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What are those black and yellow caterpillars all over Arizona?

UArizona explains the White-lined sphynx moth caterpillars across the state
A University of Arizona expert says these are the immature stage of the White-Lined Sphinx Moth, and they're feasting on all the weeds growing after a wet monsoon in Arizona.
Posted at 3:32 PM, Aug 27, 2021
and last updated 2021-08-27 18:40:14-04

TUCSON, Ariz. (KGUN) — What are those black and yellow caterpillars all over Arizona? And why are there so many of them right now?

If you've taken a stroll on the river path or looked closely at the weeds growing in your yard lately, you've no doubt spotted these squiggling critters munching on weeds or scurrying across the asphalt. You'll spot the White-lined Sphinx Moth caterpillars every year in Arizona and across North America, but a busy monsoon gave them quite a bit to chew on this year.

Gene Hall is an entomologist and the manager of the insect collection at the University of Arizona. He answered some of our questions about these critters via email -- why they're here, how long they'll stick around and what effect they'll have on our desert:

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What are these yellow and black caterpillars that we’re seeing all over Arizona right now? Are they native to our area?

These caterpillars, which come in a wide variety of body colors, are the immature stage of the White-Lined Sphinx Moth (Hyles lineata). They are native to our region and widespread elsewhere, distributed from Central America to Canada.

Are they dangerous?

No, they are harmless plant feeders, no need to fear them or be concerned.

Do they have any predators in our ecosystem?

Yes, other animals feed on the caterpillars, they are a good source of protein. Native Americans also used them as a food source.

Why are they seemingly all appearing at once?

It’s not uncommon to see these caterpillars in large numbers, especially when they migrate from one food source to another. This is the time of year we usually see the large migrations.

What’s the reason for the huge number of them this season?

While the caterpillars are known to occur in large numbers when migrating, numbers might be more increased this year due to abundant foodplants and other optimal conditions courtesy of our monsoon rains. The last couple of years we’ve experienced hot and dry summers with poor monsoons, so this might explain low numbers prior to this year, making this year’s large populations more noticeable.

When will they all start to pupate? How long will they be around?

In our region, they usually have two broods each year, and their presence will start to taper off by late summer/early fall.

What type of butterfly/moth will they become?

These are a species of moth, the White -Lined Sphinx Moth. The adult moths are important pollinators and can be seen flying at dusk and dawn. They are also known as Hummingbird Moths as they fly from flower to flower seeking nectar, similar to hummingbirds. They also fly at night, so if there are increased numbers they probably won’t be noticeable. Increased numbers of these moths is of no concern since they are harmless and important pollinators.

White-Lined Sphinx Moth
White-Lined Sphinx Moth

What impact will these have on our ecosystem?

In the natural world, there is a good balance between plants and the insects that feed on them. The caterpillars act as natural "pruners" of plants, provide a food source for other animals that feed on them, and the adult moths pollinate plants.

Any other info you would like to add?

If given the opportunity, take time to watch and observe these and other insects, learn something about their behavior, and appreciate the role they play in the natural balance of life in the desert. We are fortunate to live in a region with a rich diversity of fauna and flora.

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