With our high humidity and generally light insecticide usage, cotton aphids will again, hopefully, be only a minor problem here in 2005.
However, this “minor pest” status is not much consolation for the unlucky producers who must contend with high aphid levels and difficult treatment decisions.
Additionally, the amount of North Carolina’s cotton acreage treated for this pest seems to be trending upwards.
A review of few aphid management considerations is in order.
Bio-control success story: Management of most serious cotton insect pests via naturally-occurring predators, parasitoids and diseases often works better in textbooks than in the field.
Bio-control control of cotton aphids, however, has become our producers’ main line of defense. 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 hold aphids to sub-economic levels. Once they get a “foothold”, the wasp parasites are often able 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 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.
Cotton aphid buildups: Despite substantial potential assistance from beneficial organisms, cotton aphid populations, though sporadic, are often widespread and persistent, at times resulting in 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 typically treated with a foliar insecticide for thrips (sometimes twice), may have contributed to these aphid buildups.
Aphid Resistance: Prior to 1989, low numbers of cotton aphids were the rule in North Carolina, and could be routinely controlled by any of a number of organophosphate (OP) insecticides.
As had happened earlier in the Mid-South, by 1990 some aphid populations in North Carolina appeared to be expressing high levels of resistance to recommended OP insecticides.
Subsequently, a graduate student here confirmed both high levels of resistance in some populations and extreme variability in resistance levels throughout the state. The publicized presence of these organophosphate and later pyrethroid (Capture) resistant aphid populations appears to have contributed to producer and consultant reluctance to spray for aphids, a decision usually rewarded by effective natural control.
Finally, based upon replicated tests conducted in North Carolina in 1997, 1998 and in 2000 to 2004, it is apparent that OP-resistant aphid populations are still common.
Chemical options: With the availability of Trimax, Centric and Assail (sold as Intruder in the western states), consultants and producers have effective foliar spray options to control cotton aphids, at least for the time being.
Treatment guidelines: 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.
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 probably be sharply reduced in the coming days, and treatment should not be necessary.
Resistance avoidance: One word of caution. Cotton aphids have been notorious about developing resistance to previous insecticide classes. All of these new effective materials are in the same chemical class (choronicotinoids), and therefore the over-use of any of these materials may help confer aphid resistance to all.
Also, don’t forget that the chloronicotinoids are also the basis for the seed treatments Gaucho Grande and Cruiser, and have additional labels in many other agricultural crops. This very safe, effective class of chemistry would be a nice one to keep around for a while.
Fortunately, cotton aphids appear to be one pest species for which biocontrol is presently far and away the preferred management option.