Why are freezing temperatures good for butterflies?

We dread winter when we think of butterflies. It’s cold. Butterflies aren’t flying. We may go months without butterflies (depending on where we live). So why should we be glad for the freezes that kill back butterfly host and nectar plants? Why should we grin when we see our frozen plants?

Ice-covered passionvine

When most of these plants are frozen, they die back to the stems or to the ground. The leaves are killed and fall off the plants. THAT is the reason to be happy. The leaves are killed and fall off the plants. If that doesn’t make sense, read on!

Frozen pipevine leaves

Out of 100 eggs laid by a butterfly, only about 2 live to become adults. Nature kills the other 98. This is the way that nature is able to keep the species alive. If there were too many butterflies, they’d eat all the host plants and starve, eventually causing the species to become extinct. If you’d like to see the numbers, check out this page.

Various diseases are one of the many methods that nature uses to kill caterpillars and chrysalises. Besides diseases, nature also supplies parasites, predators, parasitoids, and a few other methods to kill eggs, caterpillars, and chrysalises.

Freezing temperatures kill most disease pathogens and most OE spores. Viruses survive freezing temperatures, though. When leaves are killed by a freeze, they eventually fall off the plant and begin decaying. As the decaying leaf washes into the soil, pathogens also wash deep into the soil, ensuring that caterpillars cannot become infected and become diseased.

One of the worst diseases to affect caterpillars is Nuclear Polyhedrosis Virus – NPV. An infected caterpillar, once it is close to dying, will crawl to the top of its plant. There, it waits. It slowly turns to liquid as it dies. The dead caterpillar begins dripping its liquid contents. This liquid falls onto and splashes all over the plant/vine. As it rains or we water our plants, the pathogens are washed off the contaminated leaves and splashed over more leaves. Once another caterpillar eats the virus pathogens, it contracts the disease and dies.

NPV killed this Gulf Fritillary caterpillar. Its insides turned to pure liquid, containing about a billion virus pathogens. The liquid drips out of the dead caterpillar, spreading the virus pathogens over the host plant.

Winter is a time of cleansing. As our gardens grow back in the spring, most of the disease pathogens have been killed or are being washed into the ground. The fresh leaves that caterpillars eat will be healthy and clean. Soon our gardens will again be full of caterpillars and adult butterflies.

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