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How to Identify a Caterpillar on a Green Bean Plant

While some farmers frown on the idea of caterpillars in the garden, you can attract butterflies to your garden as well as helping to raise the next generation of butterflies by planting vegetables that caterpillars love, such as green beans.

Over 180,000 species of caterpillars exist worldwide. Because these caterpillars come in many shapes and colors, identifying them is a matter of studying small details of their appearance and matching it to a known species.

Narrow your caterpillar species to caterpillars that are common to your geographic area. For example, the monarch caterpillars are not typically found in Northern Canada, while the gypsy moth caterpillar, an introduced species, is primarily found in the northeast portions of the United States and Canada.

Examine your caterpillar for distinguishing marks. These marks may include hair, color patterns, shape, spines, horns or size. They are frequently listed in caterpillar descriptions in books, magazines or websites where no picture is located. For example, lunar moth caterpillars are red with a black "saddle" and black dots during its first larval stage, while the woolly bear caterpillar is black with an orange band around its midsection.

Measure your caterpillar’s length. Different caterpillars average different lengths. This, combined with the caterpillar's other features, helps to identify it. A fully grown gypsy moth caterpillar may be 2 inches in length, while a cecropia moth caterpillar may be 4 inches long.

Note your caterpillar's girth to length ratio. Some caterpillars have a larger ratio than others. The cecropia moth caterpillar is nearly 3/4 inch in diameter and 4 inches long, while the hickory horned devil caterpillar is 3/4 inch wide, but may grow up to 6 inches long.

Look through caterpillar guide books such as "Caterpillars of Eastern North America: A Guide to Identification and Natural History." Books such as this include photos to see if your caterpillar matches any of the included photos.

Check caterpillar information websites including Bill Oehlke’s Caterpillar Identification Guide or Butterflies and Moths of North America. (See Resources for URLs of these and other websites.) Use the features you collected in steps 2 through 4 to find your caterpillar at one of these websites.

Take a photo of your caterpillar using a camera with a macro lens. Some experts, such as the Butterflies and Moths of North America website, offer a service in which they will identify your caterpillar if they have a good photo to look at. According to the Butterflies and Moths of North America website, in order to obtain an identification, contact your state or regional coordinator. Names of such coordinators are listed on the website.

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