DOI: 10.5884/12515
title: Effect of movement between Salix arctica individuals on Gynaephora groenlandica caterpillar growth rates at Alexandra Fiord, Nunavut, Canada, 2013

description: In June 2013, we collected 60 caterpillars (of instars 4-7), that had been active for approximately seven days, and used net bags (made from polyester, with holes of about 1.3mm2) to contain each caterpillar on a single branch of 60 haphazardly chosen Arctic willow individuals. For half of these caterpillars (n = 30) we moved them every four days to 30 new (no prior bagging) haphazardly chosen willows that were within ten metres of the previous willows, where a new willow individual was defined as any non-contiguous willow patch. This movement was done a total of three times till the time approximately when caterpillars in the field build their hibernacula (around July 6th). The remaining 30 Arctic caterpillars were kept on their original willow individuals but were moved to a new branch if all leaves and flowers on the branch had been eaten. Caterpillars of differing instars and sizes were haphazardly assigned to treatments. Willows were chosen in an area of approximately 100 m by 50 m with the condition that they had to have enough leaves for sufficient feeding and subsequent leaf trait measurements. We measured the mass of each caterpillar at the beginning and end of each four-day foraging bout. We counted the number of leaf fascicles on each netted branch at the beginning and end of each four-day foraging bout, where a leaf fascicle was an independent collection of leaves on a branch and where partially eaten leaf fascicles were quantified as a proportion remaining. We measured plant traits including specific leaf area (mm2 mg-1), leaf dry matter content (mg g-1), and carbon and nitrogen content (%) at the end of the experiment for all willows used in the moved and stationary treatments. Leaves were collected between July 4th and July 11th of 2013, and replicates for both treatments were haphazardly distributed between days to account for any phenological changes between collection days. During June 2013, we placed one sticky trap (7.6 cm by 12.7 cm, 10 cm aboveground and within 5 cm of the net bag) each next to 9 randomly chosen moved caterpillars and 9 randomly chosen stationary caterpillars. From June 17th to July 11th these sticky traps were replaced every two to three days and were moved to follow the caterpillars. Exorista thula were in such low abundances that we concentrated on Hyposoter diechmanni, identifying them to genus.
description: Purpose: Movement between host plants during the growing season is a common behaviour among insect herbivores, although the mechanisms promoting these movements are poorly understood for many systems. Two possible reasons why herbivores relocate include compensating for host-plant quantity and/or quality changes, and the avoidance of natural enemies. Gynaephora groenlandica caterpillars move several metres each day, feeding on its patchily distributed host plant, Salix arctica, and has only two natural enemies, the parasitoids Exorista thula and Hyposoter diechmanni. To evaluate the role of host-plant resource quantity, quality, and enemy avoidance on Gynaephora groenlandica caterpillar movement, we experimentally manipulated caterpillar movement between willows in the field, by moving caterpillars between Salix arctica individuals and restricting other caterpillar individuals each to a single Salix arctica throughout the active period of Gynaephora groenlandica.
description: Summary: Not Applicable