Susceptibility of Wolf Spider, Lycosa terrestris (Araneae: Lycosidae) to Chlorpyrifos.
The present study was designed to check the effect of chlorpyrifos (Lorsban) on Lycosa terrestris, the most dominant ground spider in the agroecosystem of Punjab, Pakistan. For this purpose topical and residual bioassay tests were performed. Probate analysis showed that this insecticide is highly toxic for both sexes. The calculated LD50 values were less than field dose. Results of residual bioassays showed that this insecticide is highly toxic to spiders even after one month of its exposure. Negative effects of this insecticide not only reduces the biological control potential of L. terrestris but also poses a serious threat to other natural predators of agroecosystems.
Key words: Agroecosystem, residual toxicity, ground spiders.
Spiders are the most abundant natural predators of insects in terrestrial ecosystems and consume large number of prey without damaging the plants (Lang et al., 1999; Hanna et al., 2003). Many studies have demonstrated that spiders can significantly reduce prey densities in agricultural fields (Symondson et al., 2002; Pearce and Zalucki, 2006; Schmidt et al., 2009). Agricultural fields that are frequently sprayed with pesticides often have low spider population (Amalin et al., 2000).
Cyhexatin, flubenzimine, dicofol and azocyclotin) and chlorinated hydrocarbons (endosulfan) followd by pyrethroids, organophosphates and carbamates have been reported to be most toxic the hunting spiders and web-builders (Mansour and Nentwig, 1988). Some insecticides, specificially deltamethrin, dimethoate and phosalone do not cause substantial spider mortality but can greatly disturb spider predatory activity over a variable period, depending on the insecticide and the spider species involved (Cocquempot et al., 1991). In addition to causing mortality, pyrethroid (specifically Fenvalerate and Lambda-Cyhalothrin), when applied at lower doses, can inhibit spiderling emergence from cocoons and delay web building activity (Dinter and Poehling, 1995).
Present study was designed to check the susceptibility of both sexes of wolf spider, Lycosa terrestris (Araneae: Lycosidae) to commonly applied insecticide. Chlorpyrifos by topical and residual exposure methods.
Material and methods
The male and female specimens were collected by hand picking and handy -vacuum (SIEMENS VK 20C01) from the edges of a guava orchard situated on Shahadra Road Lahore, 5 Km from Sagian Bridge from April through November, 2008. To check the effects of Chlorpyrifos on spiders, topical and residual bioassay tests were performed. Each experiment was replicated thrice. This insecticide is commonly used on grains, cotton fields, fruits, nuts, vegetable crops as well as on lawns and ornamental plants in the study area.
For topical application four concentrations of chlorpyrifos in acetone viz. 100, 200, 300 and 400 ng a.i. per individuals were tested. Before application of insecticide, spiders were anesthetized by brief exposure to CO2 gas (for 20 seconds). The insecticide was applied (0.5 ul) topically on the dorsum of each spider using a micropipette. Response of organism was recorded as unaffected (showing normal behaviour), affected (showing irregular pattern of movement), paralyzed (completely inmobilized) and dead (totally non-responsive) for each dose after 1, 2, 4, 8, 16 and 24 hours of exposure.
For dry residue testing, half inch thick layer of soil was taken in experimental pots (10 inch diameter x 5 inch high) and dried in sun before spray. Insecticide was sprayed at field rate on the pots by using Knapsack hand sprayer (THS-119428) . Control pots were sprayed only with acetone. Ten male and ten female spiders were released in the treated pots after 1, 2, 3, 5, 7, 10, 15, 20, and 30 days of spray and result was noted at 1, 2, 4, 8, 16, 24, 48, 78, 96 and 120 hours after the release. The spiders were fed with larvae of wild-type Drosophilla cultured in the laboratory. Mortality rate and other effects of insecticides were assessed daily by the examination of individuals.
For both experiments mortality was calculated for each concentration of insecticide. Results obtained from the dose response series evaluation after 8th hour for topical exposure and 48th hour for residual toxicity were subjected to Probate analysis to determine the LD50 and LD95. Dosages were expressed as nano-gram of active ingredients per treated individual (ng a.i./ individual).
Results showed that the susceptibility of chlorpyrifos is positively associated with amount of insecticide applied and duration of exposure. Topical bioassay test showed that field rate caused 100% mortality after 16 h in male individuals and after 24 h in female individuals. Double field rate caused 100% mortality after 4 h in male and after 8 h in females. The summary of Probate analysis at 8th hour assessment data is given in Table I. Resistance ratio shows that females are twice resistant than males. The percentage of mortality increases with time for all doses. The summary of Probit analysis at 48th h assessment data is represented in Table II. Results showed and chlorpyrifos remains active till 27 days for the spider specimens.
Table I.-###Estimated LD50 and LD95 for Chlorpyrifos (ng
a.i./individual) calculated at 8th h of application of both sexes of L. terristris for topical exposure method.###
Sex###LD %###Dose (Confidence limit)
Table II.- Estimated LT50 and LT95 of residual field rateof Chlorpyrifos calculated at 48th h of bothsexes of L. terristris .
Sex###LD %###Dose (Confidence limit)
In the present study males had high susceptibility than females, although the ratio of susceptibility varies in all treatments of insecticide. The high variability was consistent with the result of Baatrup and Babylay (1993) . A number of studies have reported differences in susceptibility for both sexes (Dinter and Poehling, 1995). Higher mortality in males in the present study is in accordance to Peterson (2002). High mortality in males might be due to application of high dose of insecticide relative to their body size. However, Nielson et al. (1999) reported no size effect on the susceptibility of Pardosa amentata caught from the field population.
Variations in the susceptibility levels among individuals were also recorded during the study. This might be due to the reason that spiders were collected from the field and such individuals have an unknown feeding history and determining level of starvation is very difficult. Although individuals in the present experiment were fed to satiated level prior to experimentation, variations may still be present in the degree of satiation among individuals. Peterson (2002) found starved P. amentata, or individuals fed on low quality prey to be more susceptible to the insecticide.
The post exposure after 24 h showed that reduction in mortality was very steep. The difference in mortality may be due to difference in the interaction of insecticide with the substrate and environment in the field. The high mortality in L. terrestris at field rate suggested that toxic impact of this product would be very high. Affected individuals were either paralyzed or showed un-coordinated walking. This can result in exposure to predators in actual field conditions. So the death rate
would be higher in the field than in the laboratory. Along with spiders, food will also be exposed to insecticide and will cause reduction in the population of spiders.
Long half -life of chlorpyrifos (Anonymous, 2008) suggested that this insecticide can exist for long time in the soil. Laboratory experiments also showed that chlorpyrifos remains active till 27 days for the specimens. In the field, chlorpyrifos may cause reduction in the spider density as well as diversity. Due to high toxicity of chlorpyrifos reestablishment of spider density and diversity may take longer period (even months).
It is concluded from the present work that use of chlorpyrifos is a major threat to the spider fauna in the studied area. So its use should be minimized and alternative measures should be used to control the pests in the orchards and agricultural fields. The current study also highlights the need to investigate the impact of other pesticides which are commonly used on the spider fauna of the study area in order to gain a more realistic view of what may occur in treated orchards and agricultural crops. Furthermore, field based assessments are required to provide the most reliable results to be extrapolated into real environmental situations.
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(Received .7 May 2009, revised 5 October 2010)
Department of Zoology, University of the Punjab, Lahore, Pakistan.
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|Publication:||Pakistan Journal of Zoology|
|Date:||Apr 30, 2011|
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