Do the Leaf Miners Avoid One Another?

By Carl Strang


Yesterday I described several species of tiny moths whose caterpillars mine the leaves of black/sugar maples in DuPage County, Illinois. Today I want to review some of the results of my inquiries into this system (by “system” I mean a group of interacting species in a particular environment) since 1983.


In the mid-1980’s these species all were abundant, and collectively they were removing a significant portion (in 1984 more than 20%) of the maples’ photosynthetic tissue in understory plants. We might expect that in such a situation it might benefit them to avoid one another. There are mathematical methods, called statistical tests, that allow us to say whether the co-occurrence of these herbivores on leaves is taking place randomly (no interaction evident), whether they may be avoiding one another (co-ocurrence on leaves is less than one would find in a random situation), or whether they are attracted to one another or at least to the same leaves (co-occurrence greater than in the random case).




I made these comparisons in 1984, 1985, and 1986. If there are any interactions between these leaf miner species, we would expect to see the same result showing that to be the case in each year. With four mine types, there are 6 ways any two of them can form a pair for comparison in a given year. That makes 18 statistical tests over the three years. Out of these 18, only four showed a statistically significant departure from random co-occurrence. However, only two of the four involved the same two species, Caloptilia and presumed Stigmella in 1985 and 1986. Those same two species gave a random result, however, in 1984, the year when leaf tissue loss was highest and presumably there would be the greatest benefit from avoidance. On the whole I had to conclude that these species are interacting weakly if at all.


This seems puzzling, but I have a guess as to why it’s the case. When I returned to this system in 1996 I found the populations of leaf miners very low compared to the 1980’s. Only in the case of Caloptilia at Meacham Grove did more than 10% of the leaves have one or more of these insects. Populations have remained at or below these levels ever since. It’s beginning to look like the high numbers of the mid-1980’s were unusual, so that there has not been selective pressure to benefit individuals with avoidance behavior.

%d bloggers like this: