Why most butterfly caterpillars rely on single or very few type of host plants for food?
Like some other insects, butterfly caterpillars eat only very few type of host plants. The Wanderer caterpillars only feeds on the milkweed plants and the Birdwing caterpillars only feeds on the Birdwing Vine. This single host plant phenomenon is very common in butterflies and it is not an advantage to the butterflies. We observed the following situations that contributed to this phenomenon. The following discussions can also explain why parasites are usually specialize on a single host type.
Plants Defend, Insects adapt
We, mammals have our immunity system to defend us from parasites. Plants evolved their way against being eaten by insects and other herbivores. They change their physical structures and chemical contents to avoid being eaten. Some plants may have hairs or wax on their leaves. Some plants may make themselves have a bad taste or even toxic. However, the herbivores, like caterpillars, must rely on the plants for food and they have to evolve to adapt to the plants changes. Then some insect herbivores become specialized on few type of plants. This also explains why introduced weeds can be so successful for they do not have natural herbivores in the new environment.
Adults choose, Caterpillar eat
Grasshoppers are herbivores but most of them feed on many kinds of plants. The major different between grasshoppers and caterpillars are that grasshoppers can choose their own food. If they do not like the plants they are eating, they can move to another type of plants. The caterpillars do not have the right to choose. They can only eat or die. The selection is totally up to the adult butterfly. For example, the Birdwing butterflies in Queensland, the female will mistakenly lay egg on an introduced weed, Dutchman's Pipe, which the butterflies may think it is Birdwing Vine. The Dutchman's Pipe is poisonous to the Birdwing caterpillars and this is one of the reason that drove the Birdwing to become an endangered species. Another butterfly, the Blue Triangle, are not so selective, they lay eggs on many different kinds of plants. Some of the caterpillars cannot successfully develop. However, most of them can and this not-so-selective behavior help the Blue Triangle be one of the popular butterflies in Brisbane.
Competition between species
Another reason that makes the butterflies specialize on few host plants is the competition between species. If two herbivores rely on the same host plant. Eventually one species will drive the other out of the competition. The losing species will either switch to another host plant or become extinct. Both winning and losing species will become more relied on fewer host plants.
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