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Effects of a mixture of vegetable and essential oils and fatty acid potassium salts on Tetranychus urticae and Phytoseiulus persimilis
ARTICLE IN PRESS
Ecotoxicology and Environmental Safety 70 (2008) 276–282 www.elsevier.com/locate/ecoenv
Effects of a mixture of vegetable and essential oils and fatty acid potassium salts on Tetranychus urticae and Phytoseiulus persimilis H. Tsolakis, S. Ragusa Dipartimento S.EN.FI.MI.ZO., Sezione di Entomologia, Acarologia e Zoologia, Universita` di Palermo, Viale delle Scienze, 90128 Palermo, Italy Received 21 November 2006; received in revised form 13 September 2007; accepted 2 October 2007 Available online 26 November 2007
Abstract Laboratory trials were carried out to evaluate the toxicity and the influence of a commercial mixture of vegetal, essential oils, and potassium salts of fatty acids (Acaridoil 13SLs) on the population growth rate (ri—instantaneous rate of increase) of two mite species, the phytophagous Tetranychus urticae Koch and the predator Phytoseiulus persimilis Athias-Henriot. A residue of 1.3 mg/cm2 of pesticide solution was harmless for Ph. persimilis eggs, while a moderate mortality of eggs and of larvae from treated eggs of T. urticae, was observed (53.8%). The pesticide also caused a delay in the postembryonic development of the tetranychid. Moreover, 83.4% mortality was reported for treated females tetranychids and only 24.0% for Ph. persimilis females. The pesticide influenced negatively the population growth of T. urticae which showed a very low rate of increase (ri ¼ 0.07), compared to that obtained in the control (ri ¼ 0.68). The pesticide did not affect negatively the reproductive potential of Ph. persimilis (ri ¼ 0.54 and ri ¼ 0.57 for test and control, respectively). These results suggest a considerable acaricidal activity of potassium salts of fatty acids and caraway oil on T. urticae and a good selectivity on Ph. persimilis. r 2007 Elsevier Inc. All rights reserved. Keywords: Toxicity; Caraway essential oil; Fatty acid potassium salts; Tetranychus; Phytoseiulus
1. Introduction Identifying selective pesticides for IPM programmes is necessary to protect the natural beneficial arthropod fauna and at the same time reduce environmental pollutants. A low toxicity of these products to natural beneficials is more important than a high toxicity towards the target species. This is because the action of a natural beneficial on the target species has to be added to the effect of the selective pesticide. In fact, a selective pesticide should have a low or no toxicity towards beneficials. Moreover, it should degrade rapidly in the environment. At present, very few synthetic pesticides meet these criteria, but pesticides of plant origin seem to be good candidates in this list, as they generally have a very short persistence on the plant (Riederer and Schreiber, 1995; Isman, 1997; Cabras et al., 2002), and up to now show high selectivity
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when used as extracts (Mansour et al., 1986; Tsolakis et al., 1997; Ragusa Di Chiara et al., 2007). However, the selectivity of these products has to be strictly evaluated for different species of natural enemies as deleterious or sometimes positive effects were found among the natural enemies complex (Stark et al., 1990, 1992; Tsolakis and Ragusa, 1999). The aim of this study is to evaluate the toxicity of a commercial mixture of plant essential oils (caraway oil) and potassium salts of fatty acid (FAPS), to the two spotted spider mite Tetranychus urticae Koch and its specific predator Phytoseiulus persimilis Athias-Henriot. Since their discovery as biocide agents (1947), salts of fatty acids have been reported to have a very low toxicity to humans and these salts are considered environmentally safe (Prats et al., 1999), even if their phytotoxicity limits their use on some crops (Pasini et al., 1997). Though toxic and repellent effects of the caraway essential oils have been reported on different insect and mite species (Franzios et al., 1997; Kim et al., 2004; Pavlidou et al., 2004), there are no data on their effect on predatory mites. This paper
ARTICLE IN PRESS H. Tsolakis, S. Ragusa / Ecotoxicology and Environmental Safety 70 (2008) 276–282
reports the acute toxicity, repellency, and effect on population growth of the two species of mites mentioned above when they are treated with Acaridoil 13SL (VIORYL, Greece). A demographic approach has been used because it has been shown by different authors that population growth is an important endpoint in the toxicological studies (Ahmadi, 1983; Walthall and Stark, 1997; Stark and Banken, 1999; Venzon et al., 2005). Repellency effects have also been examined as they may play an important role in the integrated pest management of the crops. 2. Materials and methods 2.1. Mite species tested T. urticae was collected from weeds and kept in laboratory cultures on bean plants (Phaseolus vulgaris L.). Ph. persimilis was collected from strawberries infested by T. urticae, reared in the laboratory on plexiglas arenas (Swirski et al., 1970), and fed with various stages of the tetranychid mite. Both species were maintained in a growth chamber set at 2571 1C, 7075% R.H., and a 16:8 h L:D photoperiod.
2.2. Chemical tested Acaridoil 13SLr (VIORYL, Greece), is a commercial product containing 13.04% (w/v) of potassium salts of fatty acids (fatty acids in Acaridoil were of plant origin), 3% (w/w) of caraway essential oil (from Carum carvi L.), 5% (w/w) mono- and diglycerides and oleic acid, 70% (w/w) vegetable oils (mainly soybean oil), and water.
2.3. Experimental unit Each experimental unit (EU) consisted of a bean leaf disc (3 cm diameter) placed with its dorsal side on wet cotton in a plastic Petri dish (100 10 mm). The cotton was saturated with distilled water daily during the test period.
2.4. Acute toxicity tests Studies were carried out separately for the eggs and the adult stages of the two mites. Eggs: 24 h old eggs were used in the tests. To obtain the same age of the eggs, 10 ovipositing females of T. urticae were transferred in each EU, and allowed to lay eggs for 24 h. Each EU contained 20 tetranychid eggs. To obtain eggs of Ph. persimilis, 15 ovipositing females were transferred into a plexiglas arena (Swirski et al., 1970) with abundant prey for 24 h. Afterwards, five eggs of the predator were transferred with a fine brush from the arena to each EU. Adults (two-spotted spider mite): in these test young females were used (max 3 days old) which had been obtained by placing 100 deutonymphs of T. urticae transferred from the cultures onto excised bean leaves placed on wet cotton in Petri dishes. Doing so, the adult mites used in our experiments were likely all fertilized. The emerged females and males were transferred onto new bean leaves for 2–3 days. Afterwards, 10 ~~ of the tetranychid were transferred to each EU and allowed to settle for half an hour before spraying. Ph. persimilis adults were obtained by transferring 100 eggs of the phytoseiid with a fine brush from the culture arenas onto a new arena with abundant prey, until attaining adulthood. The emerged females and males were transferred to a new arena with abundant prey for 2–3 days. Afterwards, 5 ~~ predators were transferred to each EU and sprayed immediately.
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Tests were replicated 5 times for the tetranychid mite and 10 times for the phytoseiid; all tests were carried out at 2571 1C, 7075% R.H., and a photoperiod of 16:8 L:D. Sprayings were done with a Potter Precision Spray Tower (Potter, 1952). We applied the concentration recommended by the manufacturer, that is, 2478 ppm of potassium salts of fatty acid and 570 ppm of caraway essential oil (concentration of the commercial formulate 19,000 ppm). For each EU, 2 ml of solution were used at 35 kPa of pressure, giving deposits of 1.3 mg of liquid per cm2. The pesticide was dissolved in distilled water. Control tests were treated with distilled water only. Sprayings were made directly on the mites. After spraying the leaf discs were placed in new Petri dishes on wet cotton with distilled water; while testing Ph. Persimilis, spider mites (all stages of T. urticae) were brushed onto a disc leaf daily to serve as food supply for the predator during the test period. The mortality of the eggs and the residual effects on young stages were recorded after 5 days for T. urticae and 3 days for Ph. persimilis. Mortality of females was recorded after 6 days for T. urticae and after 5 days for Ph. persimilis.
2.5. Repellency tests To measure the repellency of the pesticide, two-choice tests were carried out. In this case only the right half of the leaf disc was sprayed, covering the left half with a thin plastic film (mid-rib was the divisor of the disc leaf). Ten fertilized females of T. urticae or five fertilized females of Ph. persimilis were put on the mid-rib of the leaf 10 min after the spraying. Distilled water was applied in the control. Prey eggs were distributed on the whole surface of the disc leaf daily, to serve as a food supply for the predator. The mortality and the presence of adults and laid eggs on each part of the disc leaf were recorded daily at the same hour. To evaluate the effect of the pesticide on the predatory capacity of Ph. persimilis, treated eggs of T. urticae (E30 prey eggs/predatory female) were offered to the predator 10 min after spraying or 3 days after spraying. Predation and oviposition rate were registered after 24 h. Tetranychid eggs in the control were sprayed with distilled water. Tests were replicated 5 times for the tetranychid mite and 10 times for the phytoseiid at the same environmental conditions as in the acute toxicity tests.
2.6. Population growth study The effect of the pesticide on the populations growth of the two species was measured by the instantaneous rate of increase (ri) (Hall, 1964; Walthall and Stark, 1997). This rate of population growth measures the population increase or decrease and is calculated according to the following equation: ri ¼
lnðN f =N 0 Þ , Dt
where Nf is the final number of animals, N0 is the initial number of animals and Dt refers to the number of days the experiment is run. Solving for ri yields a rate of population increase similar to that obtained with the intrinsic rate of increase (rm) (Walthall and Stark, 1997). Positive values of ri show a growing population, ri ¼ 0 indicates a stable population while negative values of ri indicate a declining population directed toward extinction (Walthall and Stark, 1997). Instantaneous rate of increase was calculated after 3 and 6 days for T. urticae and after 3 and 5 days for Ph. persimilis, both on acute toxicity and repellence tests.
2.7. Statistical analysis Abbott’s formula (Abbott, 1925) was used to correct control mortality. Proportions of egg and larvae mortality, as far as distribution data, were transformed using an arcsine-square-root equation and a residual analysis has been performed in order to confirm the normality assumption prior to Student’s t-test or ANOVA. When the assumption of homogeneity of
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Table 1 Influence of Acaridoil 13SL on eggs and oviposition rate and survival of females of T. urticae and Ph. persimilis Initial number of mites
Hatched eggs
Surviving larvae
Eggs Females
100 50
71
45
Eggs Females
100 50
98
Ph. Persimilis Test Eggs Females
50 50
50
50 50
50
T. urticae Test Control
Control a
Eggs Females
Surviving females
Mortality %a mean7S.E.M.
53.7874.48 83.4076.22
7 98
Total number of laid eggs
85
Number of eggs/ female mean7S.E.M.b
0.7270.19 b
43
– –
34
8.8973.33 24.0075.95
677
3.8570.22 a
44
– –
891
4.0570.17 a
46 50
10.8170.87 a 3043
% mortality calculated according to Abbott (1925). Different letters denote significant differences for po0.01 (Student’s t-test).
b
variance (Levene test) was not met, the alternative no parametric ANOVA (Friedman test) was performed on the data. Normally distributed data were analyzed by one-way analysis of variance. When significant differences were found by ANOVA, the means were separated by Tukey’s studentized range honest significant differences (HSD) test. Student’s t-test was used to compare means of instantaneous rate-of-increase data. Differences were considered significant when 95% or 99% fiducial limits did not overlap. Analyses were computed using the software ‘‘Statistica’’ (StatSoft Inc., 2003).
3. Results 3.1. Acute toxicity tests The pesticide showed a slight ovicide effect on eggs of T. urticae and a slight residual effect on larvae from treated eggs. Dead eggs appeared dehydrated and air had penetrated inside. The total mortality of both stages was 53.78% (Table 1). It should be mentioned that after 6 days from spraying on eggs, only two larvae (4.44%) reached the protonymph stage and all individuals were in very bad condition, while in the control 100% of juveniles became protonymphs and were in good condition. On the other hand, no ovicidal effects were noted on eggs of Ph. persimilis, while a slight residual effect (8.89% of mortality) was noted on larvae (Table 1). No delay was recorded on the postembryonic development of the predator. Acaridoil 13SL caused a moderate knock down effect on young females of T. urticae after 24 h from spraying (47.6% of mortality) and a high mortality after 6 days (83.4%) (Table 1). The activity of the tetranychid females was limited and few eggs were laid (t ¼ 11.12, po0.01) (Table 1). A slight mortality was registered on females of Ph. persimilis at the end of the test (24.0%), whereas no negative effects were noted on the oviposition rate (F1,98 ¼ 0.53, p ¼ 0.46) (Table 1). As far as the population growth was concerned, the instantaneous rate of increase for T. urticae registered high
Fig. 1. Instantaneous rate of increase (mean7S.E.M.) for T. urticae and Ph. persimilis exposed to Acaridoil 13SL, 6 and 5 days (for the two species, respectively) after spraying. Different letters denote significant differences among the control and the test for each species (po0.01). Student’s t-test was performed on the data.
values in the control tests after 6 days (ri ¼ 0.68) (Fig. 1), while it was statistically lower in the Acaridoil test (ri ¼ 0.07) (t ¼ 10.67, po0.01). Statistical differences were registered for the phytoseiid population growth between the test (ri ¼ 0.54), and the control (ri ¼ 0.57), at 99% fiducial limit, but not at 95% (t ¼ 2.62, p ¼ 0.017) (Fig. 2). 3.2. Repellency tests Female tetranychid mites showed an uniform distribution on both the untreated part of the leaf and on the part sprayed with water in the control tests during the experimental period (F13,56 ¼ 1.45, p ¼ 0.23) (Table 2, Fig. 2A). Slopes of regression lines indicated a relative motility of the females, unrelated to the time, on the whole
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showed the attainment of the equilibrium after the fourth day (Fig. 3B). On the other hand, females preferred to oviposit on the untreated part of the leaf during the whole test period (F9,90 ¼ 12.65, p ¼ 0.00) (Table 2). In these tests, a mortality value similar to that obtained in the acute toxicity tests is reported (t ¼ 0.36, p ¼ 0.72) (Tables 1 and 2). The population of T. urticae in the control test had a very high rate of increase, but it was statistically lower in the pesticide two-choice test (t=8.7, po0.01) (Fig. 4). However, statistical differences were found also between the control and the test for Ph. persimilis (t ¼ 5.44, po0.01), but it should be mentioned that ri value in the test was maintained at very high levels (Fig. 4). Tests on the influence of treated prey eggs on the predatory capacity of Ph. persimilis females showed that freshly treated eggs reduce significantly both predation and oviposition rate of the predator (t ¼ 4.74, po0.01), even if both parameters were maintained at high levels (Table 3). On the other hand, the mean number of consumed prey eggs, supplied to the predator 3 days after the spraying, was similar to that obtained in the control, but a great variability was registered in these tests (Table 3). 4. Discussion
Fig. 2. Trend of probability distribution of T. urticae population on the untreated and treated parts of the leaf in the control (A) and pesticide (B) in the repellency tests.
leaf surface, even if the oviposition rate was higher on the treated part of the leaf (F11,48 ¼ 4.64, po0.01) (Table 2). However, a clear preference of tetranychid females towards the untreated part of the leaf was noted in the pesticide two-choice tests (F13,56 ¼ 59.94, po0.0001) (Table 2, Fig. 2B). Probability line indicated that this choice was maintained over time. It did not affect survival of females adversely as slight mortality was reported in these tests (Table 2). As expected, the predator mite showed a statistically uniform distribution on the leaf surface in the control tests (F9,90 ¼ 0.26, p ¼ 0.98), confirmed also by the oviposition rate (F9,90 ¼ 1.44, p ¼ 0.18), but it preferred to walk on the untreated part of the leaf in the pesticide tests (F9,90 ¼ 8.61, p ¼ 0.00) (Table 2, Fig. 3A and B). However, it should be mentioned that after the third day no statistical differences were registered between the means for the predator distribution on the untreated and treated part of the leaf (po0.01), even when probability confidential limits
The main active ingredients of the pesticide evaluated in our tests are potassium salts of fatty acid (FAPS) with caraway essential oil. FAPS are generally recognized as safe (GRAS) by the US Food and Drug Administration (21 Code of Federal Regulation 172.863) and they are used as insecticides, acaricides, herbicides, and algaecides (EPA, 1992). They probably function as contact agents since FAPS were found to penetrate the integument of arthropods disrupting cell membranes and causing dehydration and death. The caraway essential oil consists mainly of two monoterpenes: S(+)-carvone (50–60%) and S(+)-limonene (35–45%) (Hartmans et al., 1995). It is known for some plant-derived terpenoids that they can act as anti-feedants and growth inhibitors of mites (Klocke and Kubo, 1982; Murray et al., 1999; Isman, 2000). The carvanone/limonene essential oil showed high toxic effects on the poultry red mite Dermanyssus gallinae De Geer (Kim et al., 2004) and Drosophila melanogaster Meig. (Franzios et al., 1997), whereas D- and L-carvone was found to be lethal to T. urticae females (Lee et al., 1997). The exact mode of action of the essential oil remains unknown although from recent studies it was suggested that they impact the functioning of the octopaminergic nervous system (Enan et al., 1998 in Isman, 2000). Our results indicate limited ovicidal action of the product on the phytophagous mite, while it did not cause harm to the eggs of the predator. As the structure of egg chorion of the two species is similar, this difference could be due to the different exposure time. The predators’ eggs hatch within 24 h, while the phytophagous mites ones need 2–3 days. The ovicidal effect could be caused by FAPS as
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Table 2 Distribution of females and eggs of T. urticae and Ph. persimilis on the untreated and treated part of the leaf with Acaridoil 13SL
T. urticae Control
Test Ph. persimilis Control
Test
Untreated part
Treated part
Number of females and eggs registered during the test mean/day7S.E.M.a
Number of females and eggs registered during the test mean/day7S.E.M.a
Females Eggs
4.6370.26 a 14.6072.15 a
5.0670.25 a 23.3471.54 b
Females Eggs
8.5770.34 a 21.5172.64 a
0.6370.16 b 0.8970.36 b
Females Eggs
2.3170.15 a 10.4070.65 a
2.5570.17 a 11.2270.64 a
Females Eggs
2.8170.14 a 11.1070.75 a
1.3670.2 b 3.5270.44 b
Mortality (%)b
–
11.1179.94
–
17.0075.12
a
Different letters denote significant differences between the untretaed and the treated part of the leaf (po0.01). Analysis of variance (ANOVA) was performed on the data. Means were separated by Tukey’s studentized range (HSD) test. b % mortality calculated according to Abbott (1925).
Fig. 4. Instantaneous rate of increase (mean7S.E.M.) for T. urticae and Ph. persimilis partially exposed to Acaridoil 13SL (repellency tests) 6 and 5 days (for T. urticae and Ph. persimilis, respectively) after spraying. Different letters denote significant differences among the control and the test for each species (po0.01). Student’s t-test was performed on the data.
Fig. 3. Trend of probability distribution of Ph. persimilis population on the untreated and treated parts of the leaf in the control (A) and pesticide (B) in the repellency tests.
their reported mode of action aligns with our observations as do the results of others on the same mite species (Osborne and Petitt, 1985). The residual toxic effect on larvae of both species and the delay observed in the larval development of T. urticae, could be caused by the plant constituent of the pesticide (caraway essential oil), whereas the toxic effect of the pesticide on young females of the tetranychid may be caused by both FAPS and caraway essential oil. FAPS may easily penetrate the soft integument of different arthropod species, and such effects were weaker when used on species with strong integuments or covered with sclerotized shields. This could explain the different effects on tetranychid (soft integument) and phytoseiid mites (tegument with plates). It should be mentioned that Osborne and Petitt (1985) reported a greater toxicity of insecticide soaps on Ph. persimilis
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Table 3 Predation and oviposition rate of Ph. persimilis fed on T. urticae eggs treated with Acaridoil 13SL
Test 1: freshly treated prey eggs Test 2: prey eggs treated 3 days before Control a
Supplied prey eggs/predator female mean7S.E.M.a
Consumed prey eggs/predator female mean7S.E.M.a
Number of eggs/female/day mean7S.E.M.a
33.5075.77 a 31.9073.07 a 30.3876.62 a
16.8476.90 a 24.08713.97 ab 24.9971.48 b
2.9470.04 a 3.1670.07 a 3.7270.01 b
Different letters denote significant differences for po 0.01 (ANOVA Friedman test).
females than on T. urticae: LD50 of 1440 and 6600 ppm for the predator and the phytophagous mite, respectively. These differences, in our opinion, can be explained by the usage of different test methods: while Osborne and Petitt used the standardized FAO slide-dip method that allows for covering the mite body completely, we used the Potter tower for complete covering of the substrate surface on which the mites reside (IOBC/WPRS method: see Sterk et al., 1999). The high toxic effect of the product on T. urticae, observed in our tests, could also be due to the combined action of FAPS with caraway essential oil. As a matter of fact, this essential oil showed high toxic effects towards various mites and insects (Kim et al., 2004; Franzios et al., 1997; Lee et al., 1997). The pesticide showed a negative impact on the phytophagous mite over time, reducing its population growth to nearly zero. These effects were the combined result of increased mortality (direct loss of females) and of decreased fecundity (decreased production and viability of the eggs). No negative effects were noted for Ph. persimilis population growth. According to OILB/IOBC toxicological categories, this mixture of FAPS and caraway essential oil has to be considered moderately harmful for T. urticae and harmless for Ph. persimilis (Sterk et al., 1999). The very low mortality reported in T. urticae two-choice test is caused by the high repellency of the pesticide over time that restricts females on the untreated part of the leaf. In contrast, the short-lasting repellency of the pesticide towards Ph. persimilis two-choice tests, caused a mortality similar to that reported in the acute toxicity tests. Various authors found significant repellency of terpenoids to tetranychid mites (Mansour et al., 1986; Tsolakis et al., 2002; Tsolakis and Ragusa, 2004), but no data are available for Ph. persimilis. In the two-choice pesticide tests, the population growth of T. urticae was significantly reduced, while for the impact on Ph. persimilis population growth was minor. In the predation tests, using pesticidetreated prey eggs, the feeding-rate of Ph. persimilis was initially low but increased over time again albeit not to the control levels. Nevertheless, both predation rate and ovipositional rate were in principle still sufficiently high for controlling spider mites. These data indicate that the repelling effect of the caraway oil might be superable for the predator who also maintains sufficient fecundity and population growth to assume the oil could be compatible with biocontrol in the field.
In our opinion, the differences in the tetranychid tests are attributed to populations reaching the carrying capacity of the environment rather than to the sublethal effects of the pesticide. In these tests, tetranychid females lived on one half of the space in comparison to the control test. Repellence is to be considered an important target for phytophagous mite control as the aim of IPM programmes is not to destroy the phytophagous populations, but to reduce their density on crops, to permit the natural enemies’ populations to control the phytophagous ones. 5. Conclusion The mixture of potassium salts of fatty acids and caraway essential oil may be considered for usage as a selective acaricide, able to control T. urticae populations without adversely affecting Ph. persimilis. The delay in juvenile development of the tetranychid mite is very interesting but further investigations are needed to clarify the precise cause and its consequences. As no negative effects were found (given the IOBC guidelines) on the performance of individual predatory mites and their population growth, the mixture of potassium salts of fatty acids and caraway oil has serious potential as a low impact acaricide integrated with IPM programs. Acknowledgments Authors are deeply indebted to Prof. J. McMurtry and Prof. A. Lombardo who critically read the manuscript giving useful advice and to Ms E. Chiavetta who checked the English text. Anonymous reviewers are also thanked for their constructive comments and suggestions. The PRIN (2005) and PAN (2004–2006) provided financial support for this work. The present work was done in accordance with national and institutional guidelines for the protection of human subjects and animal welfare. References Abbott, W.S., 1925. A method of computing the effectiveness of an insecticide. J. Econ. Entomol. 18, 265–267. Ahmadi, A., 1983. Demographic toxicology as a method for studying the dicofol-twospotted spider mite (Acari: Tetranychidae) system. J. Econ. Entomol. 76, 239–242.
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Proceedings of the International Congress. Instituto de Biologı´ a and Facultad de Ciencias, Universidad Nacional Auto`noma de Me´xico; Sociedad Latinoamericana de Acarologı´ a. Me´xico, pp. 347–354. Riederer, M., Schreiber, L., 1995. Waxes: the transport barriers of plant cuticles. In: Hamilton, R.J. (Ed.), Waxes: Chemistry, Molecular Biology and Functions, vol. 6. The Oily Press, Dundee, Scotland, pp. 131–156. Stark, J.D., Banken, J.A.O., 1999. Importance of population structure at the time of toxicant exposure. Ecotoxicol. Environ. Saf. 42, 282–287. Stark, J.D., Vargas, R.I., Thalman, R.K., 1990. Azadirachtin: effects on metamorphosis, longevity, and reproduction of three tephritid fruit fly species (Diptera: Tephritidae). J. Econ. Entomol. 83 (6), 2168–2174. Stark, J.D., Wong, T.T.Y., Vargas, R.I., Thalman, R.K., 1992. Survival, longevity and reproduction of tephritid fruit fly parasitoids (Hymenoptera: Braconidae) reared from fruit flies exposed to azadirachtin. J. Econ. Entomol. 84 (4), 1125–1129. StatSoft Inc., 2003. Statistica, Users Manual. Stat Soft Inc., Tusla, OK. Sterk, G., Hassan, S.A., Baillod, M., Bakker, F., Bigler, F., Blu¨mel, S., Bogenschu¨tz, H., Boller, E., Bromand, B., Brun, J., Calis, J.N.M., Coremans-Pelseneer, J., Duso, C., Garrido, A., Grove, A., Heimbach, U., Hokkanen, H., Jacas, J., Lewis, G., Moreth, L., Polgar, L., Rovesti, L., Samsoe-Petersen, L., Sauphanor, B., Schaub, L., Sta¨ubli, A., Tuset, J.J., Vainio, A., Van De Veire, M., Viggiani, G., Vin˜uela, E., Vogt, H., 1999. Results of the seventh joint pesticide testing programme carried out by the IOBC/WPRS-Working Group ‘Pesticides and Beneficial Organisms’. BioControl 44, 99–117. Swirski, E., Amitai, S., Dorzia, N., 1970. Laboratory studies on the feeding habits, post-embryonic survival and oviposition of the predaceous mites Amblyseius chilenensis Dosse and Amblyseius hibisci Chant (Acarina: Phytoseiidae) on various kinds of food substances. Entomophaga 15 (1), 93–106. Tsolakis, H., Ragusa, S., 1999. Effetti dell’ olio di neem (Azadirachta indica A. Juss) su Bactrocera oleae (Gmelin) (Diptera, Tephritidae) in prove di laboratorio e di campo. Phytophaga 9 (Suppl.), 65–75. Tsolakis, H., Ragusa, S., 2004. Laboratory evaluation of plant extracts on Tetranychus urticae Koch (Acariformes, Tetranychidae), In: Weigmann, G., Alberti, G., Wohltmann, A., Ragusa, S. (Eds.), Acarine Biodiversity in the Natural and Human Sphere. Phytophaga 16, 539–548. Tsolakis, H., Leto, G., Ragusa, S., 1997. Effects of some plant materials on Tetranychus urticae Koch (Acariformes, Tetranychidae) and Typhlodromus exhilaratus Ragusa (Parasitiformes, Phytoseiidae). In: Proceedings of the ANPP—Fourth International Conference on Pests in Agriculture, Montpellier 6–8 January, pp. 239–245. Tsolakis, H., Ragusa, E., Ragusa Di Chiara, S., 2002. Effects of neem oil (Azadirachta indica A. Juss) on Tetranychus urticae Koch (Acariformes, Tetranychidae) in laboratory tests. In: Bernini, F., Nannelli, R., Nuzzaci, G., de Lillo, E. (Eds.), Acarid phylogeny and Evolution. Adaptations in Mites and Ticks. Kluwer Academic Publishers, pp. 351–362. Venzon, M., Rosado, M.C., Matiello Fadini, M.A., Iorio Ciociola Jr, A., Pallini, A., 2005. The potential of NeemAzal for the control of coffee leaf pests. Crop Prot. 24, 213–219. Walthall, W.K., Stark, J.D., 1997. Comparison of two population level ecotoxicological endpoints: the intrinsic (rm) and instantaneous (ri) rates of increase. Environ. Toxicol. Chem. 16 (5), 1068–1073.
Ecotoxicology and Environmental Safety 70 (2008) 276–282 www.elsevier.com/locate/ecoenv
Effects of a mixture of vegetable and essential oils and fatty acid potassium salts on Tetranychus urticae and Phytoseiulus persimilis H. Tsolakis, S. Ragusa Dipartimento S.EN.FI.MI.ZO., Sezione di Entomologia, Acarologia e Zoologia, Universita` di Palermo, Viale delle Scienze, 90128 Palermo, Italy Received 21 November 2006; received in revised form 13 September 2007; accepted 2 October 2007 Available online 26 November 2007
Abstract Laboratory trials were carried out to evaluate the toxicity and the influence of a commercial mixture of vegetal, essential oils, and potassium salts of fatty acids (Acaridoil 13SLs) on the population growth rate (ri—instantaneous rate of increase) of two mite species, the phytophagous Tetranychus urticae Koch and the predator Phytoseiulus persimilis Athias-Henriot. A residue of 1.3 mg/cm2 of pesticide solution was harmless for Ph. persimilis eggs, while a moderate mortality of eggs and of larvae from treated eggs of T. urticae, was observed (53.8%). The pesticide also caused a delay in the postembryonic development of the tetranychid. Moreover, 83.4% mortality was reported for treated females tetranychids and only 24.0% for Ph. persimilis females. The pesticide influenced negatively the population growth of T. urticae which showed a very low rate of increase (ri ¼ 0.07), compared to that obtained in the control (ri ¼ 0.68). The pesticide did not affect negatively the reproductive potential of Ph. persimilis (ri ¼ 0.54 and ri ¼ 0.57 for test and control, respectively). These results suggest a considerable acaricidal activity of potassium salts of fatty acids and caraway oil on T. urticae and a good selectivity on Ph. persimilis. r 2007 Elsevier Inc. All rights reserved. Keywords: Toxicity; Caraway essential oil; Fatty acid potassium salts; Tetranychus; Phytoseiulus
1. Introduction Identifying selective pesticides for IPM programmes is necessary to protect the natural beneficial arthropod fauna and at the same time reduce environmental pollutants. A low toxicity of these products to natural beneficials is more important than a high toxicity towards the target species. This is because the action of a natural beneficial on the target species has to be added to the effect of the selective pesticide. In fact, a selective pesticide should have a low or no toxicity towards beneficials. Moreover, it should degrade rapidly in the environment. At present, very few synthetic pesticides meet these criteria, but pesticides of plant origin seem to be good candidates in this list, as they generally have a very short persistence on the plant (Riederer and Schreiber, 1995; Isman, 1997; Cabras et al., 2002), and up to now show high selectivity
Corresponding author. Fax: +39 091 7028882.
E-mail address: [email protected] (H. Tsolakis). 0147-6513/$ - see front matter r 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.ecoenv.2007.10.001
when used as extracts (Mansour et al., 1986; Tsolakis et al., 1997; Ragusa Di Chiara et al., 2007). However, the selectivity of these products has to be strictly evaluated for different species of natural enemies as deleterious or sometimes positive effects were found among the natural enemies complex (Stark et al., 1990, 1992; Tsolakis and Ragusa, 1999). The aim of this study is to evaluate the toxicity of a commercial mixture of plant essential oils (caraway oil) and potassium salts of fatty acid (FAPS), to the two spotted spider mite Tetranychus urticae Koch and its specific predator Phytoseiulus persimilis Athias-Henriot. Since their discovery as biocide agents (1947), salts of fatty acids have been reported to have a very low toxicity to humans and these salts are considered environmentally safe (Prats et al., 1999), even if their phytotoxicity limits their use on some crops (Pasini et al., 1997). Though toxic and repellent effects of the caraway essential oils have been reported on different insect and mite species (Franzios et al., 1997; Kim et al., 2004; Pavlidou et al., 2004), there are no data on their effect on predatory mites. This paper
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reports the acute toxicity, repellency, and effect on population growth of the two species of mites mentioned above when they are treated with Acaridoil 13SL (VIORYL, Greece). A demographic approach has been used because it has been shown by different authors that population growth is an important endpoint in the toxicological studies (Ahmadi, 1983; Walthall and Stark, 1997; Stark and Banken, 1999; Venzon et al., 2005). Repellency effects have also been examined as they may play an important role in the integrated pest management of the crops. 2. Materials and methods 2.1. Mite species tested T. urticae was collected from weeds and kept in laboratory cultures on bean plants (Phaseolus vulgaris L.). Ph. persimilis was collected from strawberries infested by T. urticae, reared in the laboratory on plexiglas arenas (Swirski et al., 1970), and fed with various stages of the tetranychid mite. Both species were maintained in a growth chamber set at 2571 1C, 7075% R.H., and a 16:8 h L:D photoperiod.
2.2. Chemical tested Acaridoil 13SLr (VIORYL, Greece), is a commercial product containing 13.04% (w/v) of potassium salts of fatty acids (fatty acids in Acaridoil were of plant origin), 3% (w/w) of caraway essential oil (from Carum carvi L.), 5% (w/w) mono- and diglycerides and oleic acid, 70% (w/w) vegetable oils (mainly soybean oil), and water.
2.3. Experimental unit Each experimental unit (EU) consisted of a bean leaf disc (3 cm diameter) placed with its dorsal side on wet cotton in a plastic Petri dish (100 10 mm). The cotton was saturated with distilled water daily during the test period.
2.4. Acute toxicity tests Studies were carried out separately for the eggs and the adult stages of the two mites. Eggs: 24 h old eggs were used in the tests. To obtain the same age of the eggs, 10 ovipositing females of T. urticae were transferred in each EU, and allowed to lay eggs for 24 h. Each EU contained 20 tetranychid eggs. To obtain eggs of Ph. persimilis, 15 ovipositing females were transferred into a plexiglas arena (Swirski et al., 1970) with abundant prey for 24 h. Afterwards, five eggs of the predator were transferred with a fine brush from the arena to each EU. Adults (two-spotted spider mite): in these test young females were used (max 3 days old) which had been obtained by placing 100 deutonymphs of T. urticae transferred from the cultures onto excised bean leaves placed on wet cotton in Petri dishes. Doing so, the adult mites used in our experiments were likely all fertilized. The emerged females and males were transferred onto new bean leaves for 2–3 days. Afterwards, 10 ~~ of the tetranychid were transferred to each EU and allowed to settle for half an hour before spraying. Ph. persimilis adults were obtained by transferring 100 eggs of the phytoseiid with a fine brush from the culture arenas onto a new arena with abundant prey, until attaining adulthood. The emerged females and males were transferred to a new arena with abundant prey for 2–3 days. Afterwards, 5 ~~ predators were transferred to each EU and sprayed immediately.
277
Tests were replicated 5 times for the tetranychid mite and 10 times for the phytoseiid; all tests were carried out at 2571 1C, 7075% R.H., and a photoperiod of 16:8 L:D. Sprayings were done with a Potter Precision Spray Tower (Potter, 1952). We applied the concentration recommended by the manufacturer, that is, 2478 ppm of potassium salts of fatty acid and 570 ppm of caraway essential oil (concentration of the commercial formulate 19,000 ppm). For each EU, 2 ml of solution were used at 35 kPa of pressure, giving deposits of 1.3 mg of liquid per cm2. The pesticide was dissolved in distilled water. Control tests were treated with distilled water only. Sprayings were made directly on the mites. After spraying the leaf discs were placed in new Petri dishes on wet cotton with distilled water; while testing Ph. Persimilis, spider mites (all stages of T. urticae) were brushed onto a disc leaf daily to serve as food supply for the predator during the test period. The mortality of the eggs and the residual effects on young stages were recorded after 5 days for T. urticae and 3 days for Ph. persimilis. Mortality of females was recorded after 6 days for T. urticae and after 5 days for Ph. persimilis.
2.5. Repellency tests To measure the repellency of the pesticide, two-choice tests were carried out. In this case only the right half of the leaf disc was sprayed, covering the left half with a thin plastic film (mid-rib was the divisor of the disc leaf). Ten fertilized females of T. urticae or five fertilized females of Ph. persimilis were put on the mid-rib of the leaf 10 min after the spraying. Distilled water was applied in the control. Prey eggs were distributed on the whole surface of the disc leaf daily, to serve as a food supply for the predator. The mortality and the presence of adults and laid eggs on each part of the disc leaf were recorded daily at the same hour. To evaluate the effect of the pesticide on the predatory capacity of Ph. persimilis, treated eggs of T. urticae (E30 prey eggs/predatory female) were offered to the predator 10 min after spraying or 3 days after spraying. Predation and oviposition rate were registered after 24 h. Tetranychid eggs in the control were sprayed with distilled water. Tests were replicated 5 times for the tetranychid mite and 10 times for the phytoseiid at the same environmental conditions as in the acute toxicity tests.
2.6. Population growth study The effect of the pesticide on the populations growth of the two species was measured by the instantaneous rate of increase (ri) (Hall, 1964; Walthall and Stark, 1997). This rate of population growth measures the population increase or decrease and is calculated according to the following equation: ri ¼
lnðN f =N 0 Þ , Dt
where Nf is the final number of animals, N0 is the initial number of animals and Dt refers to the number of days the experiment is run. Solving for ri yields a rate of population increase similar to that obtained with the intrinsic rate of increase (rm) (Walthall and Stark, 1997). Positive values of ri show a growing population, ri ¼ 0 indicates a stable population while negative values of ri indicate a declining population directed toward extinction (Walthall and Stark, 1997). Instantaneous rate of increase was calculated after 3 and 6 days for T. urticae and after 3 and 5 days for Ph. persimilis, both on acute toxicity and repellence tests.
2.7. Statistical analysis Abbott’s formula (Abbott, 1925) was used to correct control mortality. Proportions of egg and larvae mortality, as far as distribution data, were transformed using an arcsine-square-root equation and a residual analysis has been performed in order to confirm the normality assumption prior to Student’s t-test or ANOVA. When the assumption of homogeneity of
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Table 1 Influence of Acaridoil 13SL on eggs and oviposition rate and survival of females of T. urticae and Ph. persimilis Initial number of mites
Hatched eggs
Surviving larvae
Eggs Females
100 50
71
45
Eggs Females
100 50
98
Ph. Persimilis Test Eggs Females
50 50
50
50 50
50
T. urticae Test Control
Control a
Eggs Females
Surviving females
Mortality %a mean7S.E.M.
53.7874.48 83.4076.22
7 98
Total number of laid eggs
85
Number of eggs/ female mean7S.E.M.b
0.7270.19 b
43
– –
34
8.8973.33 24.0075.95
677
3.8570.22 a
44
– –
891
4.0570.17 a
46 50
10.8170.87 a 3043
% mortality calculated according to Abbott (1925). Different letters denote significant differences for po0.01 (Student’s t-test).
b
variance (Levene test) was not met, the alternative no parametric ANOVA (Friedman test) was performed on the data. Normally distributed data were analyzed by one-way analysis of variance. When significant differences were found by ANOVA, the means were separated by Tukey’s studentized range honest significant differences (HSD) test. Student’s t-test was used to compare means of instantaneous rate-of-increase data. Differences were considered significant when 95% or 99% fiducial limits did not overlap. Analyses were computed using the software ‘‘Statistica’’ (StatSoft Inc., 2003).
3. Results 3.1. Acute toxicity tests The pesticide showed a slight ovicide effect on eggs of T. urticae and a slight residual effect on larvae from treated eggs. Dead eggs appeared dehydrated and air had penetrated inside. The total mortality of both stages was 53.78% (Table 1). It should be mentioned that after 6 days from spraying on eggs, only two larvae (4.44%) reached the protonymph stage and all individuals were in very bad condition, while in the control 100% of juveniles became protonymphs and were in good condition. On the other hand, no ovicidal effects were noted on eggs of Ph. persimilis, while a slight residual effect (8.89% of mortality) was noted on larvae (Table 1). No delay was recorded on the postembryonic development of the predator. Acaridoil 13SL caused a moderate knock down effect on young females of T. urticae after 24 h from spraying (47.6% of mortality) and a high mortality after 6 days (83.4%) (Table 1). The activity of the tetranychid females was limited and few eggs were laid (t ¼ 11.12, po0.01) (Table 1). A slight mortality was registered on females of Ph. persimilis at the end of the test (24.0%), whereas no negative effects were noted on the oviposition rate (F1,98 ¼ 0.53, p ¼ 0.46) (Table 1). As far as the population growth was concerned, the instantaneous rate of increase for T. urticae registered high
Fig. 1. Instantaneous rate of increase (mean7S.E.M.) for T. urticae and Ph. persimilis exposed to Acaridoil 13SL, 6 and 5 days (for the two species, respectively) after spraying. Different letters denote significant differences among the control and the test for each species (po0.01). Student’s t-test was performed on the data.
values in the control tests after 6 days (ri ¼ 0.68) (Fig. 1), while it was statistically lower in the Acaridoil test (ri ¼ 0.07) (t ¼ 10.67, po0.01). Statistical differences were registered for the phytoseiid population growth between the test (ri ¼ 0.54), and the control (ri ¼ 0.57), at 99% fiducial limit, but not at 95% (t ¼ 2.62, p ¼ 0.017) (Fig. 2). 3.2. Repellency tests Female tetranychid mites showed an uniform distribution on both the untreated part of the leaf and on the part sprayed with water in the control tests during the experimental period (F13,56 ¼ 1.45, p ¼ 0.23) (Table 2, Fig. 2A). Slopes of regression lines indicated a relative motility of the females, unrelated to the time, on the whole
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showed the attainment of the equilibrium after the fourth day (Fig. 3B). On the other hand, females preferred to oviposit on the untreated part of the leaf during the whole test period (F9,90 ¼ 12.65, p ¼ 0.00) (Table 2). In these tests, a mortality value similar to that obtained in the acute toxicity tests is reported (t ¼ 0.36, p ¼ 0.72) (Tables 1 and 2). The population of T. urticae in the control test had a very high rate of increase, but it was statistically lower in the pesticide two-choice test (t=8.7, po0.01) (Fig. 4). However, statistical differences were found also between the control and the test for Ph. persimilis (t ¼ 5.44, po0.01), but it should be mentioned that ri value in the test was maintained at very high levels (Fig. 4). Tests on the influence of treated prey eggs on the predatory capacity of Ph. persimilis females showed that freshly treated eggs reduce significantly both predation and oviposition rate of the predator (t ¼ 4.74, po0.01), even if both parameters were maintained at high levels (Table 3). On the other hand, the mean number of consumed prey eggs, supplied to the predator 3 days after the spraying, was similar to that obtained in the control, but a great variability was registered in these tests (Table 3). 4. Discussion
Fig. 2. Trend of probability distribution of T. urticae population on the untreated and treated parts of the leaf in the control (A) and pesticide (B) in the repellency tests.
leaf surface, even if the oviposition rate was higher on the treated part of the leaf (F11,48 ¼ 4.64, po0.01) (Table 2). However, a clear preference of tetranychid females towards the untreated part of the leaf was noted in the pesticide two-choice tests (F13,56 ¼ 59.94, po0.0001) (Table 2, Fig. 2B). Probability line indicated that this choice was maintained over time. It did not affect survival of females adversely as slight mortality was reported in these tests (Table 2). As expected, the predator mite showed a statistically uniform distribution on the leaf surface in the control tests (F9,90 ¼ 0.26, p ¼ 0.98), confirmed also by the oviposition rate (F9,90 ¼ 1.44, p ¼ 0.18), but it preferred to walk on the untreated part of the leaf in the pesticide tests (F9,90 ¼ 8.61, p ¼ 0.00) (Table 2, Fig. 3A and B). However, it should be mentioned that after the third day no statistical differences were registered between the means for the predator distribution on the untreated and treated part of the leaf (po0.01), even when probability confidential limits
The main active ingredients of the pesticide evaluated in our tests are potassium salts of fatty acid (FAPS) with caraway essential oil. FAPS are generally recognized as safe (GRAS) by the US Food and Drug Administration (21 Code of Federal Regulation 172.863) and they are used as insecticides, acaricides, herbicides, and algaecides (EPA, 1992). They probably function as contact agents since FAPS were found to penetrate the integument of arthropods disrupting cell membranes and causing dehydration and death. The caraway essential oil consists mainly of two monoterpenes: S(+)-carvone (50–60%) and S(+)-limonene (35–45%) (Hartmans et al., 1995). It is known for some plant-derived terpenoids that they can act as anti-feedants and growth inhibitors of mites (Klocke and Kubo, 1982; Murray et al., 1999; Isman, 2000). The carvanone/limonene essential oil showed high toxic effects on the poultry red mite Dermanyssus gallinae De Geer (Kim et al., 2004) and Drosophila melanogaster Meig. (Franzios et al., 1997), whereas D- and L-carvone was found to be lethal to T. urticae females (Lee et al., 1997). The exact mode of action of the essential oil remains unknown although from recent studies it was suggested that they impact the functioning of the octopaminergic nervous system (Enan et al., 1998 in Isman, 2000). Our results indicate limited ovicidal action of the product on the phytophagous mite, while it did not cause harm to the eggs of the predator. As the structure of egg chorion of the two species is similar, this difference could be due to the different exposure time. The predators’ eggs hatch within 24 h, while the phytophagous mites ones need 2–3 days. The ovicidal effect could be caused by FAPS as
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Table 2 Distribution of females and eggs of T. urticae and Ph. persimilis on the untreated and treated part of the leaf with Acaridoil 13SL
T. urticae Control
Test Ph. persimilis Control
Test
Untreated part
Treated part
Number of females and eggs registered during the test mean/day7S.E.M.a
Number of females and eggs registered during the test mean/day7S.E.M.a
Females Eggs
4.6370.26 a 14.6072.15 a
5.0670.25 a 23.3471.54 b
Females Eggs
8.5770.34 a 21.5172.64 a
0.6370.16 b 0.8970.36 b
Females Eggs
2.3170.15 a 10.4070.65 a
2.5570.17 a 11.2270.64 a
Females Eggs
2.8170.14 a 11.1070.75 a
1.3670.2 b 3.5270.44 b
Mortality (%)b
–
11.1179.94
–
17.0075.12
a
Different letters denote significant differences between the untretaed and the treated part of the leaf (po0.01). Analysis of variance (ANOVA) was performed on the data. Means were separated by Tukey’s studentized range (HSD) test. b % mortality calculated according to Abbott (1925).
Fig. 4. Instantaneous rate of increase (mean7S.E.M.) for T. urticae and Ph. persimilis partially exposed to Acaridoil 13SL (repellency tests) 6 and 5 days (for T. urticae and Ph. persimilis, respectively) after spraying. Different letters denote significant differences among the control and the test for each species (po0.01). Student’s t-test was performed on the data.
Fig. 3. Trend of probability distribution of Ph. persimilis population on the untreated and treated parts of the leaf in the control (A) and pesticide (B) in the repellency tests.
their reported mode of action aligns with our observations as do the results of others on the same mite species (Osborne and Petitt, 1985). The residual toxic effect on larvae of both species and the delay observed in the larval development of T. urticae, could be caused by the plant constituent of the pesticide (caraway essential oil), whereas the toxic effect of the pesticide on young females of the tetranychid may be caused by both FAPS and caraway essential oil. FAPS may easily penetrate the soft integument of different arthropod species, and such effects were weaker when used on species with strong integuments or covered with sclerotized shields. This could explain the different effects on tetranychid (soft integument) and phytoseiid mites (tegument with plates). It should be mentioned that Osborne and Petitt (1985) reported a greater toxicity of insecticide soaps on Ph. persimilis
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Table 3 Predation and oviposition rate of Ph. persimilis fed on T. urticae eggs treated with Acaridoil 13SL
Test 1: freshly treated prey eggs Test 2: prey eggs treated 3 days before Control a
Supplied prey eggs/predator female mean7S.E.M.a
Consumed prey eggs/predator female mean7S.E.M.a
Number of eggs/female/day mean7S.E.M.a
33.5075.77 a 31.9073.07 a 30.3876.62 a
16.8476.90 a 24.08713.97 ab 24.9971.48 b
2.9470.04 a 3.1670.07 a 3.7270.01 b
Different letters denote significant differences for po 0.01 (ANOVA Friedman test).
females than on T. urticae: LD50 of 1440 and 6600 ppm for the predator and the phytophagous mite, respectively. These differences, in our opinion, can be explained by the usage of different test methods: while Osborne and Petitt used the standardized FAO slide-dip method that allows for covering the mite body completely, we used the Potter tower for complete covering of the substrate surface on which the mites reside (IOBC/WPRS method: see Sterk et al., 1999). The high toxic effect of the product on T. urticae, observed in our tests, could also be due to the combined action of FAPS with caraway essential oil. As a matter of fact, this essential oil showed high toxic effects towards various mites and insects (Kim et al., 2004; Franzios et al., 1997; Lee et al., 1997). The pesticide showed a negative impact on the phytophagous mite over time, reducing its population growth to nearly zero. These effects were the combined result of increased mortality (direct loss of females) and of decreased fecundity (decreased production and viability of the eggs). No negative effects were noted for Ph. persimilis population growth. According to OILB/IOBC toxicological categories, this mixture of FAPS and caraway essential oil has to be considered moderately harmful for T. urticae and harmless for Ph. persimilis (Sterk et al., 1999). The very low mortality reported in T. urticae two-choice test is caused by the high repellency of the pesticide over time that restricts females on the untreated part of the leaf. In contrast, the short-lasting repellency of the pesticide towards Ph. persimilis two-choice tests, caused a mortality similar to that reported in the acute toxicity tests. Various authors found significant repellency of terpenoids to tetranychid mites (Mansour et al., 1986; Tsolakis et al., 2002; Tsolakis and Ragusa, 2004), but no data are available for Ph. persimilis. In the two-choice pesticide tests, the population growth of T. urticae was significantly reduced, while for the impact on Ph. persimilis population growth was minor. In the predation tests, using pesticidetreated prey eggs, the feeding-rate of Ph. persimilis was initially low but increased over time again albeit not to the control levels. Nevertheless, both predation rate and ovipositional rate were in principle still sufficiently high for controlling spider mites. These data indicate that the repelling effect of the caraway oil might be superable for the predator who also maintains sufficient fecundity and population growth to assume the oil could be compatible with biocontrol in the field.
In our opinion, the differences in the tetranychid tests are attributed to populations reaching the carrying capacity of the environment rather than to the sublethal effects of the pesticide. In these tests, tetranychid females lived on one half of the space in comparison to the control test. Repellence is to be considered an important target for phytophagous mite control as the aim of IPM programmes is not to destroy the phytophagous populations, but to reduce their density on crops, to permit the natural enemies’ populations to control the phytophagous ones. 5. Conclusion The mixture of potassium salts of fatty acids and caraway essential oil may be considered for usage as a selective acaricide, able to control T. urticae populations without adversely affecting Ph. persimilis. The delay in juvenile development of the tetranychid mite is very interesting but further investigations are needed to clarify the precise cause and its consequences. As no negative effects were found (given the IOBC guidelines) on the performance of individual predatory mites and their population growth, the mixture of potassium salts of fatty acids and caraway oil has serious potential as a low impact acaricide integrated with IPM programs. Acknowledgments Authors are deeply indebted to Prof. J. McMurtry and Prof. A. Lombardo who critically read the manuscript giving useful advice and to Ms E. Chiavetta who checked the English text. Anonymous reviewers are also thanked for their constructive comments and suggestions. The PRIN (2005) and PAN (2004–2006) provided financial support for this work. The present work was done in accordance with national and institutional guidelines for the protection of human subjects and animal welfare. References Abbott, W.S., 1925. A method of computing the effectiveness of an insecticide. J. Econ. Entomol. 18, 265–267. Ahmadi, A., 1983. Demographic toxicology as a method for studying the dicofol-twospotted spider mite (Acari: Tetranychidae) system. J. Econ. Entomol. 76, 239–242.
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