Effect of tillage intensity on the inter-row plant community and microlepidoptera predation rates in arid vineyards in Argentina

Microlepidoptera pose a significant threat as vineyard pests. Yet, information about inter-row management effects on natural enemies remains limited. We evaluated the impact of an inter-row tillage gradient on plant cover and the predation rate of microlepidopteran larvae and pupae as a model to estimate potential predation rates of Lobesia botrana in vineyards. The percentage of bare soil measured at the surface level in the inter-rows was the only variable that correlated with the tillage intensity gradient. Predation rates of larvae and pupae showed significant differences between tillage intensities, but did not follow the expected pattern based on the tillage gradient. Bare soil measured at the surface level had a strong influence on predation rates when the tillage gradient was simplified into high and low intensity categories. Ants were the only observed predators, with 11 species identified. They were able to respond quickly in locating and preying on the larvae and pupae offered.

Methods
Vegetation cover

To evaluate the effect of the tillage gradient on the plant community of inter-rows and on predation rates, four vineyards with different tillage regimes were selected: continuous tillage throughout the season, annual tillage at the end of winter, tillage every three years, and no tillage. The survey of the plant community present in the vineyard inter-rows was conducted using the Point Quadrat method during the summer of 2019 and fall 2020. Ten sampling sites, spaced by at least 50 m from each other, were established for each of the four tillage levels analyzed. At each sampling site, a 15-meter transect was set up, along which all plant species intercepting the line every 30 cm were recorded. Additionally, we quantified the condition of bare soil at the surface level, the total vegetation cover, and the cover of individual species. Based on traits sensitive to tillage practices we grouped species and calculated cover categories such as grasses, dicots, native, and exotic species. Here bare soil was recorded as the surface area free of vegetation and litter, whether or not there was overhead plant cover, while vegetation cover, on the other hand, refers to the presence of vegetation either at or above the surface level. The ratios of native to exotic species and dicots to grasses were calculated, with native species and dicots as the numerator. Due to the presence of many zeros in the dataset, a constant was added to each value before calculating the natural logarithm and then determining the respective proportions.

Sentinel traps

Trials using living prey were conducted using the sentinel prey technique, for this purpose, we used larvae and pupae of Plodia interpunctella (Hübner). In December 2022, 10 pairs of sentinel traps were installed in each vineyard to assess pupal predation. They were placed on the ground, at the base of the grapevine trunks. The traps consisted of a plastic Petri dish filled with sieved soil and 10 partially buried pupae. Each pair of traps was located in two consecutive grapevine rows; one of them was covered with wire mesh to exclude large predators, while the other remained exposed. This approach allowed us to differentiate between predation by arthropods and by birds or small mammals. The traps were checked every 24 hours, and the number of pupae predated was recorded as the difference between the total number of pupae offered and those remaining in the trap after 24 hours. Then pupae were replaced for another observation period of 24 hours.

In April 2023, we assessed larval predation using ten sentinel traps, each consisting of a cork affixed to a grapevine branch with five last instar larvae pinned to the final segment.  The predation rate was recorded as the difference between the larvae offered and the larvae that remained at the trap after 24 hours. After 24 hours of exposure, the remaining larvae were confirmed to be alive and active, and were recorded as non-predated. Subsequently, the trap was replaced with a new one containing five fresh larvae. In this trial, the exclusion treatment for birds and mammals was not applied.