Abdominal Discoloration and the Predatory Mite Phytoseiulus persimilis Athias-Henriot: Prevalence of Symptoms and Their Correlation with Short-Term Performance

Bimonthly shipments of Phytoseiulus persimilis were obtained from each of four commercial sources from 13 July to 15 October 1998. Predators from these shipments were assessed for symptoms and used to evaluate fecundity, survival, and voracity. An additional four shipments of P. persimilis received from 12 November to 4 December were used to evaluate predator movement. The prevalence of abdominal discoloration was determined by examining individual mites on receipt of each shipment and during a 7-day fecundity trial. Data were analyzed to determine daily and seasonal trends (from July to October) and symptoms were correlated with fecundity, survival, voracity, and movement. Three symptom groups were observed: white dot in the rectum, white Malpighian tubules, and both white dot and white Malpighian tubules. Symptoms were observed in mites from all sources. Symptoms were observed in the majority (77%; 4080/5283) of live mites that were examined immediately following shipment. There was no significant difference between the proportion of symptomatic mites on receipt and that after a 24-h acclimation period when P. persimilis were fed in conditions of excess prey. Symptoms were observed in the majority of live mites (65% of 581 to 76% of 168 individuals) on any given day of the 7-day fecundity trials; however, symptoms in individual mites often changed from one observation to the next (53% of 2111 sequential observations). The numerous transformations may be explained by the rate of egestion; a total of 198 egestions were observed during 2692 4-min observation periods. White material was excreted from the rectum during 92.4% of these egestion events, and most cases (79.1%) involved a simultaneous color change from a prominent white dot in the rectum to an asymptomatic state. The proportion of mites with white symptoms on receipt of shipment increased from 0.42 to 0.78 as the season progressed from July to October. During the 7-day fecundity trials, the number of observations of white symptoms per mite also increased from July to October. Maximum oviposition (2.84 eggs/female/day) occurred on day 3 of the trials, followed by a decline in egg production. Daily fecundity was positively correlated with survival and was substantially greater for mites surviving to day 7 of the fecundity trial. Frequent observation of white dot per individual was correlated with a 45% reduction of fecundity (from 3.8 to 2.1 eggs/female/day) for mites that lived the full 7 days. Mean daily fecundity per mite decreased 22% (from 2.87 to 2.22) from July to October, while survival increased 17% (from 4.28 to 5.01 days). Voracity was assessed during 4-h trials with three inspection periods (at 0, 2, and 4 h). Egg consumption decreased (from 4.7 to 3.3; 30%) as the number of observations of white symptoms increased from 0 to 3 observations. There was no relationship between presence of symptoms and predator movement.