PROTECTIVE BOLLWORM THRESHOLDS URGED FOR CONVENTIONAL COTTON

Jack S. Bacheler

North Carolina Extension Entomologist

Although a number of North Carolina's cotton insect pests are of minor economic concern most years, bollworms, with their potential for significant late season boll damage with no or limited compensation, must be taken seriously. The importance of utilizing the egg threshold as the trigger to initiate control of our major upcoming bollworm flight can not be overstated. An aggressive response to this flight should help insure the high fruit retention required with North Carolina's relatively short growing season.

Threshold studies conducted by our project from 1984-1989, and grower successes employing this strategy for the past 11 years point strongly toward the continued use of the egg threshold as the most profitable way of managing our major bollworm generation. These tests have consistently documented the ability of low populations of late July to mid-August bollworms to extract moderate to heavy yield losses if not controlled prior to, or at the time of egg deposition or hatch.  Because compensation for this late fruit loss is minimal at North Carolina's latitude, a very quick response to the egg and subsequent larval thresholds is must.

In the above (eight total) studies, large penalties resulted from delaying treatment. For example, on the average (with cotton set at $0.70/lb. and the cost of a pyrethroid plus application $10.00/acre), by waiting one week beyond the a 3% larval threshold, a producer could expect to lose $76 per acre compared with employing the egg threshold. On average, the best timing occurred when approximately 10 eggs per 100 terminals or 2 eggs per 100 fruiting forms were present.  Producers, consultants and scouts are urged not to overlook eggs and larvae deep within the canopy on blooms, bloom tags and bolls, especially if the weather turns dry in July and August. Egg and caterpillar data in these tests, and other observations, suggest that control is most optimum when one insecticide application was already on the cotton plants just prior to major egg hatch, with a second five days later to eliminate any hatching bollworms. Another option to consider in situations where producers employing ground rigs can not cover their acreage in six or seven days' time would be to begin with a high pyrethroid rate to extend the control time. High labeled rates in general will extend the effective 'half-life' of the insecticide,useful in the above and some other situations.

After the initial two applications (or the high rate option mentioned) at the egg threshold, our traditional larval thresholds (3 percent bollworms in squares, blooms or bolls) are employed for the remainder of the season. In rank, late, susceptible cotton, 3 to 5 or 6 treatments may be justified due to the possibility of damaging European corn borers and/or fall armyworms, in addition to the crop's higher susceptibility to bollworms. As of this writing, it's a few weeks early to predict how this year's bollworm moth flights will shape up. However, our early bollworm generations appear to be on, or slightly ahead of, schedule, and bollworm egg deposition on silking corn appears to be on the moderate to high side. Remember also that bollworms in 1997 were exceptionally light and late (perhaps the lightest the past 15 to 20 years). A repeat is unlikely. Also, if moderate to high populations of European corn borer (ECB) populations develop on field corn, our later planted cotton acreage could be in for higher ECB pressure that we have seen during these past few very light ECB years.

Surprisingly, in its 10 years of grower use in North Carolina, this "protective approach" has not resulted in an increase in insecticide use. Also, waiting for 10 eggs per 100 terminals or 2 percent eggs on squares or bolls usually means that our major economic flight is underway (most North Carolina cotton producers have light traps in their county or in adjacent counties to help confirm these flights).   One word of caution: high tobacco budworm populations (the adults of which are not readily attracted to black lights) may result in higher levels of budworm adults "sneaking" into cotton prior to the major bollworm moth flight. This can result in a treatable situation for budworms being reached prior to the bollworm egg threshold being met.  With our high present second generation budworm levels in cotton, we expect budworms to constitute a higher proportion of the bollworm/bollworm complex than we normally experience. A 3% larval threshold is suggested for the time period between the onset of blooming, but prior to the major flights. This threshold will more likely be met than was the case in 1996 and in 1997.

 Because even small numbers of bollworms can extract moderate late-season boll damage, it is imperative to get off to an economical, clean start in managing this year's major bollworm and European corn borer generations. A strict adherence to the egg threshold will provide that start, and a quick response to subsequent threshold levels of small bollworms will maximize the yield potential of this year's cotton crop.