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Understanding Insect Metamorphosis

I occasionally write about insect pests and what to do about them. However, I also know that not everyone is an entomologist, and it might be good to take a step backward this week to ensure a better understanding of insects and how they change and grow. By understanding insects better, we can also manage them more efficiently and economically. I’ll expand on that aspect toward the end of the article.

First, what is an insect? An insect has six legs and three body segments: a head, a thorax, and an abdomen. You may know that spiders, mites, and ticks have eight legs, and therefore are not insects. Sometimes we lump all these creepy crawlies under the term “arthropods,” which includes invertebrate (no spine) animals with a segmented body and jointed appendages. Examples of arthropods include insects, arachnids (spiders), and crustaceans.

Insects and their relatives are not all bad actors. Most are crucial to a healthy environment and ecosystem. One example is the importance of pollinators. It is estimated that only about 3% of insects are pests.

Metamorphosis refers to the insect’s life cycle, and how it changes and grows into an adult. Insects may have complete or incomplete (simple) metamorphosis.

The life cycle of insects with complete metamorphosis includes egg, larva, pupa, and adult. Eggs hatch to give rise to a worm-like larva. The larva feeds and molts (sheds exoskeleton, or outer skin, to become larger) until it pupates. The pupa is a transitional stage to the adult where its true legs, wings, and antennae form. In the case of a butterfly, the pupa is inside the chrysalis (other insects may spin cocoons or pupate in the soil) until the adult emerges. Adults mate and lay eggs.

An insect with incomplete metamorphosis has 3 life stages: egg, nymphs, and adult (no pupa). Eggs hatch to a small nymph, which resembles (not perfectly) what the adult will look like. Nymphs go through growth stages (instars), where they molt into bigger and bigger nymphs. The final molting reveals a fully formed adult. Adults mate and lay eggs.

It is important to understand life stages because pest controls usually target the damaging or most vulnerable stage of insect development. Timing is also a factor. We know roughly when the smallest larvae (plural of larva) are present, and the smaller they are, the more vulnerable they are. Many types of larvae feed on leaves, and control methods may be targeted to the earliest stages of damage. If they are already adults, it may be useless to apply an insecticide if all the adult does is mate and lay eggs. Some adults, however, also do damage. An example of that is the Japanese beetle.

So, if you have a pest problem, research the pest, its biology, and when the damaging life stage usually occurs. Doing a little homework may help you be more efficient with your pest control strategies, and it may also help you not throw your money away on ineffective tactics.

Keep in mind that pest control should be an integrated pest management (IPM), or holistic, approach that may include cultural controls, resistant varieties, best management practices, mechanical controls (e.g., hand-picking insects off plants), biological controls (predator insects attacking insect pests) and other strategies. You may have seen biological controls in action and not realized it. Have you ever seen a big green hornworm chewing on tomato plants with white cocoons on its back? That hornworm has been attacked by a parasitic wasp – he’s a goner, so no control is needed. (See banner photo above).

Part of IPM may include prevention or exclusion. This tactic is important for insects that enter homes. Rather than fighting armies of insects inside the house, it’s better to do what you can to seal cracks, fix screens, and possibly spray outdoor products to prevent home invasions. Chemical controls are often the last resort in pest control.

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