Jeffrey A. Davis, Katherine L. Kamminga, Arthur R. Richter and B. Rogers Leonard

Louisiana soybean growers have historically tried to protect their crop from a diverse insect pest complex consisting primarily of stink bugs, Lepidopteran defoliators (green cloverworm, soybean looper and velvetbean caterpillar), three-cornered alfalfa hopper, and bean leaf beetle. Annually, at least one of these pests causes significant problems and is responsible for limiting soybean production by reducing yield. To reduce the economic effects of these pests, growers and consultants scout weekly for insects and apply control measures – insecticides – when these pests reach damaging levels, termed action or economic thresholds.

In the past, the southern green stink bug, the brown stink bug and the green stink bug have had the greatest economic effect on Louisiana soybean producers. In addition to yield losses, stink bug feeding has caused losses in soybean quality and oil content, reduced germination and caused delayed maturity, a syndrome by which the plants retain green leaves, green stems or green pods when they reach maturity. This results in delayed harvest and decreased seed quality.

In 2000, Louisiana had a new stink bug pest arrive – the redbanded stink bug. Before 2000, the redbanded stink bug had never been an economic threat to U.S. agriculture and had been found only in Florida, Georgia and South Carolina. Since its arrival in 2000, the redbanded stink bug has rapidly spread across the state, reaching all soybean growing areas by 2006. It is now the dominant stink bug species in Louisiana soybeans.

What makes the redbanded stink bug such an important soybean pest is its damage potential. Preliminary data from caged experiments indicate the redbanded stink bug is an aggressive soybean feeder, damaging 41 percent of seeds and reducing seed weight by 33 percent within 72 hours. Current action thresholds for stink bugs in most states are nine per 25 sweeps. Because the redbanded stink bug appears to colonize soybean quickly and to reproduce faster than other stink bug species, the current threshold for the redbanded stink bug in Louisiana is six insects per 25 sweeps. In experiments conducted in Louisiana in 2009, controlling the redbanded stink bug at the old threshold of nine per 25 sweeps resulted in 50 percent of the seed being damaged.

Historically, stink bugs have been controlled effectively through insecticides. Unfortunately, the redbanded stink bug is less susceptible to products available for stink bug control on soybeans, and consequently, insecticide applications have significantly increased. LSU AgCenter-recommended guidelines for soybean production include a minimum of three insecticide applications. Prelimi-nary research indicates that even when the insecticide acephate was applied for redbanded stink bug control starting at the full-pod stage, four to six applications were needed to significantly reduce yield loss. Because the primary line of defense against this pest is multiple in-secticide applications and a minimum of three applications are being applied, resistance to insecticides is inevitable.

In short, Louisiana soybean growers need new management tactics to combat changing stink bug populations without destabilizing the system further with insecticide use. An effective integrated pest management program will be necessary to reduce stink bug effects while increasing productivity and reducing reliance on insecticides. To achieve this, alternative strategies need to be used to reduce the number of insecticide applications.

Field colonization behavior of many stink bugs is known to be concentrated. However, for the redbanded stink bug, specific field distributions and densities are not known. If this pest is concentrated within field margins, site-specific targeting of insecticide applications is possible. This could reduce pesticide applications, saving growers money while conserving natural enemies.

The first step in the experiment was to identify redbanded stink bug population densities and distributions on grower farms. LSU AgCenter research scientists, working in conjunction with their extension counterparts and Louisiana soybean producers, mapped stink bug population densities using handheld GPS devices. Then, conducting spatial analysis with GPS, researchers determined that the redbanded stink bug is concentrated along field edges (Figure 1). The distance from a treeline significantly influenced stink bug field colonization. Field edges that are 50 feet or less from a tree line are 23 to 79 times more likely to have significant stink bug pressure. Thus, fields closest to tree lots, fence rows or conservation areas have greater chances of stink bug infestations.

The next step in reducing insecticide applications for the redbanded stink bug was to use an attractant to concentrate stink bugs and thus slow field colonization and further concentrate stink bugs for site-specific targeting. Researchers at Virginia Tech found that spinosad acted as an attractant for the brown stink bug. LSU AgCenter scientists conducted laboratory experiments to determine if spinosad attracted the southern green stink bug and the redbanded stink bug. Redbanded stink bug males were attracted to spinosad, and red-banded females and southern green males and females were not.

In the quest to reduce insecticide applications, LSU AgCenter scientists next turned to site-specific insecticide applications. Because stink bugs are concentrated along field edges, spraying there could effectively control them and reduce both the amount of product used and the time required for application, reducing grower costs. One-half acre, small-plot field experiments were designed to test this hypothesis. Treatments included plots that never were sprayed (untreated check), plots that were completely sprayed (100 percent of acreage treated) and plots that had only the four rows on either side sprayed (25 percent of acreage treated). Insecticide was applied three times from pod filling to mature pods (Figure 2).

The results showed that perimeter insecticide applications compared with the untreated check delayed the redbanded stink bug plot colonization by two weeks. Treating only the perimeter also kept stink bugs below the action threshold when only one-quarter of the acreage was treated.
 
Clearly, understanding stink bug movement into fields and spraying only those areas where stink bug populations are highest can reduce insecticide applications and farmer costs. Further research will determine the effects of field applications of spinosad in combination with site-specific insecticide applications to field edges. If it works, this could reduce the number of insecticide applications for this pest even more, ensuring continued efficacy of products for stink bug control while reducing grower costs and negative environmental effects.