Does rye trigger mite outbreaks? Testing the green bridge hypothesis on commercial watermelon fields

Spider mites are an important pest on a number of crops, especially watermelon. Cover crops, such as rye, are extensively used in commercial watermelon fields to protect seedlings against wind damage but rye is also reported to act as an alternative host plant for mites and thus may act as a bridge onto the crop. However, there are little data substantiating these claims. We tested the hypothesis that watermelon growing interspersed with rye will have higher mite densities than rye-free watermelon fields.
Commercial watermelon fields with three different levels of rye cover crop (0%, 25%, 50%) across Southwest Indiana were scouted weekly, 10 fields in 2021 and 15 in 2022. Infested leaves with mites were recorded per m². Other watermelon pests, including aphids, striped cucumber beetle, spotted cucumber beetle, squash bug, and lygus bug, along with natural enemies were also recorded.
On average, 0.22 mite-infested leaves were recorded per m² of watermelon; however, 0.15 more leaves with mites were recorded when rye was present (50%) compared to fields where no rye was used (0%). In contrast, 0.42 more aphid-infested leaves were recorded in 0% rye fields compared to fields with 50% rye. Other pests did not show significant differences. These results indicate a weak link between rye and pests, although watermelons were treated with miticides/insecticides throughout the study in all fields, underpinning the low threshold for current mite/insect management. Understanding how pests are affected by rye is key to improve farming practices, which directly benefit watermelon farmers.