Molecular assessment of soil invertebrate herbivory

Trophic interactions are a key factor coining the dynamics present in floral and faunal communities. Assessing trophic interactions under natural conditions, however, is not a simple task. This is especially true for soil food webs where the opaque habitat hampers the identification of feeding relationships. Here, we propose DNA-based gut content analysis to assess how herbivorous soil insects trophically exploit below-ground plant biodiversity. Besides, stable isotope analysis and a detailed analysis of plant assemblage structure will be employed to determine the trophic links between below-ground herbivores and plants in systems harbouring different levels of plant biodiversity. We will choose Agriotes wireworms, the larvae of click beetles, as below ground herbivores, as they are generalist plant feeders, abundant in soils and a serious pest worldwide. There are four objectives to this three-year research program: (i) development of a PCR-based assay for identification of plant DNA from wireworms` gut contents, (ii) identification of plant species and groups which are fed on by different wireworm species under field conditions including determining the strength of these trophic links, (iii) tracking of seasonal patterns in wireworms` dietary choices, and (iv) assessment of the impact of plant biodiversity upon wireworm dietary choices, both, under natural conditions and within mesocosm experiments. The mesocosm experiments will help to experimentally assess the effect of plant biodiversity upon wireworms` food choices, supplementing the field-derived data and conclusions. In employing comprehensive analyses, including classical statistics as well as ordination techniques, we aim to be the first in quantifying trophic links between soil herbivores and specific plant species as well as to assess how the level of plant biodiversity effects soil herbivores` dietary choices in grasslands and arable fields. Moreover, by molecularly identifying trophic links between plants and animals, this project will take molecular analysis of trophic interactions one step further as this approach has, up to now, mainly been used for investigating animal-animal trophic interactions.

Although a considerable part of plant biomass is located below-ground, little is known on the dietary choice of soil-dwelling herbivores. While it is well known how plant identity and diversity affect the feeding preferences of above-ground herbivores, this information is lacking for soil ecosystems. This is because examining plant- herbivore feeding interactions in the soil is difficult. Unravelling these below-ground linkages and the mechanisms behind them, however, is a prerequisite for a better understanding of the relationships between herbivore and plant communities. Here we employed stable isotope analysis and developed a molecular approach to examine the dietary choice of Agriotes wireworms, the larvae of click beetles which live in the soil. These wireworms are abundant in arable land and feed on different plant species, including crops where they can inflict considerable damage. We showed that DNA of plants consumed by the wireworms can be detected and identified using diagnostic polymerase chain reaction (PCR) for at least three days after feeding. A range of PCR assays for examining wireworm feeding on plant taxa commonly occurring in Central European agricultural land were developed. Using these assays we found that Agriotes larvae, although being generalist plant feeders, do exhibit preferences for specific plant species/groups. Whether a plant was preferred or avoided was independent of plant diversity, but the extent of avoidance or preference changed with increasing plant diversity. Diverse plant communities also offer a highly attractive food source in arable fields, which can be exploited to divert the wireworms away from feeding on crops. In a field experiment we demonstrated that the wireworm-inflicted damage in maize can be reduced by 50% when a mixture of other plants is intercropped. We also found that the wireworms are not moving between crops, putatively because there is no shortage of food. Aside from examining the feeding of these soil insects on plant roots, we developed a molecular tool to identify the different species of Agriotes larvae in Central Europe. This tool was then used to examine the occurrence of Agriotes wireworms in Austrian agricultural land with regard to their association with climatic and soil parameters. This project delivered both novel methodology to examine herbivore-plant feeding interactions within the soil and new insights into the feeding ecology of soil-dwelling insect herbivores. Moreover, the current findings provide important implications for developing new means to control wireworms which threaten crops worldwide.