Protection of farm stored maize against infestation by Prostephanus truncatus (Horn) and Sitophilus species in Tanzania

J. stored Prod. Res. Vol. 26, No. 4, pp. 187-198,1990 Printed in Great Britain

0022-474X/90S3.00+ 0.00 Pergamon Press plc

PROTECTION OF FARM STORED MAIZE AGAINST INFESTATION BY PROSTEPHANUS TRUNCATUS (HORN) AND SITOPHILUS SPECIES IN TANZANIA P. &LOB’

and C. HANKS’

’ Storage Department, Natural Resources Institute, Chatham, Kent, U.K. and *Institute for Grassland and Animal Production, Hurley, Maidenhead, Berkshire, U.K. (Received for publication 12 June 1990) Abstract-Large scale trials were undertaken in Tanzania in order to assess the effectiveness of spray applications of insecticide as protectants of maize cobs against infestation by Prostephanus truncatus (Horn) and Sitophifus oryae (L.) and S. zeamais Motsch. In general, protection of cobs was not as effective as the protection afforded to insecticide-treated grain. However, in simulated farm stores excellent protection was obtained either by applying pennethrin as a dilute dust to grain or by applying it as a spray to cobs with sheathing leaves intact. Paddy husk ash also provided good protection when applied to maize grain. The treatment of maize cobs by spraying may provide the small scale farmer with a viable and more traditionally compatible alternative to the currently recommended practice of shelling maize and treating it with a dilute insecticide dust.

INTRODUCTION

Prostephunus truncatus (Horn), an indigenous pest of stored maize in Central America has recently become established on farms in East Africa, particularly in Tanzania (Golob and Hodges, 1982). Experiments carried out both in the laboratory and under field conditions in western Tanzania demonstrated the effectiveness of treating loose grain with permethrin as a method of protecting maize against this pest (Golob et al., 1985). The application of 50 g of 0.5% permethrin dust to a sack of maize grain (90 kg) has become the basis of a series of recommendations for on-farm storage of maize in P. truncatus affected areas of Tanzania. Although the use of permethrin dust by farmers has reduced storage losses caused by P. truncatus, other problems have been created which have reduced the potency of this treatment. The principal drawback has been the increased importance of Sitophilus oryzae (L.) and S. zeamais Motsch. as farmers have switched from storing maize on the cob, a form which is not so readily infested by Sitophilus spp, to storing loose maize which is readily infested by weevils. The problem has been exacerbated because permethrin is not particularly effective in controlling Sitophilus spp.

If farmers were able to store maize on the cob, the traditional method of storage, not only would problems due to Sitophilus spp be reduced but the need to shell the entire maize harvest simultaneously, a tedious, time-consuming occupation, could be avoided. Hence trials were established to determine the effectiveness of applying an insecticide spray to the external surface of the cobs with husks intact as a means of protecting grain. A single experiment in which an assessment was made of dipping the cut end of cobs in various formulations of permethrin was also carried out. In order to reduce the dependence of the farmer on synthetic insecticides for protecting stored food attempts were made to derive a treatment using a locally available substitute. As indicated below a variety of substances have been tested in western Tanzania since 1981 with little success. However, some candidate materials have shown promise and trials in which these were included are reported in this paper. Trials are also described in which the protection of shelled maize using dilute insecticide dusts was tested. These trials are similar to those reported by Golob et al. (1985), but were carried out under conditions which simulated on-farm storage much more closely, using either large sacks or traditional cribs as the storage containers. 187

P.

188

GOLOB

and C. HANKS

METHODS Climatic conditions All trials were undertaken at Tumbi Agricultural Research Station, near Tabora in western Tanzania. The Tabora area has a monomodal rainfall pattern, November to April being the rainy season. Ambient temperatures never fall below a minimum of 10°C at night and the mean minimum and maximum daytime temperatures are 18 and 25°C respectively. Maize is harvested from March onwards, mainly during May and June. As harvesting occurs after the rains have terminated drying for storage is not a problem in this part of the country and very little damage in store occurs because of fungal infestation. P. truncatus is able to infest the crop before harvest and is capable of causing heavy damage to maize during the earlier months of storage, during the dry season (Hodges et al., 1983). This behaviour differs from that of other common insect storage pests, particularly Sitophilus spp which only tend to become significant after the rains have commenced when the relative humidity of the air and the moisture of the grain has increased. Experimental materials The maize used in the trials was a mixture of improved varieties including Katumani, ICA and UCA, the last being predominant. Maize was obtained from the research station farm. The insecticides, formulations and suppliers are shown in Table 1, and detailed dosage rates are indicated with the results. Miscellaneous materials used are shown in Table 2. The traditional storage structures, “vihenge”, constructed in the shape of a cylindrical basket from woven twigs and plastered with a mixture of mud and cow dung on their internal surfaces were built by a local village craftsman. Sampling interval and sample examination Samples were examined immediately after treatment and then at 2 monthly intervals for 8 or 10 months unless otherwise stated. Each sample was coned and quartered to produce a working sample of approx. 1 kg. After shelling and sieving the moisture content was determined and assessments were made of grain damage and weight loss and of insect fecundity by measuring the numbers of adult insects produced by the F, generation. Details of the procedures used are given in Golob et al. (1985). Table

1.Theinsecticide active ingredients and formulations used in the trials

Cob spraying trials

Deltamethrin (Rousell-Uclaf): 2.5% e.c.* Deltamethrin + piperonyl butoxide: 2.5% + 25% e.c. Permethrin (Wellcome): 25% e.c. (25:75, cis : vans) Permethrin + piperonyl butoxide: 10% + 50% e.c. Pirimiphos-methyl (ICI): 50% ec. Permethrin: 25% w.p. Dust admixture

trials

(Suppliers as above) Deltamethrin: 0.2% dust Permethrin: 0.5% dust Pirimiphos-methyl: 2% dust le.c.-Emulsifiable

concentrate.

Table 2. Miscellaneous materials used in the trials* Tephrosia uogdii (African Flower Industries, Tanzania) powdered leaves Melie ozednrach (AFI) powdered leaves

Pyrethrum (AFI) crude aqueous extract Azadirachta indica powdered leaves Crotoiaria jtmcea dried seed

Washed sand Paddy husk ash Wood ash *Other materials that were used in the trials but whose results are not reported because they were ineffective included tobacco dust, maize core ash, goat dung and soda ash.

Protection of stored maize in Tanzania

189

Application of insecticide sprays to cobs

A series of four trials were undertaken between 1983 and 1987. In three of the trials cobs were laid out within an arena marked out on a well-swept area of compacted earth and the upper surfaces of the cobs were sprayed. After treatment, replicates of 60 cobs were randomly arranged and stored in small chicken-wire compartments within a large wire-mesh crib (Golob et al., 1985). In the final trial pairs of cobs were tied together by their outermost husk leaves and then suspended across horizontal bars of one of two traditional, vertical, storage frames. Each frame held 36 replicates of 36 cobs, 9 columns of 4 rows. All 4 rows of a single column were subjected to one of two treatments or were left untreated. The control and treatments were replicated 4 times. Insecticide or water sprays were applied using a Cooper Pegler CP 15 knapsack sprayer fitted with a flat fan nozzle. Both sides of the frame were sprayed. Diluted solutions were prepared so that the rate of application of the final solution was approx. 5 l/m’. Immediately treatments were applied 10 cobs were selected at random from each treatment, wrapped in aluminium foil and deep frozen to -20°C to await despatch by air to the U.K. for insecticide residue analysis. At each sampling occasion 4 replicates of each treatment and control were randomly selected from any position on the frames or in the crib and removed in entirety for laboratory examination. Treatment of cobs by dipping

Laboratory

work (Hodges and Meik, 1984) showed that maize cobs could be protected against if the cut ends of the cobs were dipped in dilute dust or solutions of permethrin. A field trial was established to test this possibility. Cobs were dipped for 4 set to a depth of 3 cm in different concentrations of permethrin emulsifiable concentrate, wettable powder or dilute dust before storage. After drying each replicate of 20 cobs was placed in a wire-mesh compartment and was examined at regular intervals as described above. P. truncatus infestation

Application of dilute insecticide dusts

Dilute dusts were applied to shelled maize by shovel mixing. The size of the replicates varied between different trials being 7.5-250 kg. Where replicates were 7.5 kg the grain was stored in small hessian sacks. Otherwise grain was stored in standard sacks of 90 kg capacity or in small vihenge of 500 kg capacity. Storage containers were filled to about half capacity. Where grain was stored in standard sacks samples of approx. 1 kg were collected from both the top and bottom of each sack to determine whether grain compaction at the bottom of the sack led to an increase in damage due to P. truncatus and if so whether it could be overcome by insecticide treatment. Storage of maize in large traditional stores

An attempt was made to compare promising practical treatments when maize was stored in large traditional vihenge. Maize grain treated with permethrin dust or maize cobs sprayed with permethrin e.c. were loaded into vihenge of approx. 1 t capacity (shelled-out weight basis). Untreated controls were stored as loose maize grain. Each structure was plastered on its internal surface with a mixture of mud and cow dung. The stores were completely enclosed, each had a small door close to the base to facilitate access to the grain. Every week either 1 kg of grain or 60 cobs were removed from each store to simulate on-farm removal of maize for consumption. Every 4 weeks the maize collected at that removal was examined for insects and insect damage. Application of miscellaneous materials to grain

Since 1981 trials have been conducted annually to estimate the effectiveness of some of the materials available locally which might prove useful as protectants of farm-stored maize. Materials (Table 2) were selected for testing because farmers were already using them as protectants or because they have had some success as protectants in other countries or on other crops. Each material was shovel-mixed with shelled grain. Replicates of 7.5 kg of grain were treated individually and then poured into small hessian sacks. Treatments and controls were replicated 4 times.

P. GOLOBand C. HANKS

190

Chemical analysis

Procedures for the analysis of pirimiphos-methyl and permethrin residues are given by Golob analysis were similar to those for permethrin except that 5.0 ml aliquots of extract were applied to a column (22 mm i.d.) containing 10 g florisil capped with l-2cm granular, anhydrous sodium sulphate and eluted with 200ml of 15% diethyl ether in hexane. The oven temperature of the GLC was 240°C. All GLC analyses were conducted using a 1 m column packed with 3% OV225 on Chromosorb W H-P, 80-100 mesh. Untreated samples “spiked” in the laboratory gave the following recovery values: pirimiphos-methyl, 96%; permethrin, 100%; deltamethrin, 84%. et al. (1985). Conditions for deltamethrin

RESULTS

All results obtained from the percentage damage assessments were transformed to arcsines and the resultant data subjected to two-way analysis of variance and multiple comparison tests using 95% confidence intervals. Unless stated otherwise all treatments were highly significantly different from controls (P < 0.001). Application of insecticide sprays to cobs

The effects of treatment on insect damage are illustrated in Fig. 1. In the first two trials (1983/84 and 1984/85) all treatments provided significant protection. This was particularly so for 32 weeks 100 ao-

1983/84

60 40 -

20 -

1984/85

60 40r

24

time

(weeks)

32

40

.

Fig. 1. The effect on grain damage during storage of insecticides sprayed onto the husk leaves of maize cobs. Curves were derived from means of 4 replicates. Application rates for individual treatments and mixtures were: permethrin (P) 0.2 g a.i./m2; deltamethrin (D) 0.05 g a.i./m2; pirimiphos-methyl (PM) 1.0 g a.i./m2. Pyrethroids were synergixed with piperonyl butoxide @) at a ratio of 1: 10. The mixture (M) was pirimiphos-methyl and permethrin applied at the same rates. Controls (C) were sprayed with water. 1983/84, 1984/85 and 1985/86 cobs were stored in small wire-mesh compartments in an open crib. 1986/87 cobs were suspended in pairs across a vertical storage frame.

Protection of stored maize in Tanzania

191 i

in the first trial where the control infestation developed relatively rapidly. During the second trial the control infestation developed much more slowly and although damage levels increased markedly in the controls after 32 weeks the damage in treatments was maintained at a very low level throughout the entire storage period of 40 weeks. There were no differences in effects between deltamethrin and permethrin nor between synergized and non-synergized components with the exception that synergized permethrin in the 1983/84 trial was significantly less effective at 40 weeks. In the 1985/86 and 1986/87 trials pirimiphos-methyl was included in the treatments both as an individual application and as a component of a mixture with permethrin. Pirimiphos-methyl alone did not provide adequate protection of cobs in the f’lrst trial. In neither trial did the mixture provide significantly better protection than was obtained with permethrin alone. Cobs stored on frames after treatment (1986/87) were not protected as effectively as cobs stored in the wire-mesh compartments of the large crib. After 40 weeks treated cobs exhibited almost as much damage as control cobs when stored on the frame. Damage to cobs stored in wire-mesh compartments was due almost entirely to P. truncatus, particularly during the early months of storage (Fig. 2; 1983/84, 1985/86). Only at the end of the trials did Sitophilus spp. become significant. However, when stored on the frames (Fig. 2; 1986/87) P. truncatus was of only limited importance throughout the storage period and most of the damage could be attributed to SitophiZus spp. It is probable that cobs stored on frames are more susceptible to attack by Sitophifus spp, adults of which would have ready access to the grain 1983/84

1985/86 100

24

weeks 24 weeks 300

200 40 weeks

100 32 weeks

u) C i

0

C

: ._

Dp

III

P

-III

C

D

Dp

P

M

Pp

-c

PM P.

0

PM

1986/87

6 ~~

D

P

Pp

0

truncatus

Sitophilus

Fig. 2. Number of live F, adults that emerged from 300 g of grain 28 days after samples were collected from cobs sprayed with different insecticides. Number of weeks shown indicates duration of storage after treatment. Details of treatments and replicates were as given for Fig. 1.

P.

192

G~LOB

and C.

HANKS

through the exposed tips of the cobs, than when cobs are stored in compact batches in the small wire-mesh compartments where cob tips would not be so readily accessible.

Treatment of cobs by dipping Insect activity remained low throughout this trial up to the final sampling occasion (Table 3). After 40 weeks’ storage, significant protection was achieved with some treatments but not by dipping in the dilute dust, the only formulation readily available to the farmer. The damage was principally due to P. truncatus. The feasibility of using this method as a means of protecting maize, even if commercially available liquid formulations could be produced for dilution on the farm, would seem to be limited.

Treatment of grain using dilute dusts All three trials in which grain was stored after treatment with pyrethroids demonstrated the manifest benefit of this method of protection (Fig. 3). It was not possisble to differentiate between the effectiveness of the treatments. It is clear that excellent protection of grain can be achieved whether the maize is stored in small lots (1983/84) or in large bulks in traditional vihenge (1984/85) where the grain would be much more consolidated, predisposing towards P. truncatus infestation. The trial with large sacks demonstrated the ability of P. truncatus to develop more effectively in compressed grain than in grain which is relatively free-moving (Fig. 4). Although the damage developed at about the same rate for the first 16 weeks, during the following 8 weeks the damage increased markedly in the bottom of the sacks because of the expansion in the P. truncatus population. By the end of the trial the damage was similar in all parts of the sack but this was principally a result of increases in Sitophilus populations; P. truncatus did not cause significant damage in the tops of the sacks at any time (Fig. 4). Observations on F, adults substantiated these conclusions (Figs 4 and 5). Very few adults were found throughout the storage period and then almost exclusively on untreated controls. Insect damage was primarily due to infestation by Sitophilus spp, not by P. truncatus.

Storage of maize in large traditional stores (vihenge) In both trials (Fig. 6) grain stored after treatment with permethrin dust and cobs stored after being sprayed were significantly less damaged than untreated grain. There was no significant difference between the two treatments. The benefit of storing sprayed cobs in an enclosed structure was much more apparent than when similarly treated cobs were stored in either wire-mesh compartments or on a frame (see above). No doubt the vihenge, themselves, provided a physical barrier against reinfestation by storage insect pests. Certainly, if stored in this manner the application of sprays would provide a valuable alternative to dust admixture with grain. Almost all the infestation in the vihenge was due to Sitophilus spp; P. truncatus was only evident in the controls (Table 4). Table 3. Pecentage number of damaged grain in samples of maize cobs diooed in different Dermethrin concentrations Duration of storage (weeks) Treatment Control (water) 25% e.c. 5% e.c. 1% e.c. 25% w.p. 0.5% dust

8

16

24

32

40

2.1 0 0 0.9 0 1.3

0.9 0.7 1.0 1.0 0.7 1.1

2.3 1.2 1.3 1.1 0.9 2.8

6.3 1.7 5.1 4.6 3.0 8.3

32.0 12.3 20.3 13.5 7.7 41.6

Each datum represents the mean of 4 replicates. e.c.-emulsifiabk concentrate; w.p.-wettable powder. 5% and 1% e.c. were prepared by dilution of the commercially available 25% concentrate.

Protection of stored maize in Tanzania

60

193

-

1984/85

80

0

16

8

24

32

60 -

40

-

8

16

time

24

32

(weeks)

Fig. 3. The effect on grain damage during storage of mixing maize grain with dilute insecticide dusts. Application rates were: permethrin (Pi) 1.5 ppm; permethrin (Pii) 2.5 ppm; deltamethrin (Di) 0.25 ppm; deltamethrin (Dii) 0.5 ppm; mixture (M) was permethrin 2.5 ppm plus pirimiphos-methyl 1Oppm. Controls (C) were untreated. In the large sack trial samples were taken from both the top (t) and bottom (b) of each sack. 1983/84-7.5 kg replicates (4) stored in small hessian sacks. 1984/85-200 kg replicates (4) stored in small vihenge. 1984/85 large sacks-50 kg replicates (2) stored in standard (90 kg) hessian sacks; samples collected from close to the bottom and top of each sack.

The effect of locally -available materials

Results obtained with substances giving significantly less damage than the control after 32 or 40 weeks are presented in Table 5. As a great deal of interest has been expressed in Tanzania in the potential use of Crotolaria juncea as a crop protectant the results obtained with dried seed are also included. Some of the ashes used were quite effective especially paddy-husk ash and ash obtained directly from the kitchen fire, i.e. wood ash. Where more than one dosage of a material was applied better protection was obtained with the higher rate of application; similar experiments in Malawi to protect maize against Sitophilus alone demonstrated the same effects (Golob et al., 1982). Wood

194

P.

GOLOB

and C. HANKS

1984/85

I 24

weeks

lIi3lP.truncatus cl

Sitophilus

25

r

Q)

-’ .-

100

1 32

2

weeks

75

50

25

Ct -

control

Cb -

control

Pt -

permethrin

Pb -

top bottom

permethrin

top bottom

0

ct

Cb

Pt

Pb

Fig. 4. Number of live F, adults that emerged from 300 g of grain 28 days after samples were collected from grain treated with insecticide dusts and stored in standard (90 kg) hessian sacks. Samples were taken from top (t) and bottom (b) of each replicate (2). Permethrin was applied at 2Sppm, controls were untreated.

ash was effective when admixed at 30% by weight, sufficient to completely cover the grain. Sand was also effective but only when applied at 20% by volume, also sufficient to cover the grain. Smaller quantities were generally not effective with the exception of paddy-husk ash which gave good protection at 5%. Plant extracts and plant parts, including seeds of C. junceu, were found to be ineffective. Although some of the treatments were significantly better than the controls the variation in their effectiveness differed from year to year (Table 5). This apparent lack of continuity and the clearly inferior protection provided in comparison to that provided by pyrethroid dusts might not give the farmer sufficient confidence to use any of these materials. Insecticide residues

Insecticide residues obtained in the 1985/86 trial, in which both cob and grain experiments were undertaken, are presented in Table 6. Residues obtained in the other trials were similar. Residues found on grain after the application of sprays were much lower than the Maximum Residue Limits established by the WHO/FAO Joint Meeting on Pesticide Residues (Snelson, 1987) but examination of the husk leaves revealed extremely high concentrations of active ingredients.

Protection of stored maize in Tanzania

195

1983/84

.****... grain dusted 16 weeks

0) = 40

150

0,

-

control

----

cobs sprayed

32 weeks

100 -

100

.> s!

50 -

80 -

0 C

Di

f-l

-

Dii

Pi

S

l-l Pii

aY 0)

M

GO-

1984/85

CO t 4

200

0

0

40

weeks

: .-

150 -

2

100 -

zz

50 -

&

80

; : 0

C

Dii

Pii

60

40

M 20

*... .* ....

EaP.truncatus cl SitoDhilus

0

0

4

8

12

16

20

25

32

36

40

time (weeks)

Fig. 5. Number of live F, adults that emerged from 300 g of grain 28 days after samples were collected from grain treated with insecticide dusts. Details of treatments and replicates are given with Fig. 3. Number of weeks shown indicate the duration of storage after treatment.

Fig. 6. Insect damage in maize stored in large traditional structures (vihenge). Each treatment was replicated twice. Permethrin was applied to grain at 2.5 ppm. Cobs with husk leaves intact were sprayed with permethrin at 0.2 g a.i./m2. Controls were untreated maize grain.

If this method of crop protection is to be practised by farmers it will be essential to ensure that the husks are not consumed by the farm livestock but are destroyed instead. Residues in grain treatment trials never reached nominal dosages applied even when all possible measures were taken to reduce loss of active ingredient. However, this did not apply to deltamethrin as nominal dosages were often obtained and even surpassed.

DISCUSSION

The data in general confirm the previous evidence (Golob et al., 1985) which indicated that much better protection of maize against P. truncates infestation is obtained if grain is stored after cobs are shelled than if cobs themselves are stored. However, it is likely that adequate protection can be obtained if undehusked cobs are sprayed and stored in a confined space such as in a traditional, mud-plastered structure, the “vihenge”. Observation of field practice and some experimental evidence suggests that if cobs are not treated almost immediately after harvest and P. truncate becomes established then spraying with insecticide may not prevent significant infestation, Insecticide must be applied as a prophylactic measure to prevent ingress of adults rather than as a remedial treatment performed to eradicate an existing population. A well plastered structure provides a good physical barrier against

P. GOL~JB and C. HANKS

196

Table 4. The number of live Fl adults that emcraed 28 davs after examination of samples collected from maize stored in vihenge v

Treatment Permethrin of storage (weeks)

Control S

Dust on grain Spray on cobs s P s P

P

1984/8g 0 0 0

8 16 24 32 40 1985@6 4 8 12 16 20 25 32 36 40

5 18 I

1 1

1 0 22 >500 >SOO 22

:i

0 0

0 0

0 0

190 55 235

0 0 235

0 0 0

0 0 13

0 0 0 0 2

2 5 31 21 69 159

0 0

0 0

I

0

1 0 1 0 4 26 156

1 0 0 1 0

2 0 0 0

2 0 0 0 1 0 2 3

386 >500

1 4 9 22

:,

Each datum is the mean of two replicates. S-Sitophilur spp; P-P. rrwcotus. Controls were untreated maize grain, dust was applied at 2.5 mg/kg and spray was applied at 0.2 g a.i./m*.

infestation and will assist the protection of maize provided it is put into store. The plastered vihenge do not prevent so not being airtight they would allow the development of untreated produce placed inside vihenge would sustain demonstrated by the controls.

the produce is free of infestation before air exchange across their surfaces and insect populations to proceed. Infested, very heavy damage with time as was

Table 5. The effect of admixing locally-available materials on damage sustained by maize grain during storage due to insect infestation Duration of storage (weeks) Treatment

SD*

24

32

40

b

1.2 3.4 1.2 3.3 2.1

1.8 49.9 0.1 1.6 32.4

16.8 53.5 1.2 15.4 91.5

3.5 0.1 2.9

::: 30.9

76.0 94.4 16.4 30.4 56.5 “’

100.0 94.8 36.5 82.3 94.5 22.1 27.9

87.0 9.3 9.4

98.9 26.3 10.5

1984pJ5

Control T. vogelii (5%)

Paddy husk ash (5%) Paddy husk ash (1%) M. azedarach (5%)

: c

Pyrethrum (1%) Wood ash (5%) Sand (20%)

: d

0.2 0.0

C

0.0

a

36.0 13.7

IsaS/fJ6 Control C. juncea (5%) Paddy husk ash (5%) Paddy husk ash (1%) Wood ash (5%) Wood ash (30%) A. in&a (1%) 19aals7 Control Paddy husk ash (5%) Wood ash (30%)

: c b d d

::; 8.6 5.3 _ 21.6 5.5 5.4

Treatments were expressed as percentage by weight of grain except for sand which was by volume. Data represent the means of 4 replicates. Absence of data is indicated by -. *Two-way analysis of variance on data transferred to amsints gave: in 1984/85 variance ratio (F) for treatments = 74.68, P < 0.001; Ffor time intervals = 417.62, P < 0.001; Ffor the interaction = 18.95, in 1985/86 F for treatments = 5.71, P < 0.01; F for time intervals = 42.53, P < 0.001; F for the interaction = 2.37. Multiple comparisons using 95% contidence intervals arc illustrated in the table; treatments followed by the same letter were not significantly different.

Protection of stored maize

191

in Tanzania

Table 6. Insecticide active ingredient of grain treated with dilute dusts, and cobs treated with sprays (1985/86) Treatment Permethrin Pirimiphos-methyl Mixture permethrin + pirimiphos-methyl Treatment

Application rate (g a.i./m2) Cob treatments 0.2 1.0 0.2 1.0 Application rate (mg/kg) Grain

Permethrin Deltamethrin Mixture permethrin+ oirimiohos-methvl

Rg~~r

(mg/kg) Husk

<0.02 0.16

4s 550

<0.02 0.12

38 438

Residues (me/kg)

treamm~ 2.5 0.5

1.38 0.53

2.5 10.0

1.28 5.25

Residues from grain treatments represent the mean of 4 replicates. Residues from cob treatments were obtained by analysing grain and husks of a number of cobs from two replicates. Permethrin residues represent the sum of cis and frans isomers (2.5:75). Samples were collected immediately after treatment and deep frozen (approx. -20°C) before being airfreighted to the U.K. for analysis of residues.

Storage of cobs in open structures predisposes them towards heavy infestation whether they are treated with insecticide or not. Exposure to wind or moving currents of air will encourage rapid metabolism of active ingredient and reduce the viability of the insecticide when compared with the use of insecticide in closed storage structures (Golob, 1984). Furthermore, cobs in open structures are subjected to greater infestation pressure because of the absence of the physical barrier. Thus if cobs are to be treated they must be stored in mud-plastered containers (or similar impervious receptables such as metal tanks or drums) unless the storage period is short when the insecticide is still biologically active. Grain must be dry (moisture content of less than 14%) if it is stored in impermeable containers to avoid anaerobic fermentation and consequent rotting. P. truncatus was almost eliminated from treated maize stored in mud-plastered vihenge, whether stored as cobs or grain (Table 4). However, in untreated grain the infestation was extremely heavy, clearly demonstrating that it is not sufficient simply to shell maize and store it untreated. Although P. truncatus develops more efficiently on cobs than on grain (Golob et al., 1985; Howard, 1983) it can very readily damage grain particularly stored in large heaps when the kernels would be consolidated. Results from the large sack trial (Fig. 4) supported this conclusion. The data suggested that under Tabora climatic conditions P. truncatus can compete favourably with S. oryzae or S. zeamais. In most of the cob and grain trials P. truncatus populations grew quicker than those of Sitophilus and only towards the end of the storage period did the Sitophilus population increase to similar levels. P. truncatus is more tolerant of hotter, drier climates (optima 30°C and 70% r.h.; Shires, 1979) than Sitophilus (optima 2527°C and 70% r.h.; Dick, 1988) and is thus able to develop relatively quickly soon after harvest in the dry, ambient conditions which exist. Only when the rains are imminent, about 5 months after harvest does the r.h. increase and the conditions become favourable for Sitophilus. When Sitophilus populations are accelerating P. truncatus is in decline so that by the end of many of the trials few P. truncatus individuals were present. It is not clear why Sitophilus populations were able to continue to increase in an environment of an ever decreasing food supply. This pattern did not occur in the vihenge trials when even at the end of storage large quantities of maize remained in the stores, sufficient to maintain the pest populations with an adequate food source for development. Rates of development of P. truncatus in control infestations differed from year to year (e.g. Fig. 1). In part this was due to minor differences in climatic conditions prevailing after harvest and also to the level of preharvest infestation. The bionomics of P. truncatus in Tabora will be the subject of a separate publication. Acknowledgements-We

wish to thank the Ministry of Agriculture and Livestock Development of the Government of the Republic of Tanzania for permitting this research to be undertaken. Thanks are due to VSO for supporting Mrs Hanks and to the Tanzanian Agricultural Research Organisation for providing staff assistance and working facilities. Our compliments to Mr Ken Kilminster for conducting residue analyses and to Mr Forbes Walker and Mr Thomas Tayari for organizing laboratory examination of samples.

198

P.

GOLOB

and C. HANKS

REFERENCES Dick K. (1988) A review of insect infestation of maize in farm storage in Africa with special reference to the ecology and control of Prostephanustruncatus.Rull. Overseas DevelopmentNatural Resources Institute, No. 18, v + 42 pp. Golob P. (1984) Improvements in maize storage for the smallholder farmer. Trop. stored Prod. Znf. 50, 1419. Golob P. and Hodges R. J. (1982) Study of an outbreak of Prostephanustruncatus(Horn) in Tanzania. Rep. Tropical Products Institute, G138, vi + 23 pp. Golob P., Mwambula J., Mhango V. and Ngulube F. (1982) The use of locally available materials as protectants of maize grain against insect infestation during storage in Malawi. J. stored Prod. Res. 18, 67-74. Golob P., Changjaroen P., Ahmed A. and Cox J. (1985) Susceptibility of Prostephanw truncatus(Horn) (Coleoptera: Bostrichidae) to insecticides. J. stored Prod. Res. 21, 141-150. Hodges R. J. and Meik J. (1984) Infestation of maize cobs by Prostephanustruncatus(Horn) (Coleoptera: Bostrichidae)aspects of biology and control. J. stored Prod. Res. 20, 205-213. Hodges R. J., Dunstan W. R., Magaxini I. and Golob P. (1983) An outbreak of Prostephanustruncatus(Horn) (Coleoptera: Bostrichidae) in East Africa. Protect. Ecol. 5, 183-194. Howard D. C. (1983) The population biology of the greater grain borer Prostephanustruncatus(Horn). Ph.D. Thesis, University of Reading. Shires S. W. (1979) Influence of temperature and humidity on survival, development period and adult sex ratio in Prostephanustruncatus(Horn) (Coleoptera: Bostrichidae). J. stored Prod. Res. 15,5-10. Snelson J. T. (1987) Grain Protectants. Australian Centre for International Agricultural Research Monograph No. 3, x + 448 pp. Canberra, Australia.