Why do predators attack parasitized prey? Insights from a probabilistic model and a literature survey.

Predators and parasitoids often encounter parasitized prey or hosts during foraging. While the outcomes of such encounters have been extensively studied for insect parasitoids, the consequences of a predator encountering parasitized prey have received less attention. One extreme example involves the potter wasp Delta dimidiatipenne that frequently provision their nest with parasitized caterpillars, despite the low suitability of this prey for consumption by their offspring. This raises two main questions: (1) why do female potter wasps continue collecting parasitized caterpillars? and (2) is this an exceptional example, or do predatory insects often suffer from fitness costs due to encounters with parasitized prey? We addressed the first question using a probabilistic mathematical model predicting the value of discrimination between parasitized and unparasitized prey for the potter wasp, and the second question by surveying the literature for examples in which the parasitism status of prey affected prey susceptibility, suitability, or prey choice by a predator. The model demonstrates that only under certain conditions is discrimination against parasitized prey beneficial in terms of the potter wasp's lifetime reproductive success. The literature survey suggests that the occurrence of encounters and consumption of parasitized prey is common, but the overall consequences of such interactions have rarely been quantified. We conclude that the profitability and ability of a predator to discriminate against parasitized prey under natural conditions may be limited and call for additional studies quantifying the outcome of such interactions.

Optimizing the releasing strategy used for the biological control of the sugarcane borer Diatraea saccharalis by Trichogramma galloi with computer modeling and simulation.

One of the challenges in augmentative biological control programs is the definition of releasing strategy for natural enemies, especially when macro-organisms are involved. Important information about the density of insects to be released and frequency of releases usually requires a great number of experiments, which implies time and space that are not always readily available. In order to provide science-based responses for these questions, computational models offer an in silico option to simulate different biocontrol agent releasing scenarios. This allows decision-makers to focus their efforts to more feasible options. The major insect pest in sugarcane crops is the sugarcane borer Diatraea saccharalis, which can be managed using the egg parasitoid Trichogramma galloi. The current strategy consists in releasing 50,000 insects per hectare for each release, in three weekly releases. Here, we present a simulation model to check whether this releasing strategy is optimal against the sugarcane borer. A sensitive analysis revealed that the population of the pest is more affected by the number of releases rather than by the density of parasitoids released. Only the number of releases demonstrated an ability to drive the population curve of the pest towards a negative growth. For example, releasing a total of 600,000 insects per hectare in three releases led to a lower pest control efficacy that releasing only 250,000 insects per hectare in five releases. A higher number of releases covers a wider range of time, increasing the likelihood of releasing parasitoids at the correct time given that the egg stage is short. Based on these results, it is suggested that, if modifications to the releasing strategy are desired, increasing the number of releases from 3 to 5 at weekly intervals is most likely preferable.