Effects of two neem insecticide formulations on the trophic interactions between the diamondback moth, plutella xylostella (L.) (Lepidoptera: Plutellidae) and its parasitoids
Akol, Anne Margaret
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The diamondback moth (DBM), Plutella xylostella (L.) is a pest of Crucifers and, of worldwide importance. Reliance on chemical insecticide control is not a substainable option therefore, new pest control initiatives emphasise the use of parasitoids and biorationals such as neem insecticides. Neemros® ( a powder formulation) and Neemroc EC® (an oil formulation) are two neem insecticides that are registered in Kenya for use against vegetable pests, including the DBM. However, the much needed information on their impact on DBM parasitoids is lacking. The aim of this study was to assess the responses of DBM parasitoids to the neem formulations when exposed to them directly or through the plant or host larvae. The formulations were applied as foliar sprays using either a knapsack sprayer or a hand-held mist sprayer fitted with a cone nozzle. In field experiments, conducted during 1999 at Juja (Kenya), Neemroc EC® checked DBM infestation but Neemros® was not as effective. Three parasitoid species namely Diadegma mollipla (Holmgren), Oomyzus sokolowskii (Kurdjumov) and Cotesia plutellae (Kurdjumov) were recovered. Parasitism by all three species did not differ significantly between Neemros® treated and control plots but was significantly lower in the Neemroc EC® treated plots during two weeks (F=3.09, d.f=8, 36; P<0.05). The individual parasitoid species also had different parasitism levels and also responded differently to the neem insecticide treatments. Cause of mortality analysis in DBM cohorts revealed that larval parasitoids accounted for <1% of the total mortality during the experimental period while the neem formulations accounted for between 15-26% of the total mortality. In laboratory tests undertaken with D. mollipla, parasitoid adults that had direct topical contact with the neem formulations showed comparable longevity to those exposed to water (control), while exposure to Karate®, a synthetic pyrethroid caused 100% mortality in less than six hours. There were no significant differences in the proportion of hosts parasitised by the neem- and water-sprayed adults (F=2.40, d.f=2, 75; P>0.05) and their foraging patterns were also similar (ANOVA, P>0.05). Also evaluated were the olfactory responses of D. mollipla to plants/plant-host complexes sprayed with the neem formulations. The parasitoids were significantly more attracted to odours from the Neemros® - sprayed plants/plant-host complexes than to a blank (clean air). In contrast, parasitoid respnse to odours from the Neemroc EC® -sprayed plants/plant-host complexes dod not differ significantly from the blank. In choice tests between infested plants sprayed with water or a neem formulation, parasitoids showed no preference for odours from water-sprayed plants over those from Neemros® - sprayed plants but showed a preference for the odours from the water-sprayed plants over those from Neemroc EC® - sprayed plants. In host acceptance and suitability tests, parasitism rates in neem- and water-sprayed hosts exposed to parasitoids at 24 hrs and 48 hrs after treatment were, with one exception, not significantly different. A high mortality level in the neem-treated parasitised host suggested their unsuitability for parasitoid development. It is concluded that the neem formulations may be relatively safe to adult DBM larval parasitoids at the doses used in the study. The establishment of plant refugia that would harbour untreated DBM larvae and, thus help to perpetuate the parasitoid population could offset the deleterious effects observed on parasitoid development.