For 18 years we have monitored two populations of the gall-forming aphid Hormaphis hamamelidis and its host plant Hamamelis virginiana (witch hazel). For most of those years we recorded the time of bud burst to determine if this was affected by climactic factors. Because the galls develop as the leaves expand we also investigated the possibility that the date of bud burst would affect the aphid population.
Results/Conclusions
There is phenological variation within and among the two sites. One population lags about 7 days behind the other. Within each population some plants consistently develop earlier than others. There is also variation from year to year depending on temperature (growing degree days). This was particularly striking during the two El Niño years of 1998 and 2010 when bud burst occurred about 2-3 weeks earlier than usual. At the site where bud burst normally occurs earlier gall density (average number per leaf) was low in both El Niño years. However, this did not hold at the second site where bud burst occurs later. A more marked pattern was apparent when we measured gall size over a 6 year period. In 1998 the galls were about half the normal height. There was a strong correlation between gall size and stem mother fecundity so that fewer aphids were produced that year.