Eileen: Hello, my name is Eileen Cullen, I’m the UW Extension Specialist in Field and Forage Crop Entomology and today we’re talking about Japanese beetle in soybean and corn. Japanese beetle has typically been and initially in Wisconsin has been more of an issue in ornamental crops, home gardens. In the last growing season or so, we really seen an upsurge in Japanese beetle defoliation in soybean and silk clipping in some cases in field corn and processing sweet corn in Wisconsin. Japanese beetle is native to Japan as you would expect, it’s an exotic insect in that way and it’s probably came in to Wisconsin through ornamental trade and then now has been moving in to field crops in agricultural settings. The Japanese beetle has one generation per year and what it initially does is it, in late June, the beetles emerge from turf and soil, grasses and they’ll start to feed on low growing plants and then they’ll eventually in, we start to see them in to July and through August in soybean and potentially in corn.
Japanese beetle is a large beetle adult, in the field it’s easily seen, it’s a metallic color, a bronze with green color and it is usually noticed by its appearance in the field and also as you’ll see in a moment it’s defoliation on the soybean leaves, for example, are silk clipping in corn. Japanese beetle has one generation per year and the way that that works is that adults emerge from the soil, usually in turf and grass areas in late June. They’ll feed on, again, ornamental plants in the area and then they’ll move in July, late June, early July and then through July and into August we can see them as we have moving in to soybean and corn fields. In August and September, the Japanese beetles will mate and then during that about mid-August and in to September they will leave the field crops and go to lay eggs in grass and turf areas. Those eggs will hatch, the grubs then will be there to over winter and they’ll actually go down into the soil profile to insulate from the cold and then that life cycle will continue the next season, adults again emerging in late June from the grass.
To determine feeding damage, for example, in soybean, we definitely do have to find the presence of Japanese beetle in the field before treatment decision is made, but keep in mind that you’re not making treatment decisions based on number of beetles per plant. We’re actually looking at the defoliation percentage that the beetle is causing to the soybean foliage. Japanese beetle will feed on soybean leaves between the leaf veins and they really cause a very characteristics skeletonizing or lace-like pattern of the soybean leaf. This is very distinctive for Japanese beetle and that differs from other defoliation that can be occurring in the fields at the same time in July and to August and through that R1 through R5 growing stage of the soybean, grasshopper, for example, will have a much more ragged feeding not the skeletonize feeding. A second generation bean leaf beetle can sometimes also be a factor although it has been, not as much of a factor in Wisconsin lately, but second generation bean leaf beetle feeding has more of a round feeding hole in the leaf. The skeletonized feeding is what you’ll be looking for with Japanese beetle.
To scout for Japanese beetle and Japanese beetle feeding damage in soybean, what you’ll want to do is be looking at the percent defoliation on the leaf tissue and there are two different thresholds for that. Keep in mind we don’t count Japanese beetles per plant as we do with some other insect pest. Certainly you need to confirm presence of Japanese beetle in the field before any treatment decision is made and that’s not difficult to do since the Japanese beetle are a large beetle and they’re easy to see in the field. To make that treatment decision you really want to look at the percent defoliation on the leaf tissue before the soybean has reach reproductive stage, so pre-bloom that percent defoliation threshold is thirty percent. After bloom and at bloom from R1 through the R5 growth stage during the reproductive soybean growth stages, the defoliation threshold is decreased and that moves down to twenty percent defoliation. This is something that you’ll get experience with over time.
One of the things that we see in judging defoliation in the fields is that oftentimes it can look worst than it is in terms of percentage. It’s very obvious to see this skeletonizing at the top of the plant and to see clusters of beetles that are very eye catching as you walk the field. One of the ways that you can fine tune that decision, there is guide in the UW Extension Publication A3646 Pest Management in Wisconsin Field Crop. In the soybean insect management section, there’s a slate of pictures that showed different percentages of leaf defoliation from insect feeding. So you can use that to compare leaves in your soybean field, whether that defoliation is coming from Japanese beetle, grasshoppers, the occasional pest status of the lepidopterous such as green clover worm or potentially second generation bean leaf beetle. Keeping in mind that thirty percent defoliation threshold in the pre-reproductive soybean growth stage and dropping that down to twenty percent defoliation in the R1 through R5 growth stages.
In addition to looking at the top of the canopy you also want to make sure that you look at the middle and the lower canopy. One way to do that is to go and find ten plants in the field that are in a representative area of the field, not all just along the edge of the field or in one area that has very noticeable damage, but get ten plants that are fairly representative randomly throughout the field and take from those ten plants, from each of those plants, a top leap, a middle canopy leaf and a lower canopy leaf and that will give you thirty leaves from the field. Again, then you can just judge the percent defoliation using the guide in UW Extension Publications A3646 if needed and that will give you an average of what percent defoliation you have going in that field at that particular growth stage, in terms of percentage, which will drive the treatment decision.
For Japanese beetle feeding in corn, consider a Foliar insecticide treatment during tasling and silking if there are three or more beetles per ear, silks have been clipped to less than half an inch and pollination in that field is less than fifty percent complete.