Developmentally-based changes in resistance to insect herbivory in the foliage of sweetgum (Liquidambar styraciflua)

Background/Question/Methods

While developmentally based changes in insect resistance have been studied in numerous herbaceous plants, less is known about how woody plant resistance to insects changes with development. We used a common dominant tree of eastern North America, the sweetgum (Liquidambar styraciflua), and a common polyphagous insect herbivore, the gypsy moth (Lymantria dispar), to test the hypothesis that resistance to herbivory, measured as insect survival and weight gain, differs in foliage from different developmental stages of canopy sized trees and saplings.

             Foliage was collected from the mature (reproductive) and juvenile (non-reproductive) zones of eight individual sweetgum trees as well as from twelve nearby saplings at Patuxent National Wildlife Refuge, Maryland, USA, for a total of 28 samples. Samples were chosen from from the sunniest location possible to minimize the effects of light on chemical defenses.  Freeze-dried, ground foliage was used to make a reconstituted diet which consisting of 20% (w/v) wheat germ-based artificial diet, 2.25% foliage, and the remainder water. For each foliage sample, three replicates each consisting of 20 neonate larvae were fed unlimited diet for 45 days while in a growth chamber at 24° C with a 16/8 light/dark cycle.

Results/Conclusions

At the end of the feeding period, larvae were weighed, dried, and reweighed. Survival varied significantly among the three treatments (One-way ANOVA on square root transformed percentages, p< 0.05), with larvae feeding on foliage from saplings having the highest survival rate (High: low: sapling, mean ± 1 SEM of % survival = 61.5 ± 3.9: 67.9 ± 3.4: 71.9 ± 2.9). Furthermore, foliage type affected larval weight gain (One-way ANOVA on log₁₀  transformed fresh masses, p< 0.05). Fresh mass was greatest for larvae feeding on sapling foliage, and lowest for mature zone feeding larvae (High: low: sapling, mean ± 1 SEM of  fresh mass (mg) = 80.3 ± 3.5: 104 ± 7.0: 143 ± 10). Fresh mass to dry mass ratio was uniform among treatments. The use of ground foliage along with artificial diet, optimized for insect growth, eliminated the effects of leaf toughness and nutrients, respectively, on the experimental outcome. Thus it appears that as sweetgum trees develop, they alter the chemical composition of foliage in a manner that makes the foliage less suitable for polyphagous insects.