Population dynamics of the diamondback moth and its parasitoids in Zimbabwe

Temporal population dynamics of the diamondback moth (DBM), Plutella xylostella (L.) and its parasitoids were studied for a continuous 13-months period at two sites in Zimbabwe, vis Henderson and Mutoko. Our results showed that mean DBM numbers of 2.2 and 2.9 per plant for Henderson and Mutoko, respectively were not significantly different (p = 0.12). At Henderson, the DBM numbers achieved a pre-winter peak in February and a post-winter peak in October, while at Mutoko the pre-winter and post-winter peaks were reached a month earlier, in January and September, respectively. A total of four parasitoid species, comprising Cotesia vestalis, Oomyzus sokolowskii, Brachymeria sp and Diadromus collaris were recorded from both Henderson and Mutoko. Cotesia vestalis predominated, accounting for over 98% parasitism at both sites. Mean parasitism rates of 40.6 and 29.3% were recorded at Henderson and Mutoko, respectively, and these were significantly different. Maximum parasitism rates were reached during the post-winter period at both sites with values of 86.4 (December) and 58% (September) for Henderson and Mutoko, respectively. For Henderson, percentage parasitism was positively correlated with both maximum and minimum temperature and DBM density was positively correlated with maximum temperature. For Mutoko neither DBM density nor percent parasitism was significantly correlated with either maximum or minimum temperature. Mean rainfall did not have any effect on host density or parasitism rates at both sites. For both sites parasitism was host density dependent. These results are significant in explaining factors limiting abundance of DBM-parasitoids and may help shape future biocontrol programmes.