Heat wave effects on an acarine predator-prey relationship

Heat waves can have fatal effects on arthropods such as insects, mites and spiders, since their heat tolerance is often lower than the diurnal maximum temperatures during a heat wave resulting in negative effects on vital traits. Theoretically, organisms can adapt to environmental stress by a genetic (change in the DNA sequence) and plastic modifications (no change in the DNA sequence). Practically, the duration of heat waves is too short to develop effective genetic adaptations. Plastic modifications, however, can rapidly result in favorable adaptations to heat waves, since these can be effective within one generation, but can also be transferred to the next generation. But what happens when a predator and his prey have different plastic adaptation potentials? Does it lead to the decoupling of the finely tuned predator-prey interaction? These questions are investigated by using the predatory mite Phytoseiulus persimilis and its prey, the spider mite Tetranychus urticae as study objects, which have prominent roles in biological control. The following assumptions will be verified: (1) plastic modifications to heat stress are possible for the predator and prey within one generation and can also be transferred to the next generation; (2) the costs of these modifications are higher for the predator than prey; and (3) the consequences of these differential adaptation potentials result in an inefficient control of the spider mites by the predator during heat waves. The costs and benefits of adaptations of the parent and offspring generation on heat waves are evaluated by the investigation of the plastic potential of the following traits of the predator and the prey: (1) predator aggressiveness and prey defense behavior; (2) learning effects on prey defense behavior; (3) the development, reproduction and survival of predator and prey, and, (4) the effects of heat waves on the population dynamics of the predator and prey. Scientific studies on plastic modifications to heat waves are scarce in relation to their consequences on predator-prey relationships. This study can therefore contribute significantly to the controversial debate, whether or not plastic modifications of arthropods are sufficient to adapt to rapidly changing thermal conditions due to climate change. In conclusion, the study can be of significant importance for the biological control of pest species, because the expected results indicate whether sufficient spider mite control is still ensured by the predator P. persimilis under extreme heat stress.

The predatory mite Phytoseiulus persimilis and the spider mite Tetranychus urticae are a prominent predator-prey pair in biological pest control, with the predator P. persimilis rapidly decimating its preferred prey T. urticae under optimal thermal conditions and preventing crop damage. However, in the course of climate change, heat waves are becoming longer, more frequent and more intense, which can have different consequences for predators and prey and subsequently for spider mite control. This was investigated in a multi-year research project. First, the development from egg to adult and the reproductive performance of predator and prey was evaluated when the mites were exposed to mild- or extreme heat waves . Both predatory and spider mites laid smaller eggs under extreme heat waves, but only the spider mites were able to produce more eggs under these conditions. Both species developed more rapidly under extreme heat waves, but the body size of adult female predators was smaller under extreme than under mild heat waves. Adult female spider mites were larger under extreme heat waves. In a further experiment, it was investigated whether these altered body proportions influenced the survival of the prey in the presence of the predator under mild (predator and spider mites approximately the same size) or extreme heat (spider mites larger than the predator). It was clearly shown that more spider mites survived under extreme heat than under mild heat. Whether the disadvantages of lower reproductive performance and reduced body size of predatory mites under extreme heat waves actually have an effect on spider mite control was investigated in a system consisting of bush beans, spider mites and predatory mites. Extreme heat waves led to an increased population growth of spider mites and thus also to a reduced bean weight, while the predatory mites were only able to control the spider mites under mild heat waves. A key aspect that probably led to inefficient spider mite control was the different spatial distribution of mites depending on the intensity of the heat waves. Under mild heat waves, spider mites and predatory mites were evenly distributed in the plant population. During extreme heat waves, the majority of spider mites were found in the upper third of the bush beans, while the predatory mites were found in the lower third. Thus, the spider mites were in an area without predators, which therefore had no access to their prey. These results are consistent with a number of studies that point to potential advantages for agricultural pests over their natural counterparts in times of climate change. To ensure future biological control of spider mites under ongoing climate change, two research-intensive options are available: (1) the search for heat-resistant predatory mite species or (2) other natural antagonists.