Allothrombium ovatum Zhang & Xin is an important natural enemy of Aphis gossypii Glover in cotton growing areas in the northern part of China. Its larvae are ectoparasitic on aphids during early spring. Larvae of A. ovatum in cotton fields were from two sources: the cotton field itself and the surrounding fields. Field experiments showed that most mites were from outside sources; open plots, allowing immigration of mites via A. gossypii alatae, had over 10 times as many mites as enclosed plots. Mite larvae migrated to cotton fields by parasitizing migrant alatae of A. gossypii; they moved with aphids into open cotton fields at the rate of 90.7 mites/m2/day, and into plastic film-mulched cotton fields at 36.0 mites/m2/day. Most migrant alatae (92.0%) had a mite load of one, 6.7% had a mite load of 2 and 1.3% carried 3. The maximum load of mites came to 26% of the host weight. Laboratory studies on the 3rd and 4th instar nymphs of A. gossypii showed that parasitized aphids suffered increased mortality and reduced reproductive rates.
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Vol. 1 • No. 1