Effect of climate change on population dynamics and damage potential of insect pests on oilseed rape

Abstract

Biotic stress such as pests can cause high yield losses in agricultural crop systems. It has been estimated that global warming will increase these rates. Thus, the impact of global warming on the distribution and abundance of insect pests caught the attention of investigators in the last years. However, little is known about the influence of climate change on the interaction between host plants and their insect pests. Current studies make only coarse assumptions about plant responses to their associated pests as consequence of climate change. The rape stem weevil (Ceutorhynchus napi GYLL.) is the first pest attacking winter oilseed rape (Brassica napus L.) in spring. Warming in spring induced by global warming may affect the pest, the plant and/or the interaction between them. The aim of this thesis was to close existing knowledge gaps about the influence of changing climatic conditions on the interaction between rape stem weevil (RSW) and its host winter oilseed rape (WOSR). Within the research framework “KLIFF – Climate Impact and Adaption Research in Lower Saxony”, several field, semi-field and laboratory trials were applied. The first aim of this study was to assess the influence of WOSR phenology in springtime on damage caused by RSW. Therefore, a field trial was conducted with the WOSR varieties ‘Elektra’ and ‘Favorite’ in an alternating split-block design over two consecutive years. Moreover, differences in the damage potential by RSW regarding the initial oviposition were analyzed. The results of this experiment provide new insights into factors influencing the interaction of RSW and its host plant. The variety ‘Elektra’ with a more rapid development in springtime was earlier infested by RSW adults during the growing season. We showed further that “stem injury index” and “damage index” are not suitable for a reliable assessment of damage potential of RSW although these parameters are used in recent studies. The emigration of RSW third instar larvae was strongly influenced by climatic conditions but not by phenology of the varieties and by the hatching time of larvae. It is almost unknown, how climate change will influence the interaction between WOSR and RSW. Therefore, a semi-field experiment with three different soil temperature regimes induced by soil warming was set up in a randomized block design. Different plant parameters and the damage potential of RSW through changes in oviposition were recorded. To study the influence of temperature on the date of RSW infestation, plots were artificially infested with RSW cocoons. The emergence of adult emergence was studied with photoeclectors. The results of this experiment suggest that global warming may lead to higher yield losses in order of an earlier beetle emergence. On plant level, the temperature and the variety were predicators for the height of the main shoot which was positively influenced by these factors. Height as well as soil temperature influenced the infestation level of the RSW larvae: taller plants and warmer plots led to an increased abundance of RSW larvae. In greater extent, studies were conducted in the laboratory to explain more in detail the biology of RSW´s. In order to analyze the effect of temperature on the oviposition, survival rate, longevity and egg development of RSW, several trials were carried out. Three average daily temperatures were assessed at constant and varying regimes. Varying or constant temperature regimes with the same mean daily temperature differed with regard to insect developmental time. Varying temperature regimes resulted in an increased oviposition by RSW adults compared to constant temperatures. Consequently, temperature regimes with the same mean daily temperature should be treated with care. Another point is the mortality rate of eggs. More eggs died at a constant temperature regime compared to a varying temperature regime. We suggest that this fact must be taken into account when trials with constant temperature regimes are performed. In the last experiment, the volatile emission of oilseed rape plants upon stress through herbivory by RSW adults or drought was compared to the emission of non-stressed plants. In addition, plant parameters of well-irrigated plants and plants exposed to drought stress were recorded. The results showed that volatile emission significantly differed between control and stressed plants. Stressed plants emitted a higher number of volatiles. Nevertheless, the volatile profiles of stress through herbivory or drought did not result in the same quality and quantity. Regardless to the volatiles, the drought stress had an obviously impact on plant height and fresh weight. The weight of stressed plants was greatly reduced compared to the weight of well-irrigated plants. In summary, this thesis presents new methodology in RSW research. In addition, knowledge gaps of detailed biological question in the interaction of RSW´s and WOSR were closed. The result of this thesis further suggests that a higher temperature may lead to an increased yield loss in WOSR caused by RSW.