Spatial and temporal variations in insect-infested acorn fall in a Quercus liaotungensis forest in North China

Three plots with different aspects and slope characteristics were surveyed in 1999 and 2000 to clarify the spatial and temporal variations in insect-infested acorn fall patterns in a Quercus liaotungensis Koidz. forest in the Dongling Mountain region, North China. There was a significant difference in the proportion of infested acorns in the three plots in a low crop year, but not in a mast year. Within oakwoods on the southeast-facing slope, the insect infestation rate on the upper slope was significantly higher than on the lower slope, but not in the northwest-facing plot. Infestation rate in the low crop year in all three plots was significantly higher than infestation in the mast year. Most of the early fallen acorns had a higher proportion of insect infestation, and in the mast year it was much more obvious than in the low crop year. The proportion of infested acorns in seed bank along the topographic gradient showed a similar decreasing trend with acorn fall time, but the proportion on the upper slope was the highest and the proportion on the lower slope was the lowest. Larval emergence from acorns commenced just after acorns fell from the trees and lasted for 40-50 days, with peak emergence occurring from 24 to 32 days after acorn rain began. We conclude that insect-infested acorn distribution in Q. liaotungensis shows spatial and temporal heterogeneity, and an early drop of infested acorns can be a short-term defensive strategy against insect infestation.