by Carl Strang
The phenomenon of the periodical cicadas raises a lot of questions. Some of these questions are ecological. Millions of the insects emerge at once every 13 or 17 years, depending on the area, and their sheer biomass suggests that they must influence the ecology of the forest as a whole. Their nymphs grow by drawing sap from tree roots. The adults damage twigs when they cut them to insert their eggs. When the masses of cicadas die, their decomposition releases nutrients into the soil. Thus, from a tree’s standpoint some of the effects are potentially beneficial, others harmful. This was the background for the paper I am reviewing today (Speer, James H., Keith Clay, Graham Bishop and Michelle Creech. 2010. The effect of periodical cicadas on growth of five tree species in Midwestern deciduous forests. American Midland Naturalist 164:173-186).
This research group used dendrochronology methods in southern Indiana, measuring growth ring widths in several tree species. They first factored out climatic influences, ruling out the effects of relatively good or bad growing seasons. Then they tested the effect of feeding by cicada nymphs, oviposition damage and the nutrient pulse following emergence.
Cicada nymphs start out very small, and grow to be relatively large, so if their numbers are impacting tree growth, the trees should grow progressively slowly through the insects’ 17-year cycle. Speer and associates could find no nymphal feeding effect in any of the 5 tree species they examined.
Sassafras, pin oak and black oak showed some growth loss from oviposition (egg-laying) damage, but these plants are less preferred for oviposition than sugar maple and white ash. The latter appeared to be hit harder, but growth rings showed no effect.
There was a nutrient pulse effect for black oak, pin oak and sugar maple, but it happened 5 years after emergence. The authors suggest that trees may need time to absorb the nutrients and produce extra wood after the adult cicada mortality event. Alternatively, this effect may result from a pulse of mortality in cicada nymphs of that age.