Combining flowering and banker plants to improve efficiency of parasitoids in the control of cabbage pests

Failures of biological control programs have been associated with factors, such as rearing systems of pest antagonists being of low quality, lack of food for pest antagonists and low efficiency of release methods among others. To contribute to the solution of these problems, techniques for improving the quality of rearing systems and strategies of habitat management were evaluated. In this context, the main goals of this study were, to evaluate different techniques for improving the quality of Diaeretiella rapae and Encarsia tricolor rearing systems (Chapter 2), to study the effect of flowering plants on the fitness and performance of the target parasitoids (Chapter 3), to study whether in the first five days after beginning of the experiment, banker plants could reduce the hot spots of pests infestation significantly and whether flowering plants improve the efficiency of banker plants (Chapter 4). In the second chapter, the approaches, type of host plant (broccoli var. Marathon F1, Brussels sprouts var. Hilds Ideal and cauliflower var. Freemont), architecture of the plant (with and without pruning) as well as cage size (0.21 m3 and 0.02 m3) were evaluated for improving the quality of the target rearing systems. The pruning involved cutting the roots, old and large leaves (>15 cm) in the 4th and 8th week after sowing. Regarding the host plant experiments, the broccoli treatment had the best effect on most measured variables. For D. rapae experiment, the broccoli treatment had 182 mummies per plant more than cauliflower. For E. tricolor
experiment, broccoli treatment had 88 mummies per plant more than cauliflower. Additionally, for both parasitoids, the incidence of powdery mildew was less in broccoli treatment compared to the other treatments. Considering the architecture experiments, pruning treatment had better performance than the control. For D. rapae experiment, pruning treatment had 106 mummies per plant more than the control, plants had no powdery mildew and parasitoids lived 2.57 days longer. For E. tricolor experiment, pruning treatment had 114 mummies per plant more than the control, plants had no powdery mildew and parasitoids
lived 12 days longer. In the cage size experiments, the large cage had better effects than the small cage in most measured variables. For D. rapae experiment, the large cage had 69 mummies per plant more than the control. For E. tricolor experiment, the large cage had 121 mummies per plant more than the control and parasitoids from this treatment lived 7.5 days more than the control. In conclusion, the use of pruned broccoli plants in large cages substantially improved the quality of both rearing systems. In the third chapter, the attractiveness between different food resources (host-plant complex and flowering plant species) was compared. Thereafter, the effect of different food resources on the parasitoid longevity in climate chamber experiments, the impact of flowering plants on the parasitoid fecundity in greenhouse experiments as well as the influence of flowering plants on the
performance of the target parasitoid in the field were measured. The parasitoid performance in the field was measured in terms of number of mummies and pest individuals, as well as the percentages of female/male parasitoids, parasitoid emergence and hyperparasitism by other species. The outcomes of the attractiveness experiments were: i) both adult parasitoids had access to the floral nectar of alyssum and buckwheat, but not to the faba beans; ii) alyssum had higher attractiveness to D. rapae compared to all other flowering plants; iii) the parasitoid D. rapae showed a higher preference for the host plant complex compared to flowering
plants, except in the case of alyssum; iv) alyssum was more attractive to E. tricolor compared to buckwheat and v) the parasitoid E. tricolor showed a similar response for the host plant complex and flowering plants. The climate chamber and greenhouse experiments showed that the fitness of both parasitoid species was substantially increased given that: i) the D. rapae longevity on sources of high quality (honey and nectar) was 4.37 times as long as on sources without sugar (control and water) and 8.91 times for E. tricolor; ii) the D. rapae longevity was 2.07 times on buckwheat treatment as long as on alyssum and 1.53 times for E. tricolor;
iii.) For D. rapae, the treatment with flowering plants (mix of alyssum and buckwheat) had 29.14 mummies more than the control, and iv) the number of mummies produced by E. tricolor did not differ significantly between the flowering plant treatment and the control. Finally, field experiments determined that the number of D. rapae and E. tricolor mummies with the flowering plant treatment were 2.1 and 1.4 times as much as the control, respectively. In conclusion, flowering plants can to attract parasitoids and play an important role in optimising the fitness of parasitoids. In the last chapter, it was first measured whether banker plants can give an opportune control at the beginning of the hot spots of the target pests and second whether the flowering plants can improve the efficiency of banker plants. The results showed that the parasitism rate in banker plant treatment was 55.5% for D. rapae and 39.9% for E. tricolor, respectively. The parasitism rate was estimated as the number of mummies related to the events of oviposition during the five days of the experiment divided by the initial number of hosts. It was also observed that flowering plants did not improve the efficiency of banker plants. Finally, it was concluded that the evaluated techniques in this study improve the fitness and performance of the parasitoid D. rapae in the field, but the efficiency of this techniques is not enough. Therefore, the optimal techniques to produce parasitoids in rearing systems as well as the role of banker and flowering plants are discussed
as strategies to optimise the efficiency of parasitoids in terms of fitness and agronomical efficiency (percentage of mummies) and to contribute to the integrated pest management.