Cotton aphid: Biocontrol success story

Although North Carolina cotton producers routinely get banged up with thrips, in recent years we’ve been fortunate that cotton aphids have only been a sporadic pest.

With our high humidity and generally light insecticide usage, cotton aphids will hopefully again be only a minor problem here 2009. However, this “minor pest” status is not much consolation for the unlucky producers who must contend with high aphid levels and difficult treatment decisions.

Also, if the pest status of cotton aphids shifts upwards as it has in other states, this tiny insect with the big reproductive potential could become a real headache.

For approximately the past 10 years in North Carolina, cotton aphid resistance to both bifenthrin (Capture, Brigade, Discipline and others) and Bidrin has rendered these insecticides fair to useless depending on the cotton aphid population encountered.

The chloronicotinoid insecticide class (Centric, Trimax Pro, and Assail — sold as Intruder west the Mississippi), our first line of defense against cotton aphids here, is also losing its activity due to resistance.

Because “chloronics” are now so routinely used on cotton (both as our primary seed treatments and as common cotton aphid and plant bug spray options) and other crops in North Carolina, it’s probably only a matter of time until cotton aphid resistance to these materials shows up in North Carolina. 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 — at least in part because our use of chloronics for aphids is limited.

Although similar in structure to chloronics, Carbine insecticide has a different mode of action, and thus resistance to this compound may be slower in coming. Testing here seems to indicate that Carbine has aphid activity similar to Centric — that is, somewhat more active than Trimax Pro and less active than Assail, though some Mid-South testing shows that aphid mortality may take somewhat longer with Carbine.

Hopefully (for us), additional Mid-South experience with resistant aphids in 2009 will tell us if this new product represents an effective chloronic alternative under further field testing.

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

Because of the low cotton acreage typically requiring treatment for insects prior to bloom here, predators and parasites (particularly two species of mummifying wasps) often limit aphids to sub-economic levels.

The fungal pathogen Neogygites fresenii can greatly limit aphid numbers, sometimes over wide areas from about mid- to late-July and often throughout the remainder of the season. Like the wasp parasites, this fungus is particularly effective in eliminating or greatly reducing moderate to large populations of aphids. As an added bonus, 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), appears to be associated with 2.5 to 12-fold higher spider mite and cotton aphid levels compared to Temik alone, based on annual consultant surveys from 2004-2005 and in 2007-2008, so scouts should pay particular attention to these situations.

With the availability of Trimax, Centric, Assail and Carbine, consultants and producers have effective foliar spray options to control cotton aphids, at least for the time being. However, foliar sprays for cotton aphids should be considered a last resort.

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 can not keep up with a rapidly 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 confirmed, the aphid infestation will often be sharply reduced in the coming week or so, and treatment is seldom necessary.

Fortunately for North Carolina producers, cotton aphids appear to be one pest species on cotton for which biocontrol is presently far and away the preferred management option. Hopefully, this biocontrol success story will continue through 2009 and beyond.

TAGS: Management
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