Impact Assessment of Apanteles plutellae on Diamond back Moth Using an Insecticide-check

Field surveys of parasitoids of diamondback moth, Plutella xylostella (L), have indicated that Apanteles plutellae Kurdj may have a suppressing effect on the population of this insect. How important the parasitoid really is and to what extent it is beneficial is, however, not exactly known. This was, therefore, evaluated using an insecticide-check method whereby four treatments, each having different selective effects, were compared, viz: Dipel (Bacillus thuringiensis Berl, selectively toxic to diamondback moth but not the parasitoid); Sevithion (carbaryl + malathion, which is toxic to the parasitoid but not to DBM); cartap (highly toxic to both host and parasitoid); and an untreated control. Cartap and Dipel treatments had significantly higher yields than either the Sevithion treatment or the control. Sevithion treatment had less yield than the control. Both the control and Dipel treatment had significantly higher A. plutellae parasitism. The parasitism rate was least in the cartap treatment while Sevithion had a moderate level of parasitism. Based on these results and on the number of parasitoid cocoons collected from the crop, there was distinct evidence of differential survival and abundance of the parasitoid as well as the diamondback moth adults between different treatments. In caparison with cartap treatment, the diamondback moth adult ratios were 1.9, 8.0 and 47.9 for Dipel, control, and Sevithion respectively. Correspondingly, the parasitoid: host ratios were 3.22, 0.64 and 0.05. These findings, in particular the high adult ratio (47.9) and the extremely low parasitoid:host ratio (0.05) for Sevithion, showed that A. plutellae is important and can contribute substantially to the suppression of diamondback moth. That this is so, and that resurgence was not due to hormoligosis or altered physiology of the host plant by Sevithion, was further confirmed in follow-up laboratory investigations.