Mixed Bag of Early Season Caterpillar Defoliators in Soybean

Over the last couple of weeks I have had numerous calls about caterpillar pest in young soybeans.  Most of these have been concerning beans that are still in the vegetative stages (V2-V5). The species that are being found are bollworm, yellowstriped armyworm, green cloverworm, granulated cutworm, alfalfa caterpillar, cabbage looper, garden webworm, and yellowstriped armyworm. In most fields defoliation has not exceeded 10% and no action will be taken, However, there are some fields or spots in fields where defoliation has exceeded threshold and required treatment but this is the exception rather than the rule.  This whole complex of caterpillar pests will act as defoliators at this stage (including bollworm) and a threshold of 35% defoliation should be used on beans that are still in vegetative stages.  In the fields where the plants are too small for sweep nets, most are scouting visually. One caution I would give you about visual sampling in small beans:  You can cover a lot of plants in a short period of time and find a lot of worms, however, the infestation may be still be very light overall and not seem that way because you can cover so much ground quickly. I would urge you to be very careful before treating this time of the year.  You would be very surprised at how well beneficial insects this time of year will keep caterpillar pests in check.  Once you make that first broad spectrum spray, your subsequent caterpillar pest problems can become much worse.  Also, most people (including me) can easily overestimate defoliation levels in soybeans.  The picture to the right shows exactly 35% defoliation of a trifoliate (scanned by a computer), first glance you may think 50%.  Also, the defoliation thresholds are for whole plants so you have to take into account all of the foliage.  In the Insect Control Guide we have pictures of various defoliation levels if you need to calibrate yourself.

When using a sweep net on green cloverworms the threshold is 75 worms per 25 sweeps 1/2 inch in size in the vegetative stages and 38 worms per 25 sweeps 1/2 inch in size in reproductive stage beans. some of the highest counts I have heard is 15-20 per 25 sweeps so far, so we are actually a long ways away in most fields from reaching a threshold and beneficial insects will likely take them out. There are a few early planted beans blooming around the state. Although I have had no reports of bollworms in flowering soybeans at any significant number yet, watch closely for them. The threshold for bollworms in reproductive stage soybean is 9 per 25 sweeps.  In reproductive stage beans, the defoliation threshold drops to 20%.  The picture to the right shows a soybean trifoliate that is 20% defoliated.

Effects of Early Season Defoliation on Yield of Soybeans:

Dr. Fred Musser and myself recently had a PhD student (Lucas Owen) who worked on verifying defoliation thresholds in soybeans in MS. Group IV soybeans had no yield loss even when defoliated 100% in theV3 stage, however when at the V6 stage, Group IV soybeans began losing yield when defoliation exceeded 50% but never exceeded 20% yield loss even at 100% defoliation levels. In Group V soybeans, yield was not significantly reduced at the V3 stage but was reduced by 16% at the V6 stage when 100% defoliation occurred.

Soybeans can tolerate a lot of damage in the early vegetative stages with no yield loss.  However as we begin reaching the later vegetative stages we can see yield loss from defoliation but this typically don’t occur unless we have extreme levels of defoliation.  Bottom line is our threshold for vegetative soybeans of 35% is about right and certainly will not get you in trouble.

Pictures of some of the caterpillar pests being found in MS soybeans.