–by Dr. Jeff Whitworth and Dr. Holly Schwarting
Dectes (soybean) stem borer damage is becoming more and more apparent as soybean harvest is delayed throughout north central Kansas. The dectes stem borer was first detected infesting soybeans in Kansas in about 1985 in south central parts of the state. Annually, the adults emerge from soybean stubble, where they overwinter as larvae, around the 4th of July. They mate and deposit eggs around soybean leaf petioles. The small larvae bore into the plants and feed, making their way into the main stem.
Dectes stem borer larvae are cannibalistic, so as these larvae come into contact with others only one survives to tunnel down the main stem to the base where they girdle around the interior of the stem, typically just prior to harvest. This girdling activity often goes unnoticed. However, the stems are weakened and wind will blow girdled plants over. Fields with significant infestations will have serious lodging, as we are seeing now in parts of NC KS. Harvesting these lodged plants is very difficult at best, and if harvest is delayed further, the grain lying on the ground may be lost to rodents, disease, etc. Infested fields should be harvested as soon as possible, hopefully before any additional lodging occurs! There is no preventative or rescue treatment available for dectes stem borers in soybeans.
For more information on the biology of the dectes stem borer, please visit Dectes Stem Borer, MF2581: https://www.bookstore.ksre.ksu.edu/pubs/MF2581.pdf
–by Dr. Jeff Whitworth and Dr. Holly Schwarting
Have received several inquiries relative to corn earworms in field corn and there does seem to be a good infestation of them throughout north central Kansas. 100% of the ears we have randomly checked currently are, or have recently been, infested with corn earworm larvae.
Many of these larvae are still relatively small and thus will be feeding for another week or two. The two most common questions received this week relative to these pests are; 1) Will a rescue treatment work? The answer – no. Once the larvae have hatched from eggs deposited on the silks and moved into the husk, they will be protected from contact insecticides.
2) Will they re-infest these corn ears? The answer – no. Field corn will be too tough by the time these larvae finish feeding, pupate in the soil, emerge as adults, mate, oviposit, hatch, and larvae initiate feeding. But, the adults of this generation will move to soybeans (soybean podworms) and/or sorghum (sorghum headworms) to oviposit and larvae can do considerable damage by feeding on soybeans within the pods and/or directly on the kernels of the heads of sorghum plants. So, the larvae currently in corn are the “spring board” for the next generations moving into soybeans and sorghum.
— by Dr. Jeff Whitworth and Dr. Holly Schwarting
Green Cloverworms in Soybeans
Remember a few weeks ago when there was considerable concern relative to all the green cloverworms causing irregular holes in leaves? Even skeletonizing some areas of some fields until treated with an insecticide and/or and entomopathogenic fungus started decimating the larval populations? Well, the surviving larvae pupated and now are annoying little aerodynamically shaped dark brown moths flying around lights at night or trying to get in through doors and windows.
These moths will mate and then begin ovipositing in soybean and/or alfalfa fields. Eggs hatch in approximately 10-14 days and the larvae will again start feeding on leaves of either crop. By this time of year, the larval feeding is usually of little consequence relative to yield. However, really late planted soybeans, and all alfalfa fields, should be closely monitored to ensure leaf feeding in either crop does not affect pod fill in soybeans or leaf area in alfalfa.
These insect pests seem to be on about the same developmental schedule as green cloverworms. So, late planted soybeans may be at risk for bean feeding within the pods.
Adult bean leaf beetles, while probably not as numerous as in past years, may still be feeding on the pods themselves. This can cause yield reductions. For more information on bean leaf beetle biology and control, please visit: https://www.bookstore.ksre.ksu.edu/pubs/MF2824.pdf
Another leaf feeder that can cause concern this time of year is woollybear caterpillars. There are several different species but all are foliage feeders although they rarely cause any economic problems.
Hopefully, most soybeans are past the stages that are succulent enough for stink bugs to be feeding on. However, there are still some late planted beans setting pods with seeds that may be vulnerable to stink bug feeding. So, until pods are turning yellow or brown, fields probably should continue to be monitored for soybean podworms, adult bean leaf beetles, and stink bugs.