2. False Signals
The more devious among the parasites can actually mimic plant growth hormones to confuse the plant for its own selfish purposes. This is true in the parasitic moth, Ectoedemia argyropeza, whose larvae parasitize the leaves of big-toothed and quaking aspen (and much less frequently eastern cottonwood). The larvae produce a chemical that mimics cytokinin, a plant growth hormone that stimulates growth, effectively telling the localized part of the leaf where the caterpillar is residing that it’s not fall and the leaf does not need to do anything to prepare for winter. While the rest of the leaf senesces, the area at the base of the leaf hosting the caterpillar has a wedge-shaped green island.
How this fits in with the moth’s life cycle
Virtually all adult moths are female, and the eggs, which are primarily produced parthenogenetically, are laid on the outside of the leaf. After hatching in the late summer/early fall, the larvae tunnel into the leaf through the petiole. Here they nestle in and begin to feed on the plant tissues. The caterpillars over winter as pupae (the cocoon stage) and emerge as adults the following spring. The challenge is that the eggs hatch late in the growing season just as aspen leaves are about to senesce. If they didn’t disrupt leaf senescence they wouldn’t have enough time to eat the green tissues to acquire enough energy to fuel transition into their pupal stage. If instead, they just cut off the entire leaf before senescence begins, while the leaf would have plenty of stored energy to support the caterpillar’s development, the leaf would also remain attached to the branch through winter and the cocoon would spend the entire winter exposed to the elements. Disrupting senescence in part of the leaf provides an optimal balance, and the moth’s success is visible this time of year as the forest floor becomes carpeted with these green and yellow leaves.