Choosing a Host Plant

Comma butterflies. Note the white “comma” on the underside of the wing (left).

The Comma Butterfly, Polygonia c-album, is one of the most polyphagous butterfly species. Butterflies frequently lay eggs, grow and develop on only a small number of plant species. Butterflies typically find host plants through visual or olfactory cues. For a species like the Comma that can survive on a multitude of plant with widely disparate odors and visual images, how does the female choose? One idea promoted by A.D. Hopkins was that the adult females might remember their larval host plant or have a chemical imprint that would lead them to oviposit on the same plant species of their larval development. This idea has several permutations, but all attempts to demonstrate a link have proved fruitless.

In one of the recent tests, a group of scientists* reared larvae on 3 host plant species, then measured the oviposition preference of the female. In two species choice tests, the females laid more eggs on Urtica dioica that the other host plants regardless of the plant the female consumed as a larva. Larvae do grow the fastest on Urtica dioica so the females lay more eggs on the better host plant and fewer on the less good host. Memory of the larval host did not occur in this test.

This result is conistent with host plant choice theory. The simplest model is that an insect makes a series of yes and no decision based on the information available and the “internal state” of the insect. In this model, the Comma Butterfly visits a plant and decides at that time whether or not to deposit an egg. If the perceived plant quality is too low or the oviposition cues too weak, the insect will leave and eventually visit another plant where the process is repeated. However, as time passes without finding a suitable plant, the insect becomes less choosy and will accept lower quality plants or plants with weaker oviposition cues. Humans have similar experience. If we have recently eaten and are satisfied, only a tempting delicacy can induce us to eat. If we are starving, many more foods are appealing. As an optimal strategy, a comma butterfly would lay eggs on Urtica dioica as best for the offspring. However, if the butterfly has not found Urtica dioica recently, that may mean it is not available. Thus, the best strategy involves laying eggs on lesser plants as they are found but continuing to search for the most desirable plants.

Alternative models to the series of yes or no decisions requires that the insect have a memory of the previous plant it visited, a means of comparing the quality of the two and the ability to locate the plants it remembers. This is a far more complicated mechanism and there is little evidence that insects use this method.

*NIKLAS JANZ, LINA SÖDERLIND and SÖREN NYLIN. 2009. No effect of larval experience on adult host preferences in Polygonia c-album: on the persistence of Hopkins’ host selection principle. Ecological Entomology. 34:50–57
DOI: 10.1111/j.1365-2311.2008.01041.x