An interesting extension article out of the Midwest last month shed light on the concept that
sometimes when you think you know your pest - you still don’t know your pest. Pest managers
in northern Indiana counties have been in the thick of the soybean
aphid battle this year, and in central and some southern areas of
the state counts are hitting threshold numbers. As with any pest
outbreak, there has been some confusion about thresholds,
pesticides, and biology. By far the most asked questions for
extension entomologists have been concerning the very small,
white aphids. These ARE soybean aphid. They ARE NOT all
“baby” aphids. They ARE NOT diseased aphids. In literature
they are referred to as “white dwarfs.” Aphids of many different
species do this in response to change. With soybean aphid, this
change in morphology may be due to hot temperatures, higher humidity, shorter day-length,
nutritional quality, predator populations, etc. Although the entomologists do not know what is
causing this, they do know that they are living, feeding, and reproducing aphids and should be
included in the total plant population count when determining treatment. It may well be that
they do not feed as heavily as the “normal” green/yellow aphid, but it has been noted that
because of their size and color, many are missed when making aphids/plant counts.
The other aspect of soybean aphid control regards insecticide efficacy. No foliar insecticide will
give residual control beyond two weeks under tough environmental conditions (i.e., sun, heat,
humidity). One week to ten days is more realistic. Second applications are becoming common,
and justified, for many fields in northern Indiana. The extension entomologists stated that if a
company representative guaranteed three to four weeks of control with their product, take them
up on their offer and hold them to the warranty. Once aphid levels reach over 100 per plant,
predators may not be able to keep ahead of the aphid’s reproductive capability. The exception
would be where aphid numbers may be on the decline. Fungal pathogens that quickly move
through aphid populations are favored by high humidity. Unfortunately, these diseases usually
only occur once aphid populations have exceeded treatment levels. (Purdue Pest & Crop,
8/12/05).
