Phenology-based recommendations for control of the litchi fruit borer ( Conopomorpha sinensis Bradley, Lepidoptera)

Abstract The litchi fruit borer ( Conopomorpha sinensis Bradley) is a major pest threatening litchi ( Litchi chinensis Sonn.) production by causing substantial fruit infestation and drop. Although pesticides are widely used, optimal application timing is debated due to imprecise descriptions of fruitlet stages and limited knowledge of pest behaviour in orchards. To address this, we aligned fruitlet development with litchi fruit borer behaviour to identify ideal pest-control timing that balances control intensity while maintaining productivity. Oviposition increased sharply during mid-development in both pesticide-treated and pesticide-free trees. Fruit abscission and fruitlet infestation rates were similar during this period, indicating that physiological factors largely determine early abscission despite borer attacks. Subsequently, pesticide-free trees exhibited higher infestation. During late development, 75–100% of shed fruitlets were damaged by the borer, suggesting the pest is the dominant factor in late fruit drop. Further analysis showed that fruit weight is the principal factor correlated with oviposition, while meteorological factors modulate the occurrence of damage. Using a fruit length of approximately 20 mm as a phenology-based indicator optimizes the timing of pesticide application. Furthermore, because insecticides primarily reduce post-oviposition survival rather than deterring oviposition itself, targeting this specific developmental stage is crucial for effectively preventing late-stage fruit loss. Significance of the Study What is already known on this subject? Litchi fruit borer ( Conopomorpha sinensis Bradley) is a major pest that causes substantial pre-harvest fruit drop and yield loss in litchi production. Currently, growers primarily rely on calendar-based pesticide applications, which often fail to align with the fruit's physiological susceptibility, leading to ineffective control, unnecessary chemical use and an increased risk of pesticide resistance. What are the new findings? Our high-frequency field monitoring combined with robust modelling identified a precise phenology-based indicator for pest control. We found that the borer predominantly causes late-stage fruit drop, whereas early-stage drop is mainly physiological. A fruitlet length of approximately 20 mm provides a practical early-warning reference for optimizing chemical interventions. Furthermore, applied insecticides were shown to primarily reduce post-oviposition survival (acting as ovicides or targeting young larvae) rather than deterring adult oviposition. What are the expected impacts on horticulture? These findings provide orchard managers with a simple, actionable visual metric (20 mm fruitlet length) to optimize the timing of pesticide applications. This approach effectively eliminates wasted early-season sprays, promotes targeted chemical selection based on ovicidal activity and supports the development of more sustainable integrated pest management strategies for litchi and potentially other tropical fruit tree crops.