“What kind of worm or caterpillar is this?” asks this reader in Northeastern New York State. His question pertains to the caterpillar-like creature pictured below, which seems to be a dark blue or purple color.
A number of these creatures were found on the back wall of a house, states our reader. They would stick to the shaded parts only, “as a fly initially”. After a couple of days, the flies “were worms or caterpillars.” Our reader has also found a few of these on his “day lillies” on his “other property 30 miles away.” Now, given the low resolution of the photo, it is difficult to make out any of the critter’s smaller characteristics, even after zooming in on the photo. Nonetheless, what we can say right away is that it is not possible for flies to become worms or caterpillars, if that is what our reader is insinuating. It is always the other way around. What is more likely is that the flies our reader spotted on the house reproduced and laid eggs on this wall. Then, after some time the eggs hatched, which spawned the worms our reader found.
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What is curious here is that, while we would have, at first, identified these as caterpillars, our reader says that he spotted flies here. This tells us that these may be sawfly larvae. Sawflies are insects that belong in the same category as bees, wasps and ants, and there are hundreds of species of them. What makes us think that these are sawfly larvae is that, not only did our reader mention that there were flies on the wall previously, but also the fact that these creatures look like caterpillars, which sawfly larvae are often mistaken for. That said, they get their name from the fact that their ovipositor resembles a saw, as well as the fact that, like a saw, they use their ovipositor to cut into the plant in which they deposit their eggs. This makes their choice of a wall to lay their eggs quite interesting, and it makes us doubt this identification. Though we suppose that, if there are suitable holes in the wall for a sawfly to lay her eggs, then this is not impossible.
Either way, while the sawfly larva, like a lot of caterpillars, can do significant damage to the plants it feeds on, they are not harmful to humans or pets in any direct way. For that reason, our reader does not really have anything to fear. If he wants to control the amount of sawfly larvae on his lilies, then he can spray the stems with soapy water, as this will make it hard for the sawflies to get a grip on the stems, and he can manually remove any egg deposits he finds (they are quite easy to spot).
In conclusion, we think that the creatures our reader has been finding are sawfly larvae. If our reader thinks that they may actually be something else, then he is welcome to contact us again with better pictures and more context. Otherwise, we wish our reader the very best!
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