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Field and Forage Crops |
Bollworms continue at moderate to high levels across much of the state, with enough to tobacco budworms mixed in with bollworms to cause higher than expected survival in some areas. Without an egg diagnostic kit to differentiate bollworms from budworms, it is difficult to tell if enough budworms are be present in a given population to justify expensive Tracer or Steward treatments. Although Steward and Tracer show greater activity against tobacco budworms, high labeled rates of these two insecticides will be needed to provide adequate bollworm control.
Beet armyworms continue to occur across much of the state, making decisions on bollworm treatments far more difficult. This has become a significant problem in many cotton fields this past week, and may get worse. Because pyrethroids are ineffective against beet armyworms and disruptive against beneficial insects, beet armyworm survival is often significantly higher following pyrethroid applications.
In cotton, which is still susceptible (not cut out) to beet armyworms, a threshold of 10 percent live beet armyworms on squares and small bolls and an armyworm found in every 2 to 3 white blooms is recommended, particularly if the cotton field were sprayed with a pyrethroid within the past 7 days. In cotton which has cut out (blooms and lateral squares difficult to find), the emphasis shifts to protecting the leaf directly under (subtending) bolls which are still filling out. Significant defoliation of these leaves can significantly reduce the filling out of these bolls, reducing yields.
Intrepid at 4.0 ounces per acre appears to be providing the most consistent beet armyworm control, with Steward also showing good activity. If bollworms are also present (often the case this past week), a pyrethroid insecticide will be need to be added to the $7.00 to $8.00 per acre cost of Intrepid; in most cases, Steward will provide acceptable bollworm control, but the high labeled rate will cost approximately $14.00 to $15.00 per acre. Tracer, which has shown mixed results against beets at lower rates, will cost $16.00 to $17.00 for the high labeled rate. Like Steward, Tracer has good tobacco budworm activity.
Fall armyworm levels appear to be still increasing on pastures and soybeans as well as cotton, though still at low levels at this time.
Cotton maturity is advancing rapidly, with an increasing number of fields within a week or less of reaching a stage no longer susceptible to bollworm damage. Thresholds for bollworms and stink bugs should be raised according to the level of large, safe bolls compared to those that are still vulnerable. For stink bugs, the suggestions for seasonably-adjusted damaged boll thresholds are provided in the Cotton Information booklet.
This week I received a telephone call about fall armyworms in crabgrass. They are also showing up in turf and about to show up in other crops. Hot, dry weather is bad for pathogens of fall armyworms, so who knows what will happen this year? Last year we had early reports in Bermudagrass pastures. Though they may prefer grasses, larvae will easily consume tender alfalfa foliage. Cutting is one alternative which often causes the larvae to leave the field in search of new food. If cutting is not an option at this time, chemicals can be used. Sevin and Lannate are the recommended choices. Check the North Carolina Agricultural Chemicals Manual for information on which insecticide might be more appropriate. Labeling is very tricky regarding forage and hay with unusual waiting periods. Consider the cost, residuals, cutting and waiting periods. Fall armyworm larvae are active during the day and night. The treatment threshold is about 50 larvae per 10 square feet (5 larvae per square foot average).
There are a lot of worms showing up in peanuts, perhaps more than we have seen in recent memory. After several years of low populations in which many fields escaped damage, it looks like this year may make up for it. More importantly is the composition of these worms. Rather than the majority being corn earworms, we are seeing fall armyworms (which are more difficult to control, but less damaging to the peanuts) and beet armyworms. Beet armyworms in peanuts at high levels are an unusual event here. The green-colored worms are often identified by the eye spot on each side of the thorax just behind the head. When small these larvae feed aggressively on the terminal, they can cause quite a bit of damage. Unlike corn earworms which are relatively easy to kill, beet armyworms represent a little tougher pest to manage. Since they are not very common, we have not had the opportunity through the years to evaluate their control in peanuts. Some information on their management is available for cotton. The typical recommendation in peanuts has been Lannate insecticide and success has been variable. I just received notice that the product Steward is now labeled for peanuts in North Carolina. I have tested this product on peanuts in North Carolina against both corn earworms and fall armyworms with good success. On other crops, good success has been achieved against beet armyworms. Growers in Georgia have been having good success with this product and it seems to provide reasonably good residual activity. So with all of this in mind, it certainly may be the product of choice as we look at the composition of the worms in the field. We do not have a good threshold for beet armyworms in North Carolina so I'm suggesting the one from Georgia. I talked with Steve Brown, my counterpart in Tifton, and he said this year they are recommending between 4 and 6 worms per row foot before they suggest treatment. That is about the same threshold we use for corn earworms. Steve Brown said that in the past he didn't think they were as damaging as corn earworms, but this year is changing his mind. Steward is made by DuPont and can be applied at 9.2 to 11.3 ounces per acre. It has a 14-day harvest restriction and one must allow 5 days between sprays. This is a different chemistry than the other products we use in peanuts; it must be ingested and requires a couple of days before the worms will die. Evaluate fields on the third day after treatment to determine success.
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Last modified on August 5, 2002 by Stephen J. Toth, Jr.