As trees become old or ‘overmature’, many become more susceptible to insects and diseases.
One of the major hypotheses generated during the early work of spruce budworm during the 1950’s was that outbreaks rose as trees matured and became a better food source for feeding spruce budworm larvae (aka the ‘Silvicultural’ hypothesis). Such increases in tree nutritional quality were speculated to allow populations to rise more quickly, thereby outstripping the ability of predators to exert control. The characteristic 35 year cycles of spruce budworm outbreak were thought to reflect the approximate 30-35 years required for maturation in balsam fir and spruce.
Whether host nutritional quality has any major influence on spruce budworm cycles has been debated vigorously during the last 50 years and there has been a general shift in emphasis towards the influence of predators. However, there may be additional insight to be gleaned from also studying how well budworm does on its host trees, particularly as populations rise.
The latest stage of experiments in the Hornes Gultch area setup the first week of June focused on trying to get some baseline information on tree quality for spruce budworm larvae. Essentially, what we do in such experiments is place lab-reared larvae on branches and enclose them in a mesh-cloth cage that looks very much like a pillow case. This cage allows us to exclude natural enemies and keep larvae on the branch. We can generally gauge how nutritious the trees are for spruce budworm based on how well they survive and how large they grow over the course of the season. If you’re travelling in the area, keep your eyes open and you may see some of our cages up in the trees!
Next week we are returning to the area to start sampling the local community of insects, including spruce budworm. These collections should allow us to get a better look at what predators are attacking spruce budworm and the extent to which they also attack other insects in the area. This information is essential as it is possible that applying pesticide to low density spruce budworm populations (i.e., as part of the Early Intervention strategy) could have unwanted effects on the predators that help to naturally keep spruce budworm at low density.