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Biocontrol may play an important role in IPM of D. suzukii, a worldwide fruit crops pest. Predation on D. suzukii has been observed in the field, but rates have not been measured. Additionally, targeting juveniles may be more effective than targeting adults for reducing population sizes. The first objective of this study was to measure naturally-occurring levels of biocontrol on D. suzukii juveniles. Furthermore, D. suzukii may either pupate inside of fruit or emerge to pupate in the soil. Knowing the location of pupae can be important for developing biological and cultural control programs. The second objective of this study was to determine the proportion of D. suzukii pupating in fruit versus soil. Predator exclusion experiments were conducted to measure the ability of natural enemies to reduce D. suzukii larvae in strawberry and blueberry fruits, and pupae in the soil underneath strawberry, blueberry, and blackberry. Funnels were used to direct larvae dropping from infested blueberry and raspberry fruits into soil-filled containers. Significantly fewer D. suzukii larvae were extracted from fruits exposed to predators, representing a reduction in larval survival of 19–49%. Significantly fewer D. suzukii pupae were recovered from soil exposed to predators, representing a 61–91% decrease in pupae. In the pupation behavior experiment, 82–93% of pupae were found in soil, significantly more than were found in fruit. Ants and spiders were commonly observed in a variety of sampling methods, and ants were observed on video to remove pupae from the soil.

Generalist natural enemies may be well adapted to annual crop systems in which pests and natural enemies re-colonize fields each year. In addition, for patchily-distributed pests, a natural enemy must disperse within a crop field to arrive at infested host patches. As they typically have longer generation times than their prey, theory suggests that generalist natural enemies need high immigration rates to and within fields to effectively suppress pest populations. The soybean aphid, Aphis glycines Matsumura, is a pest of an annual crop and is predominantly controlled by coccinellids. To test if rates of coccinellid arrival at aphid-infested patches are crucial for soybean aphid control, we experimentally varied coccinellid immigration to 1 m2 soybean patches using selective barriers and measured effects on A. glycines populations. In a year with low ambient aphid pressure, naturally-occurring levels of coccinellid immigration to host patches were sufficient to suppress aphid populations, while decreasing coccinellid immigration rates resulted in large increases in soybean aphid populations within infested patches. Activity of other predators was low in this year, suggesting that most of the differences in aphid population growth were due to changes in coccinellid immigration. Alternatively, in a year in which alate aphids continually colonized plots, aphid suppression was incomplete and increased activity of other predatory taxa contributed to adult coccinellid predation of A. glycines. Our results suggest that in a system in which natural enemy populations cannot track pest populations through reproduction, immigration of natural enemies to infested patches can compensate and result in pest control.