The bionomics ofScolothrips sexmaculatus(Pergande) (Thysanoptera: Thripidae), an insect predator of spider mites

Abstract

The biology, temperature response, and prey requirements for Scolothrips sexmaculatus were studied in the laboratory with Tetranychus pacificus as the main prey. At 26.7 C and 50% relative humidity, the mean life cycle was 8.9 (range: 7.6-10.3) days; egg, 7.2 days; 1st-stage larva, 2.8 days; 2nd-stage larva, 2.5 days; prepupa, 1.2 days; and pupa, 2.5 days. Male and female immatures developed in the same length of time, but as larvae, killed significantly different mean numbers of prey eggs, 43.0 and 56.0, respectively. Eggs of S. sexmaculatus are inserted into leaf tissue. Larval eclosion lasted a mean 7.6 min, and larvae commenced feeding a mean 40.3 min after vacating the chorion. No larval preference or nonpreference was noted for any given prey stage. Imaginal molts lasted a mean 9.1 min, and the new adult commenced feeding after a mean 92.7 min. For adult females, no preference or nonpreference was noted for any prey developmental stage. Mating was unnecessary to induce oviposition, which usually commenced on the 1st imaginal day. Unmated females produced only male progeny. Thus, S. sexmaculatus is a facultatively arrhenotokous species. Larval and adult thrips were behaviorally well adapted to preying on tetranychids which produce copious webbing. Thigmotaxes were evident in all instars, and cannibalism did not occur until prey became scarce. The lower thermal limit for incubation was between 18.3 and 23.9 C; for larval development, 12.8 and 18.3 C; for mating, 12.8 and 18.3 C; and for production of female progeny, 12.8 and 18.3 C. At 23.9 to 40.6 C, adult female thrips produced more than a mean 200 eggs during their lifetime. During the reproductive period, a mean 6.2 to 7.2 mite eggs were killed for each thrips egg laid. At 29.4 to 40.6 C, female thrips killed more than a mean 1,700 mite eggs during their lifetime. Longevity of adult females was inversely related to temperature. Production of total progeny was highest at 29.4 C, although the highest percentage of female progeny occurred at 23.9 C. Life tables were prepared from data collected at four temperatures, and intrinsic rates of natural increase were calculated therefrom. The rm values for S. sexmaculatus were 0.155, 0.232, 0.304, and 0.360 at 23.9, 29.4, 35.0, and 40.6 C, respectively. Minimums of 20 and 25 prey eggs were required for development of males and females, respectively. Mean developmental periods of both sexes were increased significantly when larvae were provided only two or three mite eggs per day. First- and 2nd-stage larvae of S. sexmaculatus effectively searched 2.45cm2 and 10.19cm2 per hr, respectively. Densities of one mite egg per 70.52cm2 and 114.59cm2 were required for development of 1st- and 2nd-stage larvae, respectively.