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Field and Forage Crops |
A significant portion of our early maturing cotton crop has thankfully become far less attractive to late season boll-damaging pests such as stink bugs, bollworms and tobacco in the past two weeks. In some cotton fields, the cotton has completely cut out" and is out of harm's way. This would a good time to assess the vulnerability of your crop to these pets and to raise thresholds accordingly, as indicted below.
Brown and green stink bugs continue to be the dominant pest of Bollgard cotton, although levels appear to be declining, as stink bugs are perhaps opting for greener pastures, like soybean fields. Plant bug nymphs and adults are also hanging around in some fields, adding to the damage of small bolls. Although the potential of further stink bug damage to bolls will likely drop drastically in most cotton fields during these first two weeks of August, because of the high level of stink bugs this year the monitoring of quarter-sized bolls is still suggested for at least the next few weeks in most cotton fields. In an untreated part of a Wayne County stink bug test that we mentioned last week, stink bug damage to bolls was considerably down from the 40 percent levels of the previous two weeks. However, 27 percent of the quarter-sized bolls showed stink bug damage this morning. That is still a lot of new damage.
On the plus side, as the level of stink bug-safe bolls (approximately 3.5 weeks after anthesis, or flowering) rises and the level of stink bug-susceptible bolls declines, common sense suggests that damaged boll thresholds may be raised. As a general rule of thumb, when the number of stink bug safe bolls (often approximately 1.25 inches in diameter) matches the number of susceptible bolls, the 10 percent internal boll damage threshold may be raised to 20 percent. Similarly, when safe bolls outnumber susceptible bolls by about 3:1, that is one out of four susceptible bolls, so the threshold may be raised to approximately 40 percent. See the following web site for details: http://ipm.ncsu.edu/cotton/insectcorner/text.html#sb. At some point in many cotton fields in the not too distant future, stink bugs will only be able to cause spotting on older bolls and not damage the inside locks and cause damage. Remember that all boll checking for a potential stink bug spray should entail the cutting open or crushing of quarter-sized bolls as the best indicator of recent damage.
For the most part, the bollworm moth flight experienced further declines this past week, and many moths are now being attracted to soybean and other hosts. As with stink bugs, as cotton fields increasingly cut out, not only are bollworm moths less attracted to cotton plants, but small bollworms are less able to become established when few squares or flowers remain. However, high levels of moths can still be found concentrating in later, lusher cotton fields, with eggs being deposited both in terminals and low in the plants. Eggs deposited on dried blooms hatch into larvae, which can feed directly on the young developing bolls. If most bolls are large and only few small bolls remain in the terminals, bollworm thresholds based on the sampling of only small bolls can be raised according to the proportion of large bolls to small bolls, often 2 to 4-fold.
Tobacco budworms have been a significant problem in many conventional cotton fields this growing season. In most years representing only a small proportion of the bollworm/budworm complex in North Carolina, this year budworm moths and their more difficult-to-control larvae have resulted in numerous cases of mediocre to poor control with the pyrethroid insecticides used to control bollworms. The time and expense of using Agdia's egg kits to identify budworms in the early part of and throughout the major bollworm moth flight in conventional cotton fields is hard to justify in most years here. This year the kits would have been a good investment for many growers.
Only occasional sightings of localized beet armyworms have been reported this past week, although fall armyworms have been a pain in a few areas and can easily be found at low levels in many cotton fields. No labeled chemicals provide good control of established fall armyworms (approximately 1/3 of an inch or larger). Cotton aphids have made a slight comeback, but will likely stay far below threshold levels for the rest of the season, with the possible exception of a cotton regrowth situation in opening cotton far down the road.
I will be out of the country attending an entomology meeting from August 10 to 23. We will have the next update on Wednesday, August 25. I hope that our major cotton insect problems are now on the wane, and that the weather cooperates. See you on the 25th.
A lots of worms are showing up in peanuts right now. Some growers are waiting to treat for these pests since vine growth is quite substantial. The damage in this is that worms eat most of what they are going to eat in the last few days before they pupate. So in other words, the damage happens hard and fast and the worms are much more difficult to control when they are large. Keep a close watch on these populations and treat when necessary.
Late instar larvae (armyworms) are developing in pastures and poised to do major damage. Keep a close watch on fields for bird activity and you may have to scout a little more closely than usual. Large larvae can damage a field almost overnight.
Specific control recommendations are difficult to relay in this note. When reading a label, armyworm refers to the true armyworm. There are also fall armyworms, yellowstriped armyworms, and beet armyworms. It is suspected that most of the populations creating problems are fall armyworms, but the other species may be present. Fall armyworm has the inverted Y on the head and four dots on the tail segments. Help in determining the species may be found in the field crops manual (AG-271). A rough threshold is 5 armyworms per square foot, but 3 armyworms per square foot may be enough at this point. Sample 10 to 12 times in different places in a field. Field edges are often more heavily infested.
Labels are tricky for these pests and pasture. Consult the North Carolina Agricultural Chemicals Manual (page 76) for help. There is some rumored dissatisfaction with using Sevin 80S, but I am not convinced that failures are not entirely due to applicator technique or weather. Sevin ground applications must use about 30 gallons of water to ensure adequate coverage. Applications are best made toward the evening, while plants are not wet, and when rain is not expected within 48 hours. (That has been tough this year.) Sevin has a 14 day grazing interval. In Bermudagrass, Lannate is labeled and works very quickly and is a restricted use chemical. Unfortunately, choices such as Warrior and Mustang Max are not labeled on bermudagrass. Malathion is also labeled in grass pasture and Warrior might be a choice in alfalfa or wheat hay.
We usually see fall armyworm problems in hot, dry summers. Perhaps the good weather is assisting a larger population. It is frustrating in that the wet weather may interfere with control. The fear is that most of you seeing any of the armyworm species will probably need to treat at some point. Consult the North Carolina Agricultural Chemical Manual for choices, and then read the label. Large larvae (1 inch or more) mean you do not have much time to make a decision. Make your decision based on scouting, larval size, grazing interval, expense, pasture composition, and weather.
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Last modified on August 9, 2004 by Stephen J. Toth, Jr.