Promoting Cotesia rubecula Marshall, 1885 (Hymenoptera: Braconidae) against the cabbage pest Pieris rapae Linnaeus, 1758 (Lepidoptera: Pieridae) through flowering plants

Conservation biological control is an approach to control pests on an environment-based manner that can contribute to a reduction in pesticide use. Providing food sources and protection for natural enemies and aesthetically enriching production landscapes in form of flower strips, promotes biodiversity and ecosystem conservation. FiBL has developed a tailored flower strip mixture for cabbage cultivation that contains the flower species Fagopyrum esculentum Moench (Polygonaceae), Vicia sativa L. (Fabaceae) and Centaurea cyanus L. (Asteraceae). It was shown to have positive effects on various antagonists of cabbage pests. In this thesis we investigated the potential of the flower strip mixture to control the cabbage pest Pieris rapae (Linnaeus 1758) (Lepidoptera: Pieridae), by promoting its antagonist Cotesia rubecula (Marshall 1885) (Hymenoptera: Braconidae).
To conduct these investigations, a successful rearing of both C. rubecula and
P. rapae had to be established. Because a complete knowledge transfer from publications is not always possible, experimental conditions were evaluated prior to testing the suitability of the flower species to attract C. rubecula and their potential to enhance both insect species in longevity and fecundity trials.
Besides exhibiting exploitable nectar, flowers should preferably be olfactorily attractive, as highly attractive flowers are easily located, reducing the time spent searching for food and subsequently increasing the per capita host searching efficiency. With a Y-tube olfactometer we found that C. cyanus and to a lesser extent V. sativa successfully attract C. rubecula. Also F. esculentum attracts C. rubecula, but only after a rewarding feeding experience. Even though not every flower offering accessible nectar is also innately attractive, it can still be suitable for conservation biological control purposes as feeding experience can change this attraction. Moreover, the application of mixtures containing attractive and rewarding flowers could help increase the success of such programs. These results support the application of the flowers in the field.
Many times flowers have been deployed for conservation biological control purposes. But rarely has it been checked whether these flowers were selective plants, enhancing beneficial insects but not the pests. The three flowering plants were suggested as selective plants for conservation biological control purposes against the cabbage moth Mamestra brassicae (Linnaeus, 1758) (Lepidoptera: Noctuidae). Here, we tested their effects on the fitness of P. rapae and its antagonist C. rubecula. We performed survival and fecundity assays in the laboratory for each insect. The longevity of the wasps was enhanced by all flower treatments and their fecundity by F. esculentum and C. cyanus as a function of longevity. But also the butterflies’ longevity and fecundity were enhanced by F. esculentum and C. cyanus. Selective plants for conservation biological control, which are suitable for a certain pest-beneficial insect complex, are not necessarily suitable for another insect complex present in the same crop. In this case the flowers should not be supplemented to the fields as a conservation biological control measure against P. rapae.
To reach desired pest control effects, flowers need to be carefully selected to promote the beneficial insect of interest, but not the pest. Although laboratory results may support or oppose the application of flowers in the field, only field trials can reveal the actual effects. In this thesis, field trials were conducted over two consecutive years to evaluate the potential of the FiBL developed flower strip mixture to control P. rapae. Because it is known that parasitism rates decrease with increasing distance from flower strips, cabbage plants were additionally intercropped with C. cyanus. The aim was to draw parasitoids into the field and retain them there, consequently increasing parasitism rates. The supplemented flowers did not lower cabbage yields nor increased P. rapae pest densities in insecticide-free plots. This indicates, that insecticide applications could possibly be spared. In fact, we found a positive correlation of parasitism rates and wild P. rapae pest densities in control fields and a negative correlation in flower supplemented fields. The selected flowers seem to be suitable for conservation biological control purposes only when pest densities are low, as they seemingly ignore hosts for nectar when pest densities are high.