Alabama Cotton, Soybeans: Moth Numbers Still on the Rise, Scouting Fields Important Before Spraying

Cotton bollworm (CBW) moth trap catches increased from the last week of July to the first week of August at the 4 sites available for comparison. However, CBW trap catches tended to be much higher a year earlier at 3 sites.

Tobacco budworm (TBW) moth trap catches increased also from the last week of July to the first week of August. Substantial numbers of soybean looper (SBL) moths continued to be trapped during the first week of August but the numbers were generally lower than during 2016.

Reports of cotton bollworms infesting Bollgard 2 and regular Widestrike (not Widestrike 3) cotton fields continue in NW AL. Based on the size of the largest CBW larvae in fields it is likely that the current CBW flight began about July 24.

The number of CBW moths trapped at the Belle Mina station in Limestone County for the 7 day period ending July 24 was 41. To date a low percentage (less than 5%) of the cotton fields in NW AL have been treated for CBW’s but the number could increase. We have had very few reports of regular Widestrike fields being treated for CBW in other parts of the state.

Inspection of fields treated earlier with an insecticide indicated less than desired control. It is extremely difficult to get chemicals to the worms that have moved down from the terminal into squares, bloom tags and bolls.

Economic thresholds are necessary to help farmers make wise investment decisions when deciding if they will get a return on money spent for an insecticide. The current recommended treatment threshold for CBW’s in cotton is 4% infested plants or 2% damaged bolls with worms present. This is a conservative threshold.

If someone comes to a farmer and tells them that they have “worms” in their cotton or soybeans and they need to spend $14 per acre for the insecticide and $7 per acre for application costs this amounts to $2100 per 100 acres sprayed or 32.3 pounds of lint per acre at 65 cents per pound and 2 bushels of soybeans/acre at $10/bushel.

Every cotton field and every soybean field will have some worms present this time of year. However, this does not mean that spraying will save more money than it costs to spray. If the farmer does not know the density of the “worms” and the farmer makes the application without knowing what percentage of his cotton plants are infested or the percentage of bolls that have a worm feeding on them then they could be wasting money.

Herbicide Resistance Info

Likewise in soybeans the farmer needs to know how many caterpillars of each species are present per sweepnet sweep or groundcloth sample to make a knowledgeable treatment decision. It is also critical to know the level of defoliation that the soybeans have undergone and the number of each species of worm present before a prudent decision can be made.

If the number of soybean loopers is low and much of the soybean defoliation is being done by green clover worms and velvetbean caterpillars then less expensive chemistry may be used to prevent yield loss.

Moth trap catch numbers indicate CBW moths are still active. Large numbers of CBW moths were reported in peanut fields recently in Monroe County. Wheat beans have been found to have close to 5 pod worms (CBW) per sweep in south Limestone County and will require treatment.

Since tobacco budworm moths are also active then some of the eggs found on cotton plants could be TBW eggs and the larvae that hatch from these eggs will die. This is one drawback to using an egg threshold in cotton.

One option for scouting for newly hatched worms is to look in the terminal and try to spot frass and live worms. Detection of these small worms could be of value in making a decision to treat a cotton field. The drawback to using this approach is that the first instar larva could die soon after feeding on the Bt cotton.

The bottom line is that when the dual gene Bt cotton technology fails and worms survive it is very difficult to keep the numbers of CBW larvae below the economic threshold. You cannot get the insecticide to the worms that are coming from eggs laid on blooms, bracts and leaves within the plant canopy.

Soybean loopers (1 per sweep in spots) were found to be increasing along with green clover worms and velvetbean caterpillars in south Pickens County last Thursday (8/3) in R5 soybeans. However defoliation was still close to 5% and the grower and consultant were wisely going to wait and see if defoliation increased before treating. The defoliation threshold is 20% for soybeans from bloom through R6.

A report from Tuscaloosa County on 8/4 indicated that looper numbers were still very low and stink bugs were the main concern. Stink bugs are increasing in test plots at the Prattville research station where untreated R5 soybeans had 3 brown marmorated and 5 southern green stink bug adults per 15 sweeps across two rows on 8/2. Total immatures for these two stink bugs numbered 5 per 15 sweeps.

The economic threshold for stink bugs is 2 per 15 sweeps from bloom to mid-pod fill. Bifenthrin at 6.4 ounces per acre gave near 100% control of these two species of stink bugs.

There are now silverleaf whiteflies present in at least one field in each of 3 Wiregrass counties. This pest has also been found in at least one peanut and one soybean field in southeast AL.

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