Although bio-control of most cotton insects is far more successful on paper than in practice, cotton aphids are often controlled naturally in North Carolina.
Because producers here have not needed to treat for the boll weevil in more than 18 years (23 years in some of our northeastern counties), and because of the low acreage typically requiring early treatment for tobacco budworms and plant bugs, predators and parasites (particularly lady bird beetles and two species of mummifying wasp parasites) have, for the most part, held aphids to sub-economic levels.
Once established, the wasp 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).
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.
Despite this assistance from beneficial organisms, during the past two years cotton aphid populations have become more widespread and persistent, resulting in some cases of maturity delays and possible yield losses in a number of cotton fields.
The high proportion of the state's cotton acreage treated with a foliar insecticide for thrips in 1999 and in 2000 (sometimes twice), appears to have contributed to recent aphid buildups. Whether this trend will continue is anyone's guess.
Prior to 1989, low numbers of cotton aphids were the rule in North Carolina, and could be routinely reduced to sub-economic levels 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.
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.
Additionally, more recent studies by a graduate student here confirmed both high levels of resistance in some populations and extreme variability in resistance levels throughout the state.
Finally, based upon replicated tests conducted in North Carolina in 1997, 1998 and in 2000, it is apparent that these variably-resistant aphid populations are still the rule.
With the recent availability of Provado 1.6 F, a product which provides fair to good control of aphids and which spares most beneficial insects, consultants and producers now have an additional option for aphid management, with other new highly effective, safe aphid products on the way.
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 if predator, parasite, or fungal levels are low, particularly if cotton is under moisture stress.
In most situations, though, if approximately 10 to 15 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.
We would again urge restraint in controlling cotton aphids with insecticides, and regard natural control as the primary, most effective and economical option for managing aphids in most situations.