RISK refers to the chance that a stand will be severely defoliated. Risk of gypsy moth defoliation is primarily based on tree species composition. Stands dominated by oaks, aspen, birch, basswood, willow, or other species preferred by gypsy moth will be at high risk of defoliation. On the other hand, risk will be lower in stands dominated by tree species that are less preferred by gypsy moth, such as maples, ash, and most conifers. Risk of defoliation may also be higher where many dead branches, bark flaps, and stem wounds are present, because these areas provide good hiding places for gypsy moth caterpillars and pupae.
Hazard depends primarily on the vigor of trees growing in the stand before defoliation.
Stand hazard is increased by factors that reduce stand vigor such as overmature trees, suppressed or diseased trees, compacted soil, or severe competition. Hazard also increases when drought, late spring frosts, flooding or other stressful environmental conditions occur. Trees growing off-site are especially vulnerable. In Michigan, tree mortality has been observed primarily in oak-dominated stands where gypsy moth defoliation coincided with drought or late spring frosts. In high hazard stands nearing economic or pathological rotation age, you may choose to log harvestable trees or stands before gypsy moth defoliation occurs.