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Field and Forage Crops |
Although cotton aphids have been widespread for several weeks, until lately these colonies have been mostly on the small side, and cotton plants are under little moisture stress. This situation has changed, with a number of cotton fields showing moderate to high levels of aphids on most plants, honeydew easily visible, wilted plants, only limited help from the aphid mummies, and no help from the fungus. Treatment is normally advised in these situations, preferably with Centric or Provado. At present, however, most cotton fields do not satisfy the above criteria.
The parasitic aphid fungus, Neozygites fresenii, was confirmed in a Faison area cotton field near the Sampson-Duplin county line. This fungal pathogen can drastically limit cotton aphid levels, and will hopefully show up on significant cotton aphid-infested acreage in the coming week or 10 days. Be certain to also consider the presence of aphid mummies, in addition to checking for the presence of the fungus. Spraying a cotton field is usually not advised once this fungus has been detected.
Although plant bugs have been a fairly minor pest in pre-bloom cotton in large areas of North Carolina in recent years, this would not be good year to overlook the possibility of plant bug damage on significant cotton acreage. With our rain-initiated, very lush vegetation drying down, plant bugs are causing more headaches for growers that usual, with some cotton fields showing both square retention counts under 80 percent and threshold levels of plant bugs. Some fields swept for plant bugs this past week have revealed up to 30 plant bugs per 100 sweeps, or approximately three times the recommended treatment threshold. Retention counts of 80 percent or more of the small terminal and upper lateral squares until a week or two into blooming would be a good indication that plant bugs are not presently a problem in that field. Because adult plant bugs can invade cotton fields rapidly, square retention counts should be taken at least weekly.
This week, an agricultural consultant found fairly significant levels of what appears to be European corn borer stem and petiole tunneling damage in a number of pre-blooming Bollgard cotton fields in Nash County and in a Johnston County cotton field. We estimated that the degree of infestation varied between 1 per 4 or 5 plants to 1 per 50 plants in the worst of the two Nash County fields (July 9), surprising for Bollgard cotton.
Bollgard cotton has shown excellent to resistance to European corn borer stem and tunneling damage since its introduction in 1996. In our annual damaged boll survey, European corn borer damage to bolls has been about 1/10 as much on Bollgard cotton treated an average of 0.9 times, compared with conventional cotton treated an average of 2.6 times. Stem tunneling by European corn borers in an Onslow County replicated 1994 test was about 100-fold less than in conventional cotton. This history makes the above observations all the more puzzling.
Whether this finding is limited in scope and has no, minimal or significant implications for our upcoming bollworm season is not presently known. Monsanto personnel are taking additional tissue samples, which can be tested in St Louis for the degree of Bollgard expression. That should shed some light on how well these (and possibly other) cotton fields will be able to withstand bollworm pressure in the coming weeks.
It appears that our major bollworm moth flights should be underway in the Rowland area of southern North Carolina between July 16 and 20. With the bollworm and stink bug season fast approaching, beginning next week the Cotton Insect Hotline Tapes (1-800-662-7301) will be available two times per week, on Tuesday and Friday afternoons.
Scouting results from many cotton fields within northeastern North Carolina are showing a significant drop in square retention during the last week. Square damage and above threshold plant bug numbers are accompanying the low square retention counts. This is not unusual. We have had an excellent early season for plant bugs (wetter and cooler than normal), plant bug nymphs have transformed (are transforming) to the adult stage, and early hosts are maturing. Early hosts include many kinds of weeds, wheat, potato, and other early crops. Currently we do not know the extent of the population and the duration of this plant bug movement period. The most common location for plant bug damage is in cotton fields adjoining potato, edges (particularly weedy edges), newly-planted soybean fields where weeds were destroyed during field preparation, and other situations where early hosts are drying down or have been destroyed (forcing the bugs to move). Growers, scouts, and consultants should be immediately alerted.
Plant bugs that colonize cotton fields will usually feed on terminals, squares, and other tender plant tissue. They will also usually mate and lay eggs. Nymphs can be very damaging later in the season. Presently, all fields with bugs will not show low square retention counts and all low square count fields will not show significant plant bug numbers. Plant bugs move a lot and may or may not choose to feed in any particular cotton field. Newly-arrived bugs may not have had a chance to damage squares and scouting results taken at bug colonization may show bugs but little damage. Bugs may move from a square damaged field and sweep counts may not show many bugs. Similarly, sweeps taken when bugs are mostly on the interior of the canopy (usually in the heat of the day and on larger plants) may not adequately catch the bugs. Movement and difficulty in sampling plant bugs may result in low confidence, frustration, and poor spray decisions.
In the mid-south, plant bugs are more common pests than in North Carolina. Often, extended colonization events require two to four sprays to keep field counts below threshold. There may be pre-flowering and post-flowering infestations. We have not seen this situation; however, in 2000 plant bugs in some areas above the Albemarle Sound showed some similarity and the situation was very different from when plant bugs were not even recognized as a cotton pest in North Carolina. The plant bug problem is obviously a changing situation and we must follow it closely. Reduced spraying of Bollgard, and now Bollgard II, will intensify bug problems that are already adapting to our new expanded cotton situation.
The 2003 Cotton Information book presents information on plant bug sampling techniques and thresholds. Insecticide choices are provided in the North Carolina Agricultural Chemicals Manual. Currently many fields are showing extensive, but low level, aphid infestations. For this reason, choosing a neonicotinoid insecticide (Trimax, Pravado, or Centric) for plant bug problems makes sense (these insecticides are active on aphids and will keep aphid populations from rapidly flaring). Other selections (e.g., Lorsban, generic acephate, Bidrin, etc.) are effective against plant bugs and are cheaper, but will likely flare up aphid populations. The later may be good choices in plant bug fields that do not have aphids.
Call backs from agents, consultants, dealers and farmers suggests that Trimax is giving mixed results on both aphids and plant bugs when used at the appropriate rate for each of these pests. This doesn't seem to be an application problem. Centric has given good results on both insects at the appropriate rates. Both of these products are pricy. Cheaper, older organophosphate insecticide products (e.g., Bidrin, Lorsban, and Orthene (acephate)) can be used successfully on plant bugs and may reduce aphids (first application of season only, not if fields were sprayed for thrips with Orthene earlier), depending on the population. But they likely will come back strong. Also, Karate, Capture, and Fury (more cheaper products) are effective on plant bugs, but Asana and Baythroid are soft. I am not sure about the other pyrethroid insecticides at this time. Aphids, if controlled, may come back strong after these products as well.
More fields are being sprayed above the Albemarle Sound and in parts of northern Washington County for plant bugs. Aphids seem to be the most serious in Hyde and Beaufort counties north of the Pamlico River.
Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
Last modified on July 14, 2003 by Stephen J. Toth, Jr.