Mechanical hedge-pruning strategies influence the abundance of a specialist aphid parasitoid in pecan orchards

Perennial fruit and nut tree systems often require tree canopy restructuring in order to increase productivity and/or keep orchards amenable to management (e.g., pesticide application and harvest). For example, in mature pecan orchards [Carya illinoinensis (Wangenh.) K. Koch], where trees are tall and canopies overlap, mechanical hedge-pruning can increase sunlight penetration and pesticide spray coverage. However, canopy modification can alter microclimatic (e.g., temperature and relative humidity) and habitat parameters (e.g., shelter), which may influence beneficial arthropod populations and overall biological control efficiency.  Thus, this study aimed to investigate the seasonal and within tree abundance of a specialist aphid parasitoid, Aphelinus perpallidus (Gahan), in commercial pecan orchards in response to: 1) hedge-pruning (hedged vs. non-hedged trees) and 2) time of hedge-pruning (summer vs. winter). Sampling occurred during two pecan phenological stages in 2020 (year of hedging) and 2021: post-pollination (June) and kernel filling (August). Yellow sticky cards were deployed in the field and left for a week at different tree heights (upper and lower canopy) for estimating parasitoid activity. Results indicated a higher abundance of parasitoids in hedged-pruned trees during the year of hedging. Interestingly, time of hedging did not affect parasitoid activity. Additionally, the upper canopies of the trees had a consistently higher abundance of parasitoids regardless of hedging, time of hedging, and pecan phenological stage. Overall, our results suggest that hedge-pruning strategies may be a potential modulator of parasitoid activity in the short term; however, no residual effects of hedging can be detected in the following season.