The Summer for Insects - Scout your Fields!

It has been a great summer for insects. The warm weather has sped up their development and insect pests are emerging at least one week earlier than usual. Also, with the lack of rain, the crops are under stress and are not putting out new leaves quickly to compensate for any injury that is taking place. I strongly recommend that you keep an eye on your crops for the next month to ensure that pests do not take over and cause even more stress for the crops.

My first concern is spidermites in soybeans. The rains have been very spotty, leaving some fields still very dry, which is ideal for spidermite activity. I saw a few fields on my way into work today that had clear signs of spidermite injury. Signs include pockets of plants, usually along the edge of the field that are turning a bronze colour. Looking more closely at these leaves, you may see white or yellow spots starting on the upper surface of the leaves. Eventually the leaves will dry up, curl and fall off. Turning the leaves over reveals fine webbing and from the naked eye very small black moving dots may be visible.

Shack these leaves onto a white piece of paper to see the mites moving around. Four or more mites per leaflet or one severely damaged leaf per plant prior to pod fill is cause for concern. If caught early enough, only a spot treatment may be necessary, treating the hot spots on the edges of the field where the mites are moving in from.

Another concern is soybean defoliators. This includes bean leaf beetles, redheaded flea beetles, grasshoppers and even corn rootworm adults. Typically we overestimate the amount of defoliation that these insects do to the plants but in a year when the plants are stressed and new leaves are slow to come out, we need to keep an eye on things.

Again, do not overestimate the amount of defoliation taking place. Soybeans have an amazing ability to
withstand quite a lot of defoliation before yield is impacted. Often someone shows me a field that they thought had at least 30% defoliation in their field, when in fact it only had 5-10%.

To determine how much defoliation there is scout in 10 areas of the field. Avoid the field edge as insect activity is usually higher there. Pick trifoliate leaves from the top, middle and lower areas of the canopy from 5 plants in these areas. Discard the least and most damaged leaflets from each trifoliate collected. Compare the leaflets that remain with the defoliation charts available in the Agronomy Guide. From R1 (early bloom) to R5 (pod fill) stage it only takes 15% defoliation to warrant control. Once the crop is beyond the R5 stage, it takes 25% defoliation and we usually instead focus on pod feeding injury that can then impact seed quality.

And finally, though I bet you thought I'd talk about soybean aphids, there is one more insect I think we need to watch for in the corn crop. Besides, I am assuming you are already scouting for soybean aphids and watching closely to see who wins the battle between the aphids and their natural enemies.

The insect I am concerned about next is corn rootworm adults. Though the adults typically do not cause enough silk clipping to warrant a spray, they have emerged one to two weeks earlier than usual this year and could hit those fields that are trying to tassel and silk. Dry conditions may keep the plants from growing more silk to compensate for the feeding injury. Field corn can withstand heavy adult activity, usually requiring at least 10 adults per ear before control is necessary but seed corn may require control if adult populations are causing extensive silk clipping, disrupting pollination. Treatment is warranted when the silks are on average being clipped down to within a 1/2-inch of the ear tip.