Use of life table statistics and degree day values to predict the colonisation success of Hydrellia lagarosiphon Deeming (Diptera: Ephydridae), a leaf mining fly of Lagarosiphon major (Ridley) Moss (Hydrocharitaceae), in Ireland and the rest of Europe

Abstract

Hydrellia lagarosiphon is a leaf mining fly of the submerged aquatic plant Lagarosiphon major and native to South Africa. With many favorable attributes this fly has the potential to be a valuable biological control agent of L. major, which has become a problematic weed in many parts of the world. Reproductive and developmental biology of H. lagarosiphon was determined at four constant temperatures (10, 13.5, 16.5, 20°C) to evaluate the rate of increase and predicted colonisation success in areas where L. major occurs and areas where its continued spread is probable. Development rates increased with decreasing temperatures and were greatest at 10°C taking 157.9days. Linear regression of developmental rate data for temperatures 10–20°C indicated that 517 degree days were required above a minimum of 7.5°C to complete development. Between two and eight generations per year were estimated across the climatic regions of Europe using the degree day model. The fitted quadratic model for the net reproductive rate (Ro) indicated that Ro falls below 1.0 at 9.9°C, suggesting a decline in population growth when fly populations are subjected to prolonged periods of temperatures below 10°C. The values of Ro for selected sites range from 0 to 13, with all but a few sites in northern Europe being suitable for the establishment of H. lagarosiphon. A minimum of two generations were required each year to sustain population growth and most biogeographical regions in Europe appeared suitable for the establishment of permanent populations of H. lagarosiphon. The implications for the release strategy of the fly are discussed.