This project will address the emerging concern of farmers on the Eastern Shore of Maryland that stink bug species use cover crops as overwintering habitats. Fall-planted cover crops have many benefits to soil health and the environment, and their use in Maryland is likely to increase in the future. However, we hypothesize that specific cover crop mixes, timing of cover crop termination relative to soybean planting, and the relative composition of soybean fields and woodlots in the farming landscape will affect stink bug populations and their potential risk to soybean production. To optimally implement a practical best management approach to this stink bug problem, we first need to address how these cultural practices and landscape factors affect stink bug populations and their movement from cover crops to soybean fields.

Several native species of phytophagous stink bugs, including the green stink bug, Acrosternum hilare (Say), brown stink bug, Euschistus servus (Say), and southern green stink bug, Nezara viridula (Linnaeus), are known economic pests of soybean. Although the brown and green stink bugs have been reported as far north as Quebec, they are more often injurious in the southern states. However, climate change resulting in warmer winters has allowed these stink bugs to overwinter successfully in more northern locations in the US. Brown and green stink bugs are commonly found in most Maryland soybean fields in late August through to harvest. Furthermore, the brown marmorated stink bug (Halyomorpha halys) is an invasive species from China that is now present in soybeans throughout Maryland, causing delayed growth at the field edges, especially next to woodlots. The economic impact of this species has been generally higher in western and central Maryland, where there are more woodlots around fields to serve as the preferred overwintering habitat and early-season food source for the 1st generation.

Stink bugs use their piercing, sucking mouthparts to feed on plant juices from the foliage and pods. Their feeding injury on soybean pods causes discolored, shriveled beans and reduces the yield and quality of the beans. In addition to the mechanical injury to the seed, they can transmit disease organisms. Feeding on younger seed pods results in a significant yield reduction. The degree of damage depends to some extent on the developmental stage of the seed when fed on by the stink bug. However, little is known about stink bug feeding injury during the early soybean vegetative stages. For instance, stink bugs infesting seedling corn can cause stunting, deformed tillers, and often plant death.

Stink bugs overwinter as adults in protected areas such as fence rows, grassy field borders, under stones, or tree bark. They become active during the first warm spring days, typically in April. They will mate, and females usually start depositing eggs in June. These eggs are laid in clusters of 20-30, primarily on leaves and stems but also on pods. Nymphs hatch from these eggs and pass through five instars before becoming adults. Approximately 5 weeks elapse between hatching and adult emergence. Adults live a long time, with most species having one generation per year in Maryland. Adult stink bugs generally reach their highest population levels in September, when they can become an economic problem for soybeans. We believe that cover crops may provide a suitable overwintering habitat for stink bugs, which then move into soybean fields after cover crop termination and thus can potentially cause similar injury to seedling soybeans.

This project addresses the emerging concern that phytophagous stink bug populations are increasing due to higher overwinter survival and are utilizing cover crops for shelter and food. We will determine the species and population densities of stink bugs that overwinter in cover crops and whether they favor certain mixtures of cover crops.

With the help of extension educators and crop advisors and in cooperation with growers, we will select at least 25 fields planted in different types of cover crops. Each field will be planted with soybeans. Fields will be equally located in the lower and upper regions of the Eastern Shore, where there have been more reports of stink bug problems. The type of cover crops and the anticipated termination timing relative to soybean planting will vary among fields and the surrounding landscape area relevant to the proportion of woodlot area.

Starting in early spring, each field will be geo-referenced using Google Maps to identify the density of woodlots and their distances to each field. Sampling of each field will begin around 45-30 days before the expected termination time of the cover crop. We anticipate that stinkbugs will have terminated their overwintering diapause behavior and are actively feeding and reproducing in the cover crop. Two biweekly visits will be made to each cover crop to assess the stink bug species present, their population densities, and whether they are actively reproducing, as evidenced by the presence of nymphs. Densities of adults and nymphs (small, medium, large) will be determined by taking 10 sweeps with a standard sweep net at five areas across each field. As an alternative sampling method, beat sampling to dislodge stink bugs from a unit area of the vegetation may be needed to estimate density, depending on the height and vegetative structure of the cover crop. Once the cover crop is terminated and the established soybean crop has reached the seedling stage, further sampling of each field by direct visual counts will be conducted at least twice to assess the presence and density of stink bugs during the early vegetative stages of soybean growth. In addition, visual observations will determine if soybean seedlings have any growth disorders attributed to stink bug feeding.

Data on stink bug density recorded in the cover crop and then later in the soybean crop in each field will be analyzed using correlation analysis to determine if stink bug populations in the soybean fields are related to densities in the cover crop. We will also use multivariate analysis to identify the influence of the cover crop mix and distance to woodlots on observed patterns in stink bug populations.

Updated July 28, 2024:
Between mid-April and mid-May, 26 cover crop fields were sampled on the Eastern Shore of Maryland. The fields sampled included 75% planted with winter wheat and 25% with a mixture of cover crops, including rapeseed, rye radish mixture, rye winter pea mixture, and a winter wheat clover mixture. Due to the low number of sampled fields planted with a mixture of cover crops, no conclusions about stink bugs’ preference for overwintering cover crops can be drawn at this time. Therefore, only wheat cover crops will be evaluated. Densities of adults and nymphs were determined by taking ten sweeps with a standard sweep net at five to ten areas across each field. Fourteen fields were sampled twice during the cover crop stage. The other fields were sampled once due to weather constraints before terminating the cover crop. Six fields were sampled again in June when soybean plants were 6-8 inches high. Sampling of stink bugs in soybeans is ongoing.

91 adult stinkbugs were found while sampling cover crops. The brown stink bug, Euschistus servus (Say), and the brown marmorated stink bug (Halyomorpha halys) made up 98% of the species found. The green stink bug, Acrosternum hilare (Say), and the red-shouldered stink bug (Thyanta curator) made up the other 2%. On average, less than 2 stink bugs were found per field. Most of the stinkbugs found in cover crops were found after May 1. During sweeps, over 200 ladybird beetles, adults, and nymphs were counted. Tarnished plant bugs (Lygus lineolaris) and aphids are other pests found on cover crops.

Early data on this project was shared with growers at the 2024 Maryland Community Classic.