How do butterflies choose where to lay their eggs.

Each kind of butterfly will only lay eggs on the correct plants. Sometimes they have a range of plants on which they lay their eggs but some species will only use a single kind of plant and then only where it is growing in the right place. This may be where there is plenty of sun for the caterpillars to get warm ( The Small Tortoiseshell does this) or it may be where there is less likelihood of the caterpillars being eaten ( The purple Emperor does this.)

The female butterfly will taste the plant with her feet and carefully choose the correct place to lay her eggs. This is determined by the presences of certain important taste chemicals in the plants. The choice of foodplant is very important. Scientists who studied a checkerspot butterfly releated to the Bay Checkerspot and the Quino Checkerspot Once found that the butterflies could tell plants from the site from which they came from apart from the same plants growing elsewhere. Even though the butterflies also occured in the other place.

The El Segundo Blue choses only one kind of plant and the caterpillars are poisoned by the foodplant used by the butterflies just a mile or two away. The butterflies are said to be just a race of the same species and there is only a small difference in the markings that is visible to people.

Sometimes butterflies can make serious mistakes. The Richmond Birdwing buttefly in Australia normally lays its eggs on a native Birthwort vine. People have planted an ornamental kind of Birthwort vine from South America. It tastes fine to the female butterflies but when the caterpillars try to eat it they are poisoned and die. The same things happens with the West Virginia White butterfly. It normally lays its eggs on toothwort plants, but a British plant called Garlic Mustard has been accindentally introduced into the woods where it lives and the females happily lay their eggs on it. The caterpillars are also poisoned and die. In some places the West Virginia White has disappeared and experts think this poisoning is the cause.