Egg laying by a butterfly on a fragmented host plant: a multi-level approach

Egg placement by herbivorous insects is an important step in their interaction with their host plants, and is the result of processes operating at different spatial and temporal scales. Although several studies have examined egg-placement patterns at different scales, this has rarely been achieved simultaneously using a multi-scale hierarchical approach. We studied egg placement in a rare European butterfly, Iolana iolas, whose larvae specifically feed on seeds of plants of the genus Colutea, using a hierarchical approach and Generalised Linear Mixed Modelling. The study was carried out in 2002 and 2003 in a ca 60 km area in southern Madrid province, Spain, where the host plant, Colutea hispanica , has a highly fragmented distribution. We monitored in detail 132 plants in 24 patches and estimated the abundance of butterflies over the whole reproductive period of C. hispanica . We measured phenological, morphological and landscape variables potentially affecting egg-placement at three hierarchical levels: fruit, plant and host plant patch. Using egg presence /absence on mature fruits as the response variable, we found that eggs were more likely to be laid on fruits aged 1 /2 weeks at the middle of the flowering period (fruit level), on large plants with a small number of shoots at the base (plant level), and in well connected host plant patches (patch level). Our results suggest that egg-placement is a process determined by factors operating at different levels: fruit, plant and host plant patch. Because egg-placement studies are often made with spatially correlated data, neglecting their intrinsic hierarchical nature could lead to equivocal conclusions.