Linda Benedict, Leonard, Billy R.
James S. Russell and B. Roger Leonard
The tarnished plant bug is becoming a major cotton pest in the mid-South states of Arkansas, Mississippi and Louisiana. In 1998, the tarnished plant bug caused a loss of more than 14,000 bales of cotton in Louisiana, even though more than $5.5 million was spent to control it.
The tarnished plant bug has become a problem in Louisiana for several reasons. The use of transgenic Bt cotton on considerable acreage has reduced insecticide application frequency for tobacco budworm, which would otherwise control tarnished plant bugs. The development of insecticide-resistant populations of this pest has made control with insecticides more variable. An increase in native host plants along field borders provides an untreated refuge for tarnished plant bugs to reproduce throughout the production season.
Tarnished plant bug injury has been primarily associated with early season pre-flowering cotton and can result in delayed crop maturity and yield reductions. Recently, tarnished plant bugs have been observed at high densities later in the season during the flowering and boll-development stages of cotton. Because of the lack of knowledge about the effects of tarnished plant bug feeding on bolls, this study was undertaken to examine the susceptibility of cotton bolls to tarnished plant bug- induced abscission (premature boll loss).
All experiments were conducted at the Macon Ridge location of the Northeast Research Station near Winnsboro. These studies were done in plots of transgenic Bt cotton (NuCOTN 33B) containing the Bollgard gene. The test areas consisted of 16 rows by 250 feet and were managed according to crop production practices recommended by the Louisiana Cooperative Extension Service.
Tarnished plant bugs were collected from cotton and native hosts in northeast Louisiana using a standard 15-inch diameter sweep net. They were held in a wire mesh cage in the laboratory for 24 hours to reduce mortality from physical injury and disease and were fed washed green beans and sugar water to maintain health. They were then placed in glass vials and transported to the field in a chilled ice cooler to eliminate mortality from heat stress.
Cotton plants were monitored twice a week until the first week of flowering. First-position white flowers (flower located on the first fruiting node of a fruiting branch from the main stem of the plant) were marked with a yellow tag on the fruiting branch between the petiole of the flower and the main stem of the plant. The date of anthesis (flower opening) was recorded on the tag in permanent ink to ascertain boll age at the time of infestation. Boll age was calculated using accumulated heat units beginning at anthesis. A heat unit (HU) is the maximum daily temperature minus the minimum daily temperature divided by two and subtracted from 60 (the minimum critical temperature for cotton plant development).
Two tarnished plant bug adults were placed into a nylon mesh bag, which was then placed over an individual boll. The opening of the bag was tightly closed around the stalk of the boll with a drawstring. The date of the infestation was also recorded on the tag for each individual boll. The non-infested bolls were also covered with bags that contained no tarnished plant bugs.
Tarnished plant bug infestations began at white flower (zero HU) and continued until bolls had accumulated 487 HU beyond anthesis. Fifty-nine different levels of accumulated HU were used within this range. The sample size for each individual HU varied from nine to 85 flowers or bolls. At 72 hours after infestation, the cages and tarnished plant bug adults were removed. The number of abscised bolls was recorded at seven days after infestation.
Boll abscission appeared to peak (76.6 percent) at 50 HU or two days after anthesis and decreased to zero at 189 HU or 7.5 days after anthesis (Figure 1). A lower incidence of boll abscission was observed for bolls that had accumulated less than 30 HU as compared to bolls that had accumulated 30 to 100 HU. On bolls with less than 30 accumulated HU, tarnished plant bugs are probably concentrating their feeding on petals and anthers before the flower naturally separates from the boll. The cotton flower normally separates at its base during the second day after anthesis and will generally fall off in a few days. Young bolls (less than 1 day old) are not exposed, and tarnished plant bugs appear to be unable to feed on these bolls and cause abscission. On older bolls, tarnished plant bugs pierce the outside skin of the boll and cause sufficient damage to result in abscission of that boll. Essentially no boll abscission was observed for bolls that had accumulated at least 189 HU.
Our study indicates that cotton bolls are safe from tarnished plant bug-induced abscission 7.5 days after anthesis. This data will be important in designing and implementing late-season integrated pest management (IPM) programs for the tarnished plant bug in the mid-South region.
James S. Russell, Graduate Assistant, Department of Entomology, LSU Agricultural Center, Baton Rouge, La.; and B. Roger Leonard, Associate Professor, Macon Ridge Location of the Northeast Research Station, Winnsboro, La.
(This article was published in the summer 1999 issue of Louisiana Agriculture.)