Movement between crops and weeds: temporal refuges for aphidophagous insects in Central Chile

Abstract

Crop edges have significant effects on populations of natural enemies,acting as source or sink habitats during the growing season. Previousobservations have shown that coccinellid species are associated withthistle (Sylibum marianum (L.) Gaertn), a common exotic weed in thecentral valley of Chile. To determine whether thistles growing at cropedges act as a putative refuge for natural enemies, the seasonalrelative abundance of aphidophagous coccinellids was estimated at 0, 10and 25 m from the edges of three alfalfa fields. Mark-recapture studieswere carried out using the trace element rubidium (Rb) to determinewhether coccinellids moved between the edges and the alfalfa. The mostcommon aphidophagous coccinellid species were Hippodamia convergens(63%), H. variegata (11%), Rhyzobius lophantae (4%) and Adaliaangulifera (4%). In mid-November, the abundance of coccinellids at theedge of (0 m from the edge) the alfalfa plot increased compared to thatat the center of the field (25 m from the edge), coinciding with areduction in the population of aphids at all sampling points. Of thecoccinellids captured at the thistle edge, 68% were marked withrubidium, suggesting movement of coccinellids from the alfalfa plot tothe thistle growing at its edges. After the thistles were removed,coccinellids returned to the crop, as shown by the presence of markedcoccinellids within the alfalfa fields at all three sampling distances.The results of this study suggest that thistles can act as a refuge forcoccinellids when aphids are not available in the alfalfa fields.