Soybean Aphid - Natural Enemies

Natural Enemies

In Asia, the soybean aphid experiences pressure from over 30 species of predators, 8 species of parasitoids, and some fungal pathogens. In Indonesia, where the soybean aphid is considered an occasional pest, evidence indicates the use of insecticides to control soybean aphids may not always be necessary due to suppression of the insect to subeconomic densities by natural enemies alone. In North America, the dominant natural enemies in soybean are generalist predators. Exclusion cage experiments have provided evidence that predators can play an important role in suppression of the soybean aphid. Impacts from predators include both the ability to suppress colony establishment early in the season as well as respond to increased densities of soybean aphids late in the season.

One of the most important predators of soybean aphids in North America is the insidious flower bug (Orius insidiosus (Say)). The insidious flower bug has its greatest impact on early to mid-season populations of soybean aphids and is often able to keep soybean aphid densities low. Fox et al. (2004) hypothesized that the impact from this predator early in the season could be attributed to small plant size and sparse canopies, which aid the insidious flower bug by reducing foraging time and decreasing the number of places soybean aphids can hide (i.e., enemy-free space). In addition, synomones released by soybean after being colonized by soybean aphids may aid the insidious flower bug in host location. When populations of soybean aphids reach very high densities, top-down pressure exerted by the insidious flower bug may fail to suppress colony growth of soybean aphids.

Another group of predators that plays a key role in suppression of populations of soybean aphids in North America is lady beetles (Coccinellidae spp.). Some prevalent species in soybean include the twospotted lady beetle (Adalia bipunctata L.), the sevenspotted lady beetle (Coccinella septempunctata L.), the spotted lady beetle (Coleomegilla maculata De Geer), the polished lady beetle (Cycloneda munda (Say)), the multicolored Asian lady beetle (Harmonia axyridis (Pallas)), the convergent lady beetle (Hippodamia convergens Guérin-Méneville), and the thirteen spotted lady beetle (Hippodamia tredecimpunctata L.).

Evidence suggests that populations of lady beetles can respond to increases in populations of soybean aphids in soybean. In addition, increases in populations of lady beetles have the ability to inhibit colony growth of soybean aphids throughout the growing season. As generalist predators, lady beetles are able to feed on alternate prey when soybean aphids are at low densities. Other characteristics of lady beetles that are advantageous in times of soybean aphid scarcity include developmental delays of certain life stages, decreased body weights, and reduced clutch sizes. One of the most competitive lady beetles in North America, the multicolored Asian lady beetle, is of exotic origin. When soybean aphids are plentiful, an adult multicolored Asian lady beetle has the capacity to consume 160 soybean aphids per day.

Other foliar-foraging predators that are present North American soybean fields that may play a role in suppression of soybean aphid populations include green lacewings (Chrysoperla spp.), brown lacewings (Hemerobius spp.), damsel bugs (Nabis spp.), big eyed bugs (Geocoris spp.), spined soldier bugs (Podisus maculiventris (Say)), hover flies (Syrphidae spp.), and the aphid midge (Aphidoletes aphidimyza (Rondani)). Another group of predators that are present in soybean fields is ground beetles (Carabidae spp.); however, field experiments have shown limited to no impact from these predators on populations of soybean aphids due to the fact that ground beetles rarely scale soybean plants for prey. While parasitoids of the soybean aphid have a large impact on colonies in Asia—Lysiphlebia japonica (Ashmead) can have a soybean aphid parasitism rate as high as 52.6% in China—parasitoids are thought to exert only minimal pressure on soybean aphids in North America.