Soybeans typically harbor an incredible diversity of insects that feed on the plants from the time they start to crack the soil until the last leaf has dropped in the late summer. And somewhere between these two extremes, insect and spider mite populations occasionally build up to economically damaging populations. When this happens, often the most effective pest management tactic for most growers and crop managers is to apply a foliar insecticide to reduce the population.

There are several insecticides, and seed treatments, labeled for use in Iowa soybeans. The more commonly available products for the three major soybean arthropod pests (the insects and mites), along with the pre-harvest interval, are listed in Table 1. There are several notes of caution, however, necessary to understand the effects of insecticides:

Bean leaf beetles

Broad-spectrum insecticides sprayed against the overwintered population of bean leaf beetles will knock out early-season beneficial insects and may increase mid-season populations of soybean aphids and possibly spider mites. Seed treatments should suppress overwintered populations of bean leaf beetles, thereby resulting in a smaller first generation in July. But if the overwintered population is large, then the benefits of a seed treatment may be minimal and there will be no carryover effect on the subsequent generation.

Soybean aphids

Any broad-spectrum insecticide in Table 1 will provide acceptable control of aphids if applied with sufficient water carrier and complete coverage of the plant canopy. The seed treatments do not provide adequate soybean aphid control. In most of Iowa, aphids arrive too late in the growing season for the seed treatment to have any residual effect as the insecticide lasts only about 60 days. If soybeans are planted in April, then when the aphids arrive in late June or early July, the residual of the seed treatment will not be adequate to kill the insects. The exception would be in northeastern Iowa where aphids typically arrive earlier in the growing season. Here seed treatments may be valuable for suppression of early soybean aphid populations.

We have reported in previous editions of the ICM newsletter on the efficacy of these foliar products. In this article, you can see how the various products compare regarding knockdown, residual activity, and yield protection.

Spider mites

The pyrethroids (Asana® XL, Baythroid®, Pounce®, and Warrior™) will kill all the beneficial insects without killing all the spider mites. This may cause the spider mite population to rebound, thereby making the problem worse. Spider mites are best controlled in soybeans by using an organophosphate (dimethoate or Lorsban®, or its generic equivalent).

Figure 1. Bean leaf beetles feeding on an untreated soybean leaflet in an insecticide evaluation. (Marlin E. Rice)

In 2004, several insecticides were evaluated against bean leaf beetles. Potted V1-stage soybean plants were sprayed in a mechanical spray chamber, the insecticides were allowed to dry, and then sprayed leaflets were removed from the plants and placed inside a sealed Petri dish along with five beetles that had been starved for 8-24 hours (see Figure 1). The beetles were allowed to feed for 24 hours and then their mortality was recorded. The residual effects of the insecticide also were tested at 7 and 14 days post application by using new leaflets and freshly collected beetles from the field, again that were starved before the experiment. The seed treatments (Cruiser® and Gaucho®) were evaluated by removing leaflets from seed treated plants that were the same age as plants that were sprayed.

Several insecticides (Lorsban®, Mustang™, and Warrior™) provided very good knockdown of adult bean leaf beetles one day after application. The residual effects of the insecticides 7 days after application in causing beetle mortality was only evident in the two rates of Lorsban. The combination of Lorsban® and Pounce® in this test did not improve the performance of this insecticide mixture (see Table 2).

Table 1. Commonly available insecticides labeled for use in Iowa soybeans, 2007.

* restricted use pesticide; read and follow all label directions.

† rate per 100 lbs. of seed applied prior to planting.

Table 2. Evaluation of insecticides on bean leaf beetle mortality 1, 7, and 14 days post application. Iowa State University, 2004.

* means in the same column and followed by the same letter are not significantly different, P = 0.05.