1. Herbivores inducing host-plant trait changes can indirectly affect the performance, distribution, abundance and behaviour of other herbivores, even when they are temporally or spatially separated. However, it is unclear whether this occurs at scales relevant for applied purposes such as pest control in agricultural crops.
2. We studied the indirect effects of a minor pest, the mirid bug Helopeltis sulawesi, on the major pest of cacao Theobroma cacao in Southeast Asia, the pod-boring moth Conopomorpha cramerella. For 2 years, we surveyed herbivore damage and yield in 10 focal trees in each of the 43 cacao plantations and analysed patterns of co-occurrence of the two herbivores. In a two-choice experiment, we tested whether gravid females of C. cramerella searching for oviposition sites discriminate against pods damaged by H. sulawesi.
3. The proportion of pods affected by both pest species was significantly lower than expected. This pattern could not be ascribed to differential responses to environmental or management variables, but was because of avoidance of H. sulawesi damage by ovipositing C. cramerella females as shown in a two-choice experiment. The reduction in co-occurrence of damage by the two herbivores was found at tree and at plot scale and held across three harvest seasons.
4. The differential yield impacts by the two herbivores and the avoidance by C. cramerella of pods damaged by H. sulawesi lead to a yield optimum at a H. sulawesi incidence of 51%.
5. Synthesis and application. Plant-mediated indirect interactions between minor and major pest insects can be important drivers of yield loss at agriculturally relevant spatial and temporal scales. In cacao, the mirid bug H. sulawesi, a minor pest, generates conspicuous damage which often triggers pest control with insecticides. This practice may be counterproductive, because decreasing H. sulawesi damage benefits the main pest, the cacao pod borer C. cramerella resulting in a marketable yield optimum at intermediate densities of the minor pest. Pest control recommendations should take into account the relative effect of control measures on interacting herbivores to avoid replacing one pest problem with another, potentially more serious one, during the course of a fruiting season.