Plant Bugs: A Minor Pest In North Carolina- For The Time Being ?

Jack S. Bacheler, North Carolina Extension Entomologist

Thankfully, fewer individuals from points south with an `agronomic' background appeared at last winter's producer meetings with local horror stories of lost fruiting positions measured by the foot (up the plant), alarming maturity delays and significant yield penalties due to plant bugs or Lygus. On the other hand, 3.5% of our state=s cotton acreage was treated last year, up from our longer-term average of an average about 0.5 to 1%. Additionally, a significant portion of our cotton acreage near by or adjacent to Irish potatoes must be treated annually. Fortunately this acreage is relatively small - though this is of little consolation for producers in this situation.

Plant bug populations vary greatly from area to area across the cotton belt. In Mississippi, entomologists estimated the percent of cotton acreage treated for Lygus averaged 84% in 1996-1997 (Cotton Insect Losses: Proc. Beltwide Cotton Conferences). In northern Alabama, the treated acreage was approximately 81% for the same two years. However, the average treated acreage for plant bugs in the southeastern coastal states was much less: Georgia was 8%, South Carolina 4.3%, and Virginia weighed in at 0%. As you can see, in Georgia, the Carolinas and Virginia, treatments for plant bugs is far more the exception than the rule.

In North Carolina, the most sensible way to assess the need for plant bug treatment is to keep up with the percentage of squares being retained by the plants. Research conducted here and elsewhere suggests that if cotton plants are retaining 80% or more of their squares, additional sweeping or other monitoring for plant bugs is unnecessary. In most years, percent square retention is often in the low to high 90's until blooming begins. To assure that fruit losses due to plant bugs are recent, be sure to check at least two newly developing squares per plant, selecting one in the terminal area and one on a lateral fruiting branch. The pinhead squares monitored are often about an eighth inch in length, including the bracts, are often the smallest squares which are easy to see, and are associated with a leaf about the size of a dime through a quarter. Hotter, drier weather seems to push plant bugs down in the canopy, and at times, first and/or second position square loss on a lateral branch can predominate. Checking two squares each per plant from 50 randomly-selected plants from throughout the field would constitute an adequate sample size in most cases. If square retention is less than 80%, and square damage from tobacco budworms has been ruled out, then sweeping is normally advised

[budworm damage to squares is often associated with a tiny penitration hole, webbing, and/or some adjacent feeding on tender leaves; however budworms can also make a single penitration hole which becomes nearly invisable as the square dries up. These blacked squares can be difficut to differentiate from plant bug injury. Fortunately, budworm damage to squares in the 20% range higher is uncommon in North Carolina.]

For fields with square retention of less than 80%, taking ten, 25- sweep samples throughout the field for adult and immature plant bugs should indicate if treating is advisable (though other factors may effect this decision). In North Carolina, expect adults to make up the vast majority of the population under most circumstances. Lygus numbers can then be compared to appropriate plant growth category of the plant bug threshold table (pg.22 of the Cotton Insect Scouting Guide, or pg. 137 of the 1998 Cotton Information Booklet).

Plant Bugs in Perspective: It is imparative that producers stick to management strategies tailored to their own circumstances, particularly in areas of lower plant bug pressure where advice from outside the area tends to address population levels much higher than in the Carolinas and treatment responses which are much more protection-oriented than in our region. In North Carolina, with the boll weevil a thing of the past and cotton aphids almost entirely controlled by predators, parasites and fungi, early season insecticide use is almost entirely directed against June second generation tobacco budworms on limited acreage. Scattered pockets of economic levels of plant bugs are mostly confined to a few of our northeast counties. The unnecessary treating of sub-economic levels of plant bugs would likely greatly increase the level of early tobacco budworm sprays, a second unneeded expense with potentially more dire consequences- resistant budworms. Hopefully, North Carolina producers will continue to base their treatment decisions on scouting, proven thresholds and wise council.