Western Tent Caterpillars Prefer the Sunny Side of the Tree, but Why?

The egg masses and tents of the western tent caterpillar, Malacosoma californicum pluviale Dyar, are conspicuously clumped on and among trees of suitable host species. By recording the location of egg masses and tents at four sites, we determined that more egg masses and tents occur on the south facing branches of trees, and on trees that are isolated and exposed to the sun for most of the day. Survival of early instar caterpillars was not influenced by the location of tents on the tree, but at two sites, significantly more tents on the south side of the trees had remains of dead caterpillars. Laboratory assays indicated little difference in food quality in leaves from the sunny (south-facing side) and the shaded (north-facing side) of the trees, although male pupae were significantly heavier when fed leaves from the sunny sides of trees. These leaves were also significantly higher in nitrogen content and lower in water content. Tent caterpillar moths oviposit on twigs in late evening. Therefore, the concentration of eggs on the parts of trees that receive the most sun could be associated with slight light or temperature differentials at this time of day, or could result from some cue arising from the twigs such as the increased release of volatiles following a day of exposure to the sun. We suggest that environmental influences on oviposition behavior may be very important in determining patterns of oviposition and subsequent herbivory, and question the common assumption that the distribution of insects indicates food quality variation.