Susceptibility of Potted Sweetgum Seedlings to Insect Herbivore Damage as Influenced by Fertilization
We report the influence of fertilization on the susceptibility of sweetgum (Liquidambar styraciflua L.) seedlings to naturally occurring insect herbivores. Thirteen-week-old potted sweetgum were placed in a pasture near the margin of a hardwood forest containing scattered sweetgum trees. Groups of 14 seedlings were treated weekly with either no (0), low (1.5 g/L), medium (3.0 g/L), or high (6.0 g/L) concentrations of 20-20-20 soluble fertilizer. Seedling performance was assessed by measuring height and leaf number and rating seedling condition throughout the 11-week experiment. Types and numbers of insect herbivores were recorded at 2- to 3-week intervals for each seedling throughout the experiment and cumulative damage was estimated for each seedling at the end of the experiment. Herbivore damage was generally light, except for weeks 2 through 5, when seven seedlings were defoliated and killed. Six of these seedlings were in the medium and high fertilizer treatments. Defoliations were due to late instar larvae of the yellowstriped armyworm (Spodoptera ornithogalli Gn.). Other insect herbivore species were detected, but their impacts were minor. Eleven seedlings received no detectable herbivore damage during the experiment; 10 of these received zero and low fertilizer treatments. Results suggest that the susceptibility of potted sweetgum seedlings to insect herbivore damage was associated with fertilizer concentration. This effect was not related to differences in seedling size because major defoliation events occurred before significant differences in seedling size among fertilizer treatments were detected. Yellowstriped armyworm has potential as a pest of sweetgum seedlings in intensively managed plantings.