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Cotton aphids: Biocontrol actually working

With our high humidity and generally light insecticide usage, cotton aphids will hopefully be only a minor problem here again in 2007.

For the past decade in North Carolina, cotton aphid resistance to both bifenthrin (Capture, Brigade, and Discipline) and Bidrin has rendered these insecticides fair to useless depending on the cotton aphid population encountered.

Because chloronicotinoids are now so routinely used on cotton (Gaucho Grande, Cruiser, Avicta, Aeris as seed treatments; Centric and Trimax Pro for cotton aphids and plant bugs, etc.) in North Carolina, resistant cotton aphid populations will likely show up here as they have in the Mid-South. So the clock may be running out for materials like Centric, Trimax Pro and Assail for cotton aphid control.

As far as we know, these materials are still effective here. Although similar in structure to “chloronics”, Carbine insecticide may have a different mode of action, and thus resistance to this compound may be slower in coming.

Testing so far in North Carolina seems to indicate that Carbine has aphid activity similar to Centric — that is, somewhat more active than Trimax and less active than Intruder. Mid-South experience with resistant aphids in 2007 will hopefully tell us if this new product represents an effective chloronic alternative under field conditions.

Management of most serious cotton insect pests via naturally-occurring predators, parasitoids and diseases, often works better in textbooks than in the field. Biocontrol of cotton aphids, however, has become our producers’ primary option in reducing economic levels of cotton aphids.

Because of the low cotton acreage typically requiring treatment for early tobacco budworms and plant bugs, predators and parasites (particularly two species of mummifying wasp parasites) often limit aphids to sub-economic levels. In most cases, these parasites are often able to control cotton aphids even during the two to three pyrethroid treatments our producers typically use to control the major mid-July to mid-August bollworm generation(s) on conventional cotton.

Also occurring from about mid-July and often throughout the remainder of the season, the fungal pathogen Neogygites fresenii can greatly limit aphid numbers, sometimes over wide areas.

Like the wasp parasites, this fungus is particularly effective in eliminating or greatly reducing moderate to large populations of aphids. Additionally, insecticides do not adversely impact fungi.

Despite substantial potential assistance from beneficial organisms, cotton aphid populations are sometimes widespread and persistent, and if present at high levels, may cause maturity delays and possible yield losses, particularly if this damage occurs when the cotton is under drought stress. (Yield losses resulting from cotton aphid feeding have been hard to document in the Southeast, however).

The high proportion of our state's cotton acreage now treated with a foliar insecticide for thrips behind seed treatments (sometimes twice), may be contributing to these aphid buildups.

With the availability of Trimax, Centric, Assail (sold as Intruder in the western states) and Carbine, consultants and producers have effective foliar spray options to control cotton aphids, at least for the time being. Foliar sprays for cotton aphids should be considered as a last resort, however.

In cases where aphid colonies are present on most plants in high enough numbers to result in honeydew and/or wilted terminals, treatment may be needed, particularly if cotton is under drought stress.

However, scouts should also assess predator, parasite, and fungal levels, and also take moisture into account. Less moisture means more plant stress.

Beneficial insects such as ladybird beetle and green lacewing larvae and adults, syrphid fly larvae, and others also help limit cotton aphid buildups, but sometimes cannot keep up with an expanding aphid population.

An experienced scout or consultant comes in handy, as these assessments can be difficult. In most situations in North Carolina, if approximately 10 percent to 20 percent of the aphids are in the form of the round, brownish paper-like mummies, or if any level of the parasitic fungus is present, the aphid infestation will often be sharply reduced in the coming week or so, and treatment is most often not necessary.

Fortunately, cotton aphids appear to be one pest species for which biocontrol is presently far and away the preferred management option.

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