Abundance and mortality of a specialist leafminer in response to experimental shading and fertilization of American holly

Phytomyza ilicicola (Diptera: Agromyzidae), a univoltine specialist leafminer, typically reaches higher population densities on cultivated host planted in sunny, urban sites than it does on native trees in the forest understory. I tested the hypothesis that environmental factors, more specifically differences in light intensity and soil fertility, are responsible for this observed pattern, either by affecting leaf morphology or nutritional quality, or indirectly through early leaf abscission. Clonal trees were planted at a common site and experimentally shaded and fertilized for 14 months before being exposed to oviposition by adult flies. Differences in leaf thickness, nitrogen and water status, and in the abundance, developmental rate, survival, pupal weights and abscission-related mortality of leafminers were compared among trees. Leaves from experimentally shaded trees were larger and thinner, with only two palisade cell layers as compared to three to four layers in leaves from unshaded trees. Crystals, probably of calcium oxalate, are abundant in the abaxial cell layer, and it was hypothesized that these might set mechanical constraints on larvae feeding within shaded leaves. However, there was little or no difference in leafminer abundance, developmental rate, survival to pupation, area of finished mines, or pupal weight between shaded and unshaded trees. Leafminers compensated for feeding within thinner, shaded leaves by consuming portions of the abaxial and adaxial palisade layers and leaving a thinner roof on the mine. Furthermore, there was no difference in thickness between leaves with successful or aborted mines in either shade or sun. Leaves from fertilized trees contained 37% higher total nitrogen than those from unfertilized control trees. Fertilization did not significantly affect leafminer abundance, developmental rate, mine area or pupal weight, but survival to pupation was slightly lower on fertilized trees. Abscission-related mortality of P. ilicicola was much higher in full sun than for shaded trees, opposite of what would be predicted if early abscission per se were a primary determinant of relative abundance between habitats. These results indicate that structural constraints on leafminer larvae within shaded leaves, differences in leaf nitrogen related to soil fertility, and/or differential mortality resulting from early leaf abscission are probably not the proximate causes of density variation of this leafminer between woods and urban habitats.