"Telenomus laeviceps" Förster, 1861 (Hymenoptera: Scelionidae), a potential biocontrol agent against the cabbage pest "Mamestra brassicae" (Linnaeus, 1758) (Lepidoptera: Noctuidae)

In agriculture, the widespread use of insecticides with ecotoxicological side effects has become an increasing problem, stressing the importance of alternative solutions. Invertebrate biocontrol agents are becoming increasingly important in the attempt to reduce or even replace the use of these insecticides. Different families of egg parasitoids were taken into consideration and their potential as biocontrol agents evaluated. The greatest emphasis has been placed on the genus Trichogramma. However, of the known 200 species of Trichogramma, only 19 species have been mass reared and used in augmentative biological control programs. These numbers point out the difficulties of developing new biocontrol agents and the importance of accurate feasibility studies to evaluate the potential of the new candidates, increasing their chance to land on the market.
This thesis investigated the biology of the egg parasitoid Telenomus laeviceps Förster, 1861 (Hymenoptera: Scelionidae) to build a stable rearing in order to conduct field efficacy trials, evaluating its performance in the control of the cabbage moth Mamestra brassicae (Linnaeus, 1758) (Lepidoptera: Noctuidae). Laboratory trials were conducted to evaluate the influence on the parasitation performance of T. laeviceps of (I) the number of females parasitizing the same egg clutch, (II) temperature and (III) the egg deprivation time (the time until mated females come into contact with host eggs). This last point had the greatest effect on the parasitation rate and the proportion of female offspring produced, with egg deprived females performing better then newborn females.
An important aspect that determines the performance of this parasitoid, in the laboratory and in the field, is the provision of an exploitable food source. The presence of nectar near the release point in the field was shown to increase the effectiveness and persistence of released biocontrol agents, such as diverse Trichogramma spp.. We conducted laboratory experiments to test the influence of cornflower, Centaurea cyanus L. (Asteraceae); buckwheat, Fagopyrum esculentum Moench (Polygonaceae) and common vetch, Vicia sativa L. (Fabaceae) on fecundity and longevity of T. laeviceps. Furthermore, the olfactory attractiveness of these flowers was evaluated in olfactometer trials. These flowers are the main components of a tailored flower strip implemented in Switzerland in the production of cabbage crops. In the presence of these three nectar providers, T. laeviceps survived significantly longer then when provided with water only. In addition, its fecundity was enhanced by C. cyanus and F. esculentum. These two flowers were further proved to be olfactory attractive for T. laeviceps.
The geographic distribution of a biocontrol agent is extremely important to determine its potential market. In fact, it is easier to obtain permits to release a biocontrol agent if it is native for the country of interest. To this end, we conducted field trials in three European countries to collect egg parasitoids. The sampled egg parasitoids were determined at the genus level to discriminate between Trichogramma spp. and Telenomus spp. (morphological species determination) and at the species level to identify T. laeviceps (molecular species determination). For the molecular species determination, specific qPCR markers were developed during the project.
Together with our commercial partner, a field delivery system was developed to effectively release T. laeviceps in the field. Preliminary field trials were conducted in 2015 to test the release method and to determine the control potential of two densities (120’000 and 240’000 parasitoids/ha) of the biocontrol agent. Results were very promising, with a mean parasitation rate for both densities of 36 % and maximal values of 70 %. However, the production of such a high density was, with the former production system, economically not feasible. Therefore, a density of 65’000 parasitoids/ha was calculated based on the costs of one application of the insecticide spinosad. Efficacy field trials were conducted in 2016 and 2017 to test the plant protection potential of this density compared to standard insecticides applied in organic agriculture. The results showed that the parasitation performance of the released parasitoids was not enough to efficiently control the cabbage moth.
Finally, over the same two years, separate field trials took place to test the potential of the provision of flowering plants in the promotion of T. laeviceps. Conservation biocontrol includes measures applied at the field level, aiming to promote different ecosystem services such as pest control or pollination. Released Trichogramma spp. benefit from conservation biocontrol, showing an increased parasitation performance and persistence in the field. Here, we tested the influence on the parasitation performance of T. laeviceps of flower strip and cornflowers as companion plants (2016 and 2017), flower strip only (2016) and control without provision of flowers (2016 and 2017). In 2016, released T. laeviceps and natural occurring T. laeviceps and Trichogramma spp. took advantage from the provided flowering plants. Further, the total control of cabbage moth eggs, due to egg parasitoids and predators, was 15 % higher in field with flowers than in control fields.