Alfalfa Living Mulch Advances Biological Control of Soybean Aphid

Despite evidence for biological control in North America, outbreaks of the invasive soybean aphid, Aphis glycines</it> Matsumura (Hemiptera: Aphididae), continue to occur on soybean (Glycine max</it> L. Merr.). Our objectives were to determine whether natural enemies delay aphid establishment and limit subsequent population growth and whether biological control can be improved by altering the within-field habitat. We hypothesized that a living mulch would increase the abundance of the aphidophagous community in soybean and suppress A. glycines</it> establishment and population growth. We measured natural enemy and A. glycines</it> abundance in soybean grown with and without an alfalfa (Medicago sativa</it> L.) living mulch. Soybean grown with an alfalfa living mulch had 45% more natural enemies and experienced a delay in A. glycines</it> establishment that resulted in lower peak populations. From our experiments, we concluded that the current natural enemy community in Iowa can delay A. glycines</it> establishment, and an increase in aphidophagous predator abundance lowered the rate of A. glycines</it> population growth preventing economic populations (i.e., below the current economic threshold) from occurring. Incorporation of a living mulch had an unexpected impact on A. glycines</it> population growth, lowering the aphids’ intrinsic rate of growth, thus providing a bottom-up suppression of A. glycines</it>. We suggest future studies of living mulches or cover crops for A. glycines</it> management should address both potential sources of suppression. Furthermore, our experience suggests that more consistent biological control of A. glycines</it> may be possible with even partial resistance that slows but does not prevent reproduction.