Jack S. Bacheler
North Carolina State University
Southeast Farm Press, July 7, 2004
Cotton Aphid Outlook: With our high humidity and generally light insecticide usage, cotton aphids will again likely be only a minor problem here in 2004. That's not much consultation, however, for the unlucky producers who will contend with high aphid levels and difficult treatment decisions. A review of few aphid management considerations is in order.
Biocontrol: Management of most serious cotton insect pests via naturally-occurring predators, parasitoids and diseases seems to work better in textbooks than in the field. Biocontrol control, however, has become our producers' main line of defense against cotton aphids. Because of the low acreage typically requiring early treatment for early tobacco budworms and plant bugs, predators and parasites (particularly lady bird beetles and two species of mummifying wasp parasites) often hold aphids to sub-economic levels. Once established, 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.
Cotton Aphid Buildups: Despite substantial assistance from beneficial organisms, during the two of the past four years cotton aphid populations, though sporatic, were often widespread and persistent, resulting in some cases of maturity delays and possible yield losses in a number of cotton fields. (Yield losses resulting from cotton aphid feeding have been very hard to document in the Southeast, however). The high proportion of our state's cotton acreage treated with a foliar insecticide for thrips in 1999 through 2002 (sometimes twice), may to have contributed to these aphid buildups. Both thrips treatments and cotton aphid levels appeared to have dropped somewhat in 2003, however. So far in 2004, thrips spraying appears to have been up to pre-2003 levels.
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 NC 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 a 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, 2001, and in 2003, it is apparent that OP-resistant aphid populations are still widespread.
New Chemical Options: With the availability of Trimax), Centric and Assail (sold as Intruder in the western states), consultants and producers now have very effective foliar spray options to which cotton aphids have not yet developed resistance.
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 levels of predator, parasite, and fungal levels, and 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 to 20% 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. All of these new effective materials are in the same chemical class (choronicotinoids), and therefore the use of any of these materials may help confer aphid resistance to all. Plus, cotton aphids have been notorious about developing resistance to previous insecticide classes. So count this pest as one for which biocontrol is typically far and away the preferred management option.