over-baiting the population. Application rates should be reevaluated at the same sites as the initial evaluation. The control process is reiterated for each management area until all management areas have been assigned a ``no action'' goal. This means that crops in the region no longer support population densities at damaging or potentially damaging levels, and the control procedure can be terminated. (14) Accurate and rapid methods for estimating rodent damage in agricultural crops need to be developed and generally, these will be dependant on crop type. Additional cost±bene®t analysis should then be undertaken to determine the ecacy of the control campaign and hence, the savings to the industry (e.g. Caughley et al., 1998).

Fig. 6. Hypothetical population densities at 10 management areas 1 month following application of management strategies. lated relative to the average densities in crops with signi®cant damage (line labeled A) (see text for details).

1

Densities are calcu-

196

D.S.L. Ramsey, J.C. Wilson / International Biodeterioration & Biodegradation 45 (2000) 183±197

3. Conclusion The control process presented here has been developed primarily to cope with rodent populations that are in¯icting signi®cant damage or will potentially in¯ict signi®cant damage to agricultural crops before harvest. It is not designed as a framework to provide long-term cost-e€ective solutions to rodent problems in agriculture such as EBRM (Singleton et al., 1999). Nevertheless, acute rodent problems can vary widely in their e€ects both spatially and temporally and the control process presented provides an example of an ecologically based framework that can be used to optimise rodenticide use in an emergency situation. Incorporating an understanding of the population dynamics and ecology of the pest species into baiting campaigns allows more ecient targeting and timing of rodenticide use which should reduce the unnecessary under- or over-baiting prevalent in many baiting campaigns. This approach should result in more cost-e€ective baiting strategies and hence, result in reduced crop losses to the industry.

Acknowledgements The authors are indebted to Steve Cantrill who provided invaluable assistance in both the development and preparation of the control process presented here. The study was jointly funded by the Wheat Research Committee of Queensland and the Barley Research Committee of Queensland.

References Adamczyk, A., Walkowa, W., 1971. Compensation of numbers and production in a Mus musculus population as a result of partial removal. Ann. Zool. Fennici. 8, 145±153. Alibhal, S.K., 1985. E€ects of diet on reproductive performance of the bank vole (Clethrionomys glariolus ). J. Zool. Lond. (A) 205, 445±452. Andrewartha, H.G., Birch, L.C., 1954. The Distribution and Abundance of Animals. University of Chicago Press, Chicago. Begon, M., Harper, J.L., Townsend, C.R., 1986. Ecology: Individuals, Populations, Communities. Blackwell Scienti®c Publications, Oxford. Bomford, M., 1987a. Food and reproduction of wild house mice. I. Diet and breeding season in various habitats on irrigated cereal farms in New South Wales. Aust. Wildl. Res. 14, 183±196. Bomford, M., 1987b. Food and reproduction of wild house mice. II. A ®eld experiment to examine the e€ects of food availability and food quality on breeding in spring. Aust. Wildl. Res. 14, 197±206. Breed, W.G., 1982. Control of mammalian and avian reproduction in the Australian arid zone, with special reference to rodents. In: Barker, W.R., Greenslade, P.J.M. (Eds.), Evaluation of the Flora and Fauna of Arid Australia. Peacock Publications, South Australia.

Bronson, F.H., 1979. The reproductive ecology of the house mouse. A. Rev. Biol. 54, 265±299. Brown, P.R., Hung, N.Q., Hung, N.M., van Wensveen, M., 1999. Population ecology and management of rodent pests in the Mekong river delta, Vietnam. In: Singleton, G., Hinds, L., Leirs, H., Zhang, Z. (Eds.), Ecologically-based Rodent Management, ACIAR Monograph No. 59, p. 494. Bullard, R.W., 1985. Isolation and characterisation of natural products that attract or repel wild vertebrates. In: Acree, T.E., Soderland, D.M. (Eds.), Semiochemistry: Flavours and Pheromones. W. de Gruyter and Co, Belrin. Cantrill, S., 1992. The population dynamics of the house mouse (Mus musculus L.) in an unstable agricultural ecosystem. Ph.D. Thesis, Queensland University of Technology. Caughley, J., Bomford, M., Parker, B., Sinclair, R., Griths, J., Kelly, D., 1998. In: Managing Vetebrate Pests: Rodents. Bureau of Resource Sciences and Grains Research Development Corporation, Canberra. Christian, J.J., 1950. The adreno-pituitary system and population cycles in mammals. J. Mammal. 31 (3), 247±259. Delong, K.T., 1967. Population ecology of feral house mice. Ecology 48 (4), 611±634. Dubock, A.C., 1982. Pulsed baiting Ð a new technique for high potency, slow acting rodenticides. In: Proc. 10th Vert. Pest Conf. Monterey, CA. Fordham, R.A., 1971. Field populations of deermice with supplemental food. Ecology 52 (1), 138±146. Fraps, R.M., 1962. E€ects of external factors on the activity of the ovary. In: Zuckerman, S., Mandl, A., Eckstein, P. (Eds.), The Ovary. Academic Press, New York, pp. 34±35. Galef, B.J., Mason, R.J., Prett, G., Bean, N.J., 1988. Carbon disulphide: a semiochemical mediating socially-induced diet choice in rats. Physiology and Behaviour 42, 119±124. Hone, J., 1980. Probabilities of house mouse (Mus musculus ) plagues and their use in control. Aust. Wildl. Res. 7, 417±420. Kay, B.J., Twigg, L.E., Nichol, H.I., 1994. The strategic use of rodenticides against house mice (Mus domesticus ) prior to crop invasion. Wildlife Research 21, 11±19. Kozel, R.M., Fleharty, E.D., 1979. Movements of rodents across roads. The Southwestern Naturalist 24 (2), 239±248. Krebs, C.J., 1994. Ecology: The Experimental Analysis of Distribution and Abundance, 4th ed. Harper Collins, New York (800 pp.). Krebs, C.J., Chitty, D., Singleton, G.R., Boonstra, R., 1995. Can changes in social behaviour help to explain house mouse plagues in Australia? Oikos. 73, 429±434. Krebs, C.J., Myers, J.H., 1974. Population cycles in small mammals. Advances in Ecological Research 8, 267±399. Lee, A.K., McDonald, I.R., 1985. Stress and population regulation in small mammals. Oxford Reviews of Reproductive Biology 7, 261±304. Leung, L.K.P., Singleton, G.R., Sudarmaji, Rahmini., 1999. Ecologically based population management of the rice-®eld rat in Indonesia. In: Singleton, G., Hinds, L., Leirs, H., Zhang, Z. (Eds.), Ecologically-based Rodent Management, ACIAR Monograph No. 59, p. 494. Lund, M., 1988. Nonanticoagulant rodenticides. In: Prakash, I. (Ed.), Rodent Pest Management. CRC Press, FL, USA. Miller, J.G., 1974. The signi®cance of preference in laborotory bait acceptance studies. In: Johnson, W.V. (Ed.), Proc. Vert. Pest Conference. Davis, CA. Mutze, G.J., 1989. E€ectiveness of strychnine bait trails for poisoning of mice in cereal crops. Aust. Wlldl. Res. 16, 459±465. Mutze, G.J., 1981. Mouse plagues in South Australian cereal-growing areas. III. Changes in mouse abundance during plague and non-plague years, and the role of refugia. Wildlife Research 18, 593±604.

D.S.L. Ramsey, J.C. Wilson / International Biodeterioration & Biodegradation 45 (2000) 183±197 Mutze, G.J., Veitch, L.G., Miller, R.B., 1990. Mouse plagues in South Australian cereal growing areas. II. An empirical model for prediction of mouse plagues. Aust. Wildl. Res. 17, 313±324. Myllymaki, A., 1984. Ecacy of a number of toxic baits and baiting against the voles Microtus agrestis and Arvicola terrestris. In: Proc. 11th Vert. Pest Conf. Sacramento, CA. Newsome, A.E., 1969a. A population study of house mice permanently inhabiting a reed bed in South Australia. J. Anim. Ecol. 38, 361±377. Newsome, A.E., 1969b. A population study of house mice temporarily inhabiting a South Australian wheat®eld. J. Anim. Ecol. 38, 341±359. O'Connor, J.A., 1948. The use of blood anticoagulants for rodent control. Research Supplement 7, 334±336. Olsen, P., 1981. The stimulating e€ect of phytohormone, gibberellic acid, on reproduction of Mus musculus. Aust. Wildl. Res. 8, 321± 325. Oxley, D.J., Fenton, M.B., Carmody, G.R., 1974. The e€ects of roads on populations of small mammals. J. Appl. Ecol. 11, 51± 59. Pech, R.P., Hood, G.M., Singleton, G.R., Salmon, E., Forrester, R.I., Brown, P.R., 1999. Models for predicting plagues of house mice (Mus domesticus ) in Australia. In: Singleton, G., Hinds, L., Leirs, H., Zhang, Z. (Eds.), Ecologically-based Rodent Management, ACIAR Monograph No. 59, p. 494. Prakash, I., 1988. Bait shyness and poison aversion. In: Prakash, I. (Ed.), Rodent Pest Management. CRC Press, FL, USA. Redfern, R., Gill, J.E., Hadler, M.R., 1976. Laboratory evaluation of WBA 8119 as a rodenticide for use against warfarin-resistant and non-resistant rats and mice. J. Hyg. 77, 419±426. Redhead, T.D., 1982. Reproduction, growth and population dynamics of house mice in irrigated and non-lrrigated cereal farms in New South Wales. Ph.D. Thesis, Australian National University. Redhead, T.D., 1988. Prevention of plagues of house mice in rural Australia. In: Prakash, I. (Ed.), Rodent Pest Management. CRC Press, FL, USA. Redhead, T.D., Enright, N., Newsome, A.E., 1985. Causes and predictions of outbreaks of Mus musculus in irrigated and non-irrigated cereal farms. Acta. Zool. Fennica 173, 123±127. Richards, C.J.G., 1983. Cost e€ective use of high potency anticoagulant rodenticides. In: Proc. 6th British Pest Control Conf. Cambridge, UK.

197

Richards, C.J.G., Huson, L.W., 1982. Towards the optimal use of anticoagulent rodenticides. Acta Zoologica Fennica. Rowe-rowe, D.T., Meester, J., 1982. Population dynamics of small mammals in the Drakensberg of Natal, South Africa. Z. Saugetierkunde 47 (6), 347±356. Sadlier, R.M.F.S., 1969. The Ecology of Reproduction in Wild and Domestic Animals. Methuen, London. Saunders, G.R., Giles, J.R., 1977. A relationship between plagues of the house mouse, Mus musculus (Rodentia: Muridae) and prolonged periods of dry weather in South, Eastern Australia. Aust. Wildl. Res. 4, 241±248. Singleton, G.R., 1985. A demographic and genetic study of house mice, Mus musculus, colonising pasture haystacks on a cereal farm. Aust. J. Zool. 33, 437±450. Singleton, G.R., 1989. Population dynamics of an outbreak of house mice (Mus domesticus ) in the Mallee wheatlands of Australia Ð hypothesis of plague formation. J. Zool. Lond. 219, 495±515. Singleton, G.R., Leirs, H., Hinds, L.A., Zhang, Z., 1999. Ecologically-based management of rodent pests Ð re-evaluating our approach to an old problem. In: Singleton, G., Hinds, L., Leirs, H., Zhang, Z. (Eds.), Ecologically-based Rodent Management, ACIAR Monograph No. 59, p. 494. Stickel, L.F., 1979. Population ecology of house mice in unstable habitats. J. Anim. E. col. 48 (3), 871±887. Stoddart, D.M., 1988. The potential for pheromonal involvement in rodent control programs. In: Prakash, I. (Ed.), Rodent Pest Management. CRC Press, FL, USA. Twigg, L.E., Singleton, G.R., Kay, B.J., 1991. Evaluation of bromodialone against house mice (Mus domesticus ) populations in irrigated soybean crops. I. Ecacy and control. Wildlife Research 22, 717±731. Whisson, D., 1996. The e€ect of two agricultural techniques on populations of the cane®eld rat (Rattus sordidus ) in sugarcane crops of north Queensland. Wildlife Research 23, 589±604. White, J., Wilson, J., Horskins, K., 1997. The role of adjacent habitats in rodent damage levels in Australian macadamia orchard systems. Crop Protection 16, 727±732. Wilson, J.C., 1978. Studies on the population dynamics of the house mouse (Mus musculus L.) on the Darling Downs, Queensland. M. App. Sc. Thesis. Queensland Institute of Technology. Wilson, J., Whisson, D., 1987. Towards the control of rodents in sugarcane. In: Proc. 8th Australian Vertebrate Pest Control Conference.