Clutch size and larval performance of pollen beetles on different host plants

Discrepancies found between preference and performance for herbivorous insects are often explained using a variety of interpretations. In this study the relationship between clutch size and larval performance is considered in the context of preference. Sequential choice trials were used to determine the preference ranking of the pollen beetle (Meligethes aeneus) for five different plant species. Performance estimated as larval weights after one week was measured in a separate experiment on the same plant species. M. aeneus showed a strong preference for three Brassica species (Brassica napus, B. juncea and B. nigra) when compared to Sinapis alba and Eruca sativa. Overall larval performance on B. napus was highest followed by E. sativa, B. nigra, B. juncea and S. alba (in descending order). Thus discrepancies between preference and performance were demonstrated. Some of these differences could be explained when clutch size and larval competition were considered. Clutch size per bud differed among plant species, from the highest on E. saliva to the lowest on B. nigra. Competition among larvae in a bud was demonstrated on B. nigra, B. juncea and S. alba. On these species larvae from buds containing four or more larvae weighed less than larvae from buds with a single larva. Reduction in weight suffered by larvae from multiple clutches was correlated to the expected number of larvae per bud for each plant species. Larvae on plant species receiving small clutch sizes and sustaining high larval mortality suffered most from competition. On plant species with larger clutch sizes and little larval mortality no competition among larvae was demonstrated. Clutch size was correlated to mean bud size on the different plant species. It is suggested that the pollen beetle modifies clutch size according to expectation of larval competition on different host plant species.