Differential effects of the acylurea insect growth regulator teflubenzuron on the adults of two endolarval parasitoids of Plutella xylostella, Cotesia plutellae and Diadegma semiclausum

The acylurea chitin synthesis inhibitor, teflubenzuron ('Nomolt' SC) showed no apparent activity against adult males of Diadegma semiclausum Hellen or Cotesia plutellae Kurdj. when applied to foliage at a rate (identical with 50 g a.i. ha-1) approximating to 2 X field usage. In contrast, the pyrethroid cypermethrin ('Cymbush' EC) showed appreciable foliar activity (at rates identical with 10-50 g a.i. ha-1) to D. semiclausum but showed low activity against C. plutellae. When single adult female D. semiclausum were pre-treated with teflubenzuron (identical with 50 g a.i. ha-1) and then exposed to 5, 10, 25, 50 or 100 second-instar host larvae, the number of parasitoid cocoons formed was significantly reduced compared with controls except at the lowest host density (P < 0.05). However, there was no evidence that teflubenzuron had a host density-dependent effect on parasitism by D. semiclausum. Teflubenzuron had no significant effect on host parasitism in an equivalent experiment with C. plutellae (P > 0.05) except at the median host density. When batches of ten host larvae were introduced at hourly intervals to female D. semiclausum pre-treated with teflubenzuron (identical with 50 g a.i. ha-1), the number of cocoons formed in the first two batches was not significantly different from untreated controls (P > 0.05), whereas teflubenzuron significantly reduced the number of cocoons in the next three batches of host larvae (P < 0.05). This suggests that teflubenzuron may affect chitin synthesis in developing eggs within female D. semiclausum but has little or no effect on pre-formed eggs which are more likely to be oviposited first. Teflubenzuron had no significant (P > 0.05) effect on parasitism by C. plutellae when presented with batches of host larvae. When female D. semiclausum and C. plutellae were placed in the same host arena, the former showed a 3-fold greater rate of parasitism. Pre-treatment with teflubenzuron significantly reduced parasitism by D. semiclausum (P > 0.05) but had no significant effect with C. plutellae (P > 0.05) although the latter species failed to exploit the reduction in parasitism by D. semiclausum. The differential response of D. semiclausum and C. plutellae to insecticides is discussed in relation to the relative rates of establishment of these species in Malaysia.