Iowa’s most significant soybean insect pest, soybean aphid, has host-alternating biology. This species has multiple, overlapping generations on soybean in the summer. Fall migration to buckthorn is based on senescing soybean, and decreasing temperatures and photoperiod. For the majority of the year, soybean aphids are cold-hardy eggs near buckthorn buds. As spring temperatures warm up, soybean aphid eggs hatch and produce a few generations on buckthorn before moving to soybean.
For many aphids that overwinter as an egg, hatching often happens when the host resumes spring growth. This makes biological sense because the aphids feed on phloem from actively-growing tissue. If egg hatch happens too soon, they can suffer mortality from starvation. Research has confirmed soybean aphid egg hatch happens around buckthorn bud swell. Soybean aphid egg hatch occurs between 147-154 degree days (base 50°F) and buckthorn bud swell happens shortly after that (165-171 degree days). Based on air temperatures in 2020, I expect egg hatch is complete in northern Iowa, where most of the buckthorn in Iowa is located.