Soybean Loopers and the Effect of Defoliation on Yield

We have been getting calls on loopers showing up in soybean for about 2 weeks. In recent days there have been reports of defoliation exceeding 20% or numbers exceeding threshold in some areas in the central to southern part of the state. The bulk of our soybean loopers typically show mid to late August or even into September so this is a little early but certainly not unusual. Soybean loopers are migratory and work their way north each year from southern latitudes. Early arrival could be due to a number of reasons.

Soybean Looper or Cabbage Looper: As a general rule of thumb, the loopers that we see in June and early July are usually cabbage loopers. This time of year populations are generally switching over to soybean loopers in our area. However, we can have cabbage loopers mixed in with them. This can sometimes cause confusion about control options since cabbage loopers are very easy to control and soybean loopers are difficult to control. Can you tell them apart? Most people believe that soybean loopers have black legs (front legs) and cabbage loopers have white legs. However, a paper published in the Journal of Entomological Science 33(4) 421-425 in 1998 by Doug Jost and Henry Pitre showed that this can be an unreliable method of distinguishing between soybean and cabbage loopers. In 1995, 99% soybean loopers collected from cotton and soybeans had black legs and 31% of the cabbage loopers collected in cotton had black legs. The only cabbage looper they were able to collect out of soybeans in 1995 had black legs. In 1996, only 50% of the soybean loopers collected from cotton had black legs and only 55% of the soybean loopers collected out of soybeans had black legs. The safe thing to do is assume they are the more difficult to control soybean loopers if they are showing up any time after mid-July and treat them as such. Of course you can pull the mandibles and check but this is cumbersome for consultants and not easy to do.

Soybean Looper Biology: The life cycle of soybean and cabbage loopers are nearly identical. On average it takes about 3 days for the eggs to hatch. The larval stage last 13-14 days then they pupate on the underside of the foliage. The pupa stage last for about 7 days and then they go through a pre-oviposition stage for about 3.5 days. From egg to adult is about 26-28 days on average. (Handbook of Soybean Insects: Leon G. Higley and David J. Boethel). Remember with most defoliating caterpillars, they will consume 90% of the foliage that they will eat in their lifetime in the last 3 days of their life. This is why our threshold of 19/25 sweeps also has the clause of ½ or greater in size or 20% defoliation. So if you are checking a field and find numerous newly hatched loopers, you will have time to wait until your next check before they get enough size to cause extensive defoliation. This also allows natural control factors such as disease and beneficial insects to do.

Understanding the effects of defoliation on yield loss in soybeans: A few years ago, Dr. Lucas Owen, conducted an extensive research project on effects of defoliation on soybean yield at different growth stages and where the defoliation occurred in the plant canopy. The graph below shows expected yield loss as a percentage of the untreated or non defoliated plots at R3, R5, and R6 growth stages for whole plant defoliation. Whole plant defoliation is what our thresholds are based on. We looked at the effects of the defoliation in the top half or bottom half only of the plant and they were not significantly different than each other. In fact, if you had 100% defoliation in the top of 50% of the plant, the yield was the same as 50% defoliation in the whole plant. This is important because bean leaf beetles tend to feed in the top half of the plant and  loopers tend to start in the bottom and move up but we must remember to average it out across the whole plant. Another important finding is that yield loss can occur even as late as R6 stage from foliage loss, however, it takes at least 50% defoliation before we start seeing significant yield loss. Once we get 7-10 days past R6 (R6.5) it would be very difficult to show a yield reduction from ANY amount of defoliation from pests of soybean. For example, looking at the graph below, 60% whole plant defoliation at R6 would result in a 10% yield loss. We currently have a new student, Mr. Ben Thrash, who is working on yield loss associated with sub-threshold incremental levels of defoliation.

Click to Enlarge

Terminating Defoliator Sprays in Soybeans: As mentioned above, once we get 7-10 days past R6 (R6.5) it would be very difficult to show a yield reduction from defoliating pest of soybean even if it was 100% defoliation. At R6.5 the seed has separated from the membrane that holds it to the pod wall and easily comes out when shelled by hand. Therefore it is in the drying down process and is no longer getting nutrients from the pod wall, so foliage is no longer playing a role in yield.

Control Options:  Soybean loopers have a long history of resistance to pyrethroids, organophosphates, and carbamates insecticides. Until recently we relied heavily on a short list of products such as Steward or Intrepid to control this pest. These products do indeed still work but have almost exclusively been replaced by the Diamide class of chemistry because of the long residual and broad spectrum control of nearly all caterpillar pests in soybeans.  Products in the Diamide group labeled in soybeans include Belt, Besiege, and Prevathon. Besiege is actually a premix of Chlorantraniliprole and lambda-cyhalothrin (the use rate of 7 oz./acre gives your the equivleant of 14 oz. of Prevathon and 1:90 Karate Z)Last year Dow AgroScience also received registration on Intrepid Edge which is a premix of methoxyfenozide and spinetoram. These products have also shown very good performance with extended residual on soybean loopers. Each of these products work extremely well but are more costly. Most growers feel the extra cost is worth the money due to extended residual control. It is highly recommended to avoid pyrethroids and/or acephate for soybean looper control. I have seen these products actually flare loopers. On occasion, I have seen suppression when tank mixing the two together but this is a risky game to play.

Use Rates. See product labels for more information.

Intrepid 4-6 oz. per acre

Steward 5.6-11.3 oz. per acre

Belt – 2-3 oz. per acre

Besiege 7-9 oz. per acre

Prevathon 14-20 oz. per acre

Intrepid Edge 4-6 oz. per acre (Also a 2ee label for a 3 oz. rate once crop gets past R5 growth stage)