Phytoseiulus persimilis is mass-reared throughout the world for biological control of two-spotted spider mites. Greenhouse growers in British Columbia have reported reduced efficacy of the predator and correlated this reduction with white abdominal discoloration. We surveyed shipments of the predator from four insectaries for one season to determine symptom prevalence, daily variation in symptom expression, fecundity, survival and predator voracity. Symptoms were observed in more than 75% of predators on receipt from the four insectaries. This percentage did not change with 24 h acclimation in feeding dishes, but there were significant trends in symptom expression during the subsequent 6 d: the proportion of predators with white dot in the rectum decreased and the proportion with white Malpighian tubules increased. These results and daily observations of symptom change in an individual - occasionally associated with egestion - suggests that white abdominal discoloration is a natural part of the excretory process. Fecundity was not correlated with white symptoms on receipt of a shipment, but the proportion of predators with symptoms nearly doubled from July to October and fecundity decreased 22% during the same period. Fecundity was negatively correlated with the frequency of observation of white abdominal symptoms during fecundity trials. Because the substance causing the white discoloration is composed mainly of potassium, we examined the effect of fertilizer concentration - applied to the host plants for the spider mite prey - on discoloration and fecundity in the predator. Higher fertilizer concentrations resulted in higher proportions of observations of white abdominal discoloration in the predator, but fecundity was not affected (4-5 eggs per female per day for 10 d following mating in all treatments). The results suggest that white abdominal symptoms may not necessarily indicate reduced predator performance.
Species 1: Acari Phytoseiidae Phytoseiulus persimilis
Species 2: Acari Tetranychidae Tetranychus urticae
Keywords: abdominal discoloration
The ESA 2001 Annual Meeting - 2001: An Entomological Odyssey of ESA