One reason for soybean lodging is girdling by the soybean stem borer. Larvae of this longhorned beetle girdle the soybean plants just above the soil line in preparation for overwintering in the lower portion of the stem. The problem can be easily identified by looking for tunneling in the soybean stems. If the main stem of the plant is tunneled look for a legless, yellowish cream colored larva, 1/2 to 5/8 inch long larva in the base of the girdled plants. For more information and pictures of this pest see the publication MF2581, “The Soybean Stem Borer” www.oznet.ksu.edu/library/entml2/MF2581.pdf
Look for dying leaves (refer to the web link at the end of this answer). You can obtain an idea of how serious a mid-season infestation of soybean stem borer larvae in soybeans can be soon after the larvae have entered the main stems. Take a yardstick, broom handle, pool cue, or other long, thin pole and orient it parallel to the row, against the stems above ground level but beneath the canopy. Raise it upwards and push the pole against the canopy so you can see into it as the plants are bent back. Dying mid-canopy leaves, well above the normal leaf abscission zone may be visible; their dark, wilted appearance contrasting sharply with the healthy leaves making up the majority of the canopy. In most instances, there will be only one dead leaf per plant, though sometimes there are more. Pull the leaf free from the plant and split the petiole. It should show evidence of having been tunneled and there will be some insect waste products (frass) present within the tunneled area. Look closely at the point where the petiole attached to the plant. An entry hole should be visible, showing where the larva tunneled through the petiole base into the stem. This action is typically the event that killed the leaf. At this stage, many or most larvae will be somewhere inside the stem, chewing their way down to ground level. Splitting the stem should reveal a legless, yellow larva which, in Kansas, is the immature soybean stem borer. If your field inspections are well timed, these dying leaves will be readily visible. For pictures and more images, see http://www.oznet.ksu.edu/entomology/extension/KIN/KIN_2005/kin-18/05ksnew18.htm#Soybean_Stem_Borer_–_Look_for_Dying_Leaves
Until other control options are developed, fields should be sampled before maturity for tunneling and live larvae. Fields with high percentages of infested stems should be harvested as soon as possible to avoid girdling and lodging. For more information and pictures of this pest see the publication MF2581, “The Soybean Stem Borer” www.oznet.ksu.edu/library/entml2/MF2581.pdf. If a large percentage of soybean plants at mid-season show evidence of soybean stem borer infestation (death of individual mid-canopy leaves, above the normal abscission layer, with evidence of insect tunneling within the leaf petiole), avoid delaying harvest after the plants reach physiological maturity. Once a stem borer larva reaches the soil line, it will chew out an overwintering chamber and (just above that point) will girdle the stem from the inside, plugging the tunnel with frass between the girdling location and the overwintering chamber. Girdled plants are very susceptible to lodging and can separate completely from the base of the stem. These larvae are not social. The overwintering chamber will usually only host one living larva. A large percentage of lodged plants can create conditions that lead to a very frustrating, slow and often inefficient harvest. The longer harvest is delayed, the greater the chance that stem-borer infested plants will break off and fall to the ground where their pods will shatter and the beans become lost. Thus far, if the larvae have made it into the stem, we (in Kansas) have not identified registered insecticides that can be applied to stop further tunneling and girdling from occurring. In fact, spraying currently registered products earlier in the year in an attempt to control egg-laying adults has generally produced questionable results in Kansas research trials where heavy stem-borer pressure was present.
These beetles (on a population basis) can be active for about 8 weeks. Thus far, Kansas researchers have not had overwhelming success in achieving that degree of persistence with insecticides registered for soybeans. While the population can be reduced, the residual activity of a single insecticide application has not always prevented significant populations of tunneling larvae from infesting the plants. On-going research may eventually determine how this can be achieved at some point in the future.
“For more information on this or other topics related to soybean production, contact your state extension specialist or your local extension educator/agent.”
The following is the link to the soybean extension specialists: state extension soybean specialist
