Agronomic Crop Insect Scouting

David Owens, Extension Entomologist, owensd@udel.edu

Soybean

Continue scouting for pod feeders and defoliation. The most active insects include corn earworm (very low numbers), stink bugs, bean leaf beetles, grasshoppers, and green cloverworm. Cloverworm populations this year seem to be less than last year. Soybean defoliation thresholds in the reproductive stages are 20%, these thresholds are conservative. When evaluating defoliation, it is important to consider the entire plant and the entire canopy, not just focusing on the leaflets with holes in them. It is easy to overestimate defoliation impact; keep in mind that 20% canopy-wide defoliation looks downright awful.

This brings up an interesting question though: is it worth treating soybean with a fungicide/insecticide tank mix? There are several factors to consider. First – defoliation. Are you approaching the 20% defoliation mark? Last year we had a couple of fields approach this level of defoliation due to unusually abundant green cloverworm. I have not received any reports of large numbers of green cloverworm this year, and in the fields we have been scouting they are present in low numbers. Most typically, the answer to this question is no.

Second – pod feeders. Are stink bugs present in any sort of significant number? One could argue that the cost of application is very low because you are already going across the field, and that this should mean a much lower number of stink bugs per sweep sample would offset the cost of the insecticide. Observations from Virginia over many years suggest that pyrethroid application not only helps control stink bugs, but also prevents further colonization, possibly due to a repellent effect. On the other hand, full season fields in our area often do not develop significant stink bug populations. Where you may see the most stink bugs are immediately adjacent to corn and to wood lines with abundant wild cherry.

Third – risk for flaring other pests. This is more of a consideration in states to our south where corn earworm and soybean looper are more reliable. I think the risk of flaring up either pest in our area is fairly low.

Fourth – are pollinators present? While soybean does not require insect pollination, yield can be significantly greater where honeybees work on the flowers. This can be heavily influenced by variety and by surroundings. Is your field near a registered apiary (find out more information at https://fieldwatch.com/)? If so, it might be worth pausing.

Finally – how risk adverse are you in your operation and your landscape? Are you able to scout the fields to know what the current insect scenario is for this season as opposed to field surroundings or field history? If someone asked you to give them $1.50 x # soybean acres ready to be treated, would that come out to a significant sum (a hundred dollars, a thousand dollars)? Is your field very close to surface waters that a pyrethroid could drift into and potentially harm fish? I do not think there is a right or wrong answer to several of these questions, there are times when this application might break even or be useful. I think in the typical field, it will not. But on the other hand, IF a return trip is necessary that MIGHT have been headed off, we could cause some damage to yield by driving back into the field. The problem is one of resolution: how can I demonstrate differences in yield and profit down to a tenth of a bushel? If the planter sneezed when putting those beans in, or a deer walked through the very corner of a plot, or one spot in that plot is sandier or more nematody than the rest, or an entomologist tripped a few times while scouting a field, that is going to influence yield far more than the $1.50 generic lambda-cyhalothrin.

Corn

Some observations of spider mite colonies on dryland corn and dry pivot corners came in earlier this week. Often, mite colonies will largely disappear to the row that the irrigation gun starts. Our best mite guidance comes from the Midwest, and the below information was gleaned from Virginia Tech’s Pest Management Guide: “Spider mite populations often seem to explode as plants reach the grain-fill period, especially during extended hot, dry weather when the plants are stressed. If corn has not dented, treatment may be warranted if mite colonies are present along the midribs on the lower surfaces of one-third to one-half of the leaves on 50 percent of the plants.” Labeled materials include Oberon, Portal, and Zeal, along with bifenthrin and Hero. Getting good yield data from mite infestations can be difficult, and you have to ask yourself how large the infestation is, where the infestation is present, and is it worth spot treating or putting a boom over the edge of a field.

In non-rotated corn fields, check for the presence of western or northern corn rootworm feeding on silks. If the stalks are goosenecking near the ground, and if you see one beetle per plant, control measures for next year are recommended. Either rotate out of corn, use a transgenic variety targeting rootworm, or apply additional insecticide in-furrow. There is no benefit to spraying adults.

 

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