Posted 31 August 2016. PMN Crop News.
Numerous Caterpillars Showing up in Cotton
Source: Mississippi State University Press Release. www.cals.msstate.edu
Mississippi State, Mississippi (August 13, 2016)--Over the last couple weeks, we have received numerous calls about multiple caterpillar pests in Bt cotton. By far, the most common and widespread species has been bollworm, but populations of fall armyworm are also high and we have even heard of a few beet armyworms at treatable levels. We will talk a little bit about each one of them in more detail.
It has been years since we have seen significant beet armyworm infestations in cotton. However, we have received a few reports of them in Bollgard II cotton this year. They can be distinguished from other armyworm species by a black spot on the second thoracic segment just above the second true leg. In general, beet armyworms are one of the most difficult species to control in cotton. The diamides, Prevathon and Besiege, and Diamond are all good options to control beet armyworm. We generally think of Diamond as a plant bug material, but it is actually one of the very best insecticides we have tested against armyworms.
In contrast to beet armyworm, we usually see some fall armyworms in Bollgard II every year. Fall armyworms are identified by an inverted Y on their head capsules and 4 small black spots on the last 2 abdominal segments that form a square. Overall, fall armyworm populations have been very high this year in multiple crops and cotton is no different. Feeding by small larvae usually shows up as grazing on square and boll bracts. As they grow, fall armyworm larvae bore into squares and bolls and will completely consume the anthers of white flowers. Management of fall armyworm is similar to that of beet armyworm.
In general, the Cry1F protein in Widestrike cotton provides good control of armyworms and we generally donít see populations of either fall armyworm or beet armyworm in those varieties.
Several things have likely contributed high populations of armyworms in cotton this year. First, fall armyworms and beet armyworms do not overwinter in Mississippi and must migrate into the state from southern locations every year. The mild 2015-2016 winter likely contributed to populations surviving further north than usual. Because of that, populations migrated into Mississippi earlier than normal and that has allowed them to complete multiple generations and build up to higher populations.
Another factor that may have contributed to higher populations in cotton has been the lower densities of plant bug populations. Remember that we mentioned above that Diamond is one of the best insecticides that we have tested against armyworms. Because of the fewer sprays for plant bugs in 2016, we have used less Diamond in cotton than we have in several years. In general, those Diamond applications that we have been making for plant bugs has been helping to keep armyworm populations low in cotton.
Bollworms are an annual pest of both Widestrike and Bollgard II cottons. We generally average between 1 and 3 applications to manage bollworms in these technologies every year. It is looking like 2016 is going to be closer to 3 applications than 1 application. We donít have an egg threshold published for Bt cottons in the control guide, but spraying on egg counts can be beneficial under high populations. Over the last week to 10 days, we have heard of some very high egg counts (multiple eggs per plant on nearly every plant). Knowing that a large percentage of those eggs are bollworm and the current stage of the cotton, spraying on eggs is probably warranted at this time.
We know that most of the eggs being laid right now are primarily bollworm because we are seeing high levels of survival after they hatch in Bt cotton. Also, most of the cotton is just beyond peak bloom and is starting to approach cutout. We know that as cotton begins to mature that levels of the Bt proteins decline. This creates the perfect situation for bollworms to survive and cause damage in Bt cotton, especially when populations are high.
At what level of eggs do I need to spray Bt cotton?
This is a difficult question to answer because it is a very difficult question to research. After numerous conversations among ourselves and with numerous consultants, we have decided that it is best to spray at around 30% eggs. As we said, this is not based on research, but is our best guess at what level of eggs will result in enough damage to warrant a spray.
In terms of control, a well timed spray with acephate plus a pyrethroid can provide adequate control in Bt cotton. The closer the spray is to when most of the eggs will hatch, the better it will do. Once the eggs hatch and the larvae begin to feed and increase in size, it is less likely that this combination will provide adequate control. The diamide insecticides that contain chlorantraniliprole (Prevathon or Besiege) will usually provide the best control of bollworms in cotton.
Another important question right now is about the rainfastness of these insecticides. First, acephate has very poor rainfastness. It needs at least 6 hours after a spray to not get washed off and 8 hours is probably better. In contrast, we have very little concern about the diamides being washed off. As a rule, if the diamides have time to dry, they should provide good control regardless of rain.
Finally, an important question is when to stop spraying for worms in cotton. In the control guide, we suggest terminating insecticide sprays when cotton reaches nodes above white flower 5 plus 350 heat units. Not surprisingly, most people donít have a clue when their cotton reached this point. As a result, we have started doing some research to develop a new method for determining when to terminate insecticide applications in cotton. That research is still ongoing, but we have determined that most varieties will reach that point around the 5th week of flowering so that should provide a good guideline about when to stop spraying.