Eugene Burris and Eugene M. Holman
As field corn acreage has increased in the mid-South, consultants and farmers have often noted high tarnished plant bug populations in cotton fields adjacent to corn. The origin of the infestations in these cotton fields is often perceived to be a result of early population buildup in the pre-tassel stages of corn. In these troublesome fields, economic thresholds for insecticide treatments are usually reached earlier than in cotton fields not bordered by corn.
Plant mapping data taken from cotton fields adjacent to corn indicate a gradient in plant damage similar to insect population gradients detected by sweepnet. At cotton field margins adjacent to corn, square loss at nodes 4 to 9 ranged from 48 percent to 85 percent, indicating severe damage occurred shortly after the pin-head square stage of cotton. Cotton will compensate for moderate levels of early season square loss, depending upon water availability and late-season environmental conditions, but early season square losses of more than 30 percent can reduce yield, delay maturity and usually require expensive insecticide treatments. These studies were conducted to determine the origin of the tarnished plant bug infestations in cotton fields next to corn and to study the peak times for immigration of the insects.
Tarnished plant bugs in corn
Evaluation of tarnished plant bug movement into pre-tasseling corn and pre-squaring cotton was accomplished by using sticky traps, sweepnet samples and whole plant observation. Population surveys in 1998 used 40 sticky traps placed on 1-inch by 1-inch garden stakes in a grid pattern. Ten cards were placed at the corn/cotton margins, and these were replicated four times so that the grid pattern covered about 400 by 1,000 feet.
Regardless of the sample method used, low densities of tarnished plant bug adults were recorded in pre-tasseling corn, and no reproduction could be associated with the corn. Previous research also indicated that only limited plant bug reproduction occurs in corn. In these studies, plant bug adults ranged from a high of 85 per acre to a low of 11 per acre. Samples taken in the corn crop in mid-April with sweepnets confirmed that tarnished plant bug densities were very low.
Tarnished plant bugs in weeds Tarnished plant bugs have numerous plant hosts, including many weed species common to Louisiana. Sweepnet samples of plant bugs in weeds near corn and cotton fields revealed large densities of plant bugs in all stages of development. From March to June, it was not uncommon to collect eight to 10 adults per 10 sweeps.
In mid-April, all stages of nymphs were present in the sweepnet samples in very high numbers. This indicates a key time in the life cycle of the tarnished plant bug, because the nymphs observed in mid-April represent the immature insects that would eventually infest cotton as new adults.
The weedy areas harboring large numbers of plant bugs included roadside ditches, field drains, point rows, highway medians, utility poles and other non-tillable areas such as wooded areas adjacent to fields. At one farm site, a waterfowl conservation site had floral vegetation after drainage and produced large numbers of plant bugs. The principal weeds that supported high populations of plant bugs were vetches, clovers and docks, but many broadleaf weeds that flower in spring were observed to harbor plant bugs.
Direction for future research
These surveys indicate that plant bug densities in pre-tassel corn are too low to cause significant economic damage in adjacent cotton fields. Furthermore, the level of reproduction in pre-tassel corn was not high enough to account for the consequent damage. In contrast, development of plant bugs in weed habitats near these fields supported extremely high densities of tarnished plant bug adults. The peak numbers of adults and nymphs observed by mid April probably account for the first infestations in cotton, however, the economic infestations occurring in cotton adjacent to corn cannot be attributed solely to weed refuges near corn, because weed refuges infested with tarnished plant bugs are abundant near fields far removed from corn and without similar problems.
One possible explanation is that corn is a barrier to local migration, because the flight of plant bugs is about 3 feet above the ground. Too, plant bugs may prefer cotton over corn as a food source. Most of the recorded hosts for plant bugs are broadleaf plants, and only a few are grasses. These are hypotheses only. The reasons for the high populations of plant bugs in cotton adjacent to corn need further evaluation.
The information gathered suggests that vegetation management of the weedy areas associated with high populations of tarnished plant bugs would have merit and could probably be accomplished with selective herbicides and mowing. A project has been initiated for the year 2000 with the U.S. Department of Agriculture’s Agricultural Research Service to evaluate early herbicide programs and their effect on tarnished plant bug populations and their movement into cotton.
(This article was published in the winter 2000 issue of Louisiana Agriculture.)