Soybean aphid numbers creep up
As reports of soybean aphids slowly increase, younger soybeans (R1-R2) that lack a canopy and need more rain may be at risk for yield loss.
Soybean aphid numbers have crept up in some fields, especially in southwest Michigan. Also, there are reports in some fields in Ionia County. The aphids in the pictures I saw were fat, green, happy and concentrated on new growth (lots of nitrogen). In some fields, numbers were up to 200 to 300 on some plants. Aphid numbers remain low in the fields my lab samples at MSU and outlying farms in Montcalm and Saginaw counties, so the infestation is patchy. High numbers in some areas may be related to recent rain events, if winged aphids were carried from other locations and dropped in Michigan. But, some of the reports are from areas with sandy soil or low K, a predictable risk factor for soybean aphids.
The threshold for soybean aphids is 250 per plant (not on a single plant, the average across a field). In essence, if every plant has soybean aphids, with several-hundred concentrated on the new growth, that is 250. Note that 250 is simply a spray guide with a five- to seven-day lead time built in. Damage from soybean aphids does not actually start until there are more than 600 per plant (a very reliable, well-tested threshold).
If the soybeans are well into the R stages (R4-R5), the canopy is full and moisture is adequate, beans can easily handle the current levels of aphids and wouldn't get sprayed unless numbers increase dramatically. Aphids living under a full canopy will eventually experience high humidity, and in the next few weeks, get infected with beneficial aphid-killing fungus. Also, there are a lot of beneficial insects in the fields we monitor and these will reduce infestations.
Beans that are younger (R1-R2), lack a canopy and need more rain are at most at risk now for yield loss.
Please only treat fields that really need to be sprayed. We do not need to flare spider mites in August.
Thanks to Charles Scovill, Syngenta; Bruce Mackellar, Michigan State University Extension; and other alert samplers for the aphid information.