Below ground efficiency of a parasitic wasp for Drosophila suzukii biocontrol in different soil types

The parasitoid wasp Trichopria drosophilae is promising as a biocontrol agent for controlling the ubiquitous pest Drosophila suzukii (Matsumura). Crucial for the successful implementation of any biocontrol agent is a high parasitisation rate by the parasitoid. Most studies investigating the parasitisation rate of D. suzukii pupae have focused on parasitisation in the fruit or in a petri dish. However, the predominant pupation site of D. suzukii in the field is the soil. Unfortunately, little is known on how well parasitoid wasps can detect and parasitise pupae of D. suzukii buried in the soil. Therefore, the researchers conducted soil parasitisation experiments of T. drosophilae on D. suzukii pupae using two pupation depths in three different soil types (loamy sand, loam, and clay). In all three soil types, the researchers found generally low D. suzukii pupae parasitisation rate by T. drosophilae, independent of the pupation depth. The pupation behaviour of D. suzukii and the parasitisation behaviour of T. drosophilae are discussed in detail. For pest control in most soil types, our results mean that the number of D. suzukii larvae pupating in the soil should be reduced, e.g., by adding a layer of sandy soil or covering the soil with plastic mulch. This might increase the probability of success when using T. drosophilae as a biocontrol agent.

The cosmopolitan fruit pest Drosophila suzukii, also called spotted wing drosophila, is still a major challenge for farmers worldwide. Especially in years with favourable conditions for the pest, the risk of total yield losses can be high. Therefore, functional integrated pest management methods are essential to control the pest. Consequently, extensive knowledge is required for a broad range of different possible control methods. One promising candidate is larval and pupal parasitoids. Parasitoids, mostly wasps, lay their eggs in or on a host, for example in the larvae or pupae of D. suzukii. The larvae of the parasitoid then feed on the host and eventually kill it.

One advantage of parasitoids as pest control is that they can be mass-reared and released at a certain date. Thus, population growth can be controlled if release is early in the growing season9. Especially for D. suzukii, early control is necessary because its population can be high in the surrounding habitats and the insects thus mass invade the fruits when they are nearly ripe. In field and laboratory studies, naturally occurring parasitoids, such as the pupal parasitoids Pachycrepoideus vindemiae and, especially, Trichopria drosophilae, have proved promising results in controlling D. suzukii. The pupal parasitoid T. drosophilae can parasitise the pupae of D. suzukii during the entire pupal development time. A crucial ability of the parasitoid during parasitisation is locating the host pupae. The location of the pupae of D. suzukii can be directly in the fruit, but especially in the field, the large majority of the larvae pupate in the soil underneath the fruit plant. This location means that the parasitoid needs to be able to locate the pupae in or near the fruit and in the soil matrix.

Guillén, et al. found that P. vindemiae could only locate pupae of the Mexican fruit fly Anastrepha ludens when the pupae were on the soil surface. They could not locate them when they were in the soil. In contrast, another study found that P. vindemiae and T. drosophilae can parasite D. suzukii pupae in the soil. In their study, Wang, et al. studied the parasitisation rate of D. suzukii pupae in fruits and the soil. However, it is unclear whether the pupae were actually buried in the soil or lay accessible on the soil surface. Furthermore, neither Guillén, et al. nor Wang, et al. studied the parasitisation rate in different soil types. Therefore, it is still unclear in what soil type and to which soil depth T. drosophilae is capable of finding and parasitising the pupae of D. suzukii.

To answer this question, the researchers investigated the pupation behaviour of D. suzukii and the parasitisation rate of T. drosophilae in three different standardised soil types (loamy sand, loam, and clay) with the same soil moisture and at two soil depths (0–6 mm and 7–12 mm). Furthermore, the hatching rate of D. suzukii was assessed under these soil conditions.

Read more at https://www.nature.com/articles/s41598-022-12993-w