The bird poop caterpillar is the larval stage of the Giant Swallowtail butterfly (Papilio cresphontes), one of the largest butterflies in North America. Its mottled brown-and-white coloring makes it look almost exactly like a fresh bird dropping sitting on a leaf, a disguise so convincing that most predators pass right over it.
Why It Looks Like Bird Droppings
The coloring isn’t random. The caterpillar’s body is dark brown or olive with irregular patches of white and cream, mimicking the wet, uneven texture of real bird feces. The pattern works because predators like birds, lizards, and wasps have no reason to investigate something that looks like waste. It’s a form of protective camouflage called masquerade, where an animal resembles an inedible object rather than blending into its background.
All Papilio (swallowtail) caterpillars share this bird dropping appearance during their early life stages. The disguise holds through roughly the first four instars, which are the growth phases a caterpillar passes through between molts. Once the caterpillar reaches its fourth or fifth instar, it grows large enough that the disguise becomes less convincing. At that point, some species shift to green coloring with eyespots that make them look like a small snake head instead.
A Backup Defense: The Osmeterium
Looking like bird poop isn’t the caterpillar’s only trick. Tucked behind its head is a bright orange, forked gland called an osmeterium. When the caterpillar feels threatened, it shoots this gland outward like a fleshy snake tongue. The sudden flash of color and the organ’s shape can startle birds and small reptiles on its own, but the real weapon is chemical. The osmeterium releases a liquid that smells like spoiled cheese, potent enough to send ants, spiders, and mantids scrambling.
What’s particularly clever is that the chemical cocktail changes as the caterpillar grows. Younger, smaller caterpillars produce compounds called terpenoids that are especially effective against ants, their biggest threat at that size. Once they’re large enough that ants are no longer a serious danger, they switch to producing butyric acids, which work better against vertebrate predators like birds and lizards. The caterpillars manufacture these chemicals internally rather than absorbing them from the plants they eat.
Where You’ll Find Them
Bird poop caterpillars feed almost exclusively on plants in the citrus family (Rutaceae). If you live in the southern United States and grow citrus trees, there’s a good chance you’ve already encountered one. The caterpillar is so commonly found on orange trees that it has a second nickname: the “orangedog.” Beyond commercial citrus like sweet orange, they also feed on native plants including prickly ash, hoptree, sea torchwood, and Hercules-club, along with ornamental citrus relatives like Mexican orange and white sapote.
Giant Swallowtails range across the eastern United States, down through Mexico, and into Central America, with their territory expanding northward in recent decades. You’re most likely to spot the caterpillars on the tender new growth of their host plants, where the young leaves are easier to chew.
From Egg to Butterfly
A female Giant Swallowtail lays single eggs on the leaves of host plants. The eggs hatch in four to ten days depending on temperature. From hatching, the caterpillar spends about three to four weeks eating and growing, passing through its series of molts until it reaches roughly two inches long. At that point, it forms a chrysalis and pupates. The adult butterfly that emerges is striking: black with bands of yellow spots, with a wingspan that can reach five to six inches.
Should You Remove Them From Citrus Trees?
If you find bird poop caterpillars on a mature citrus tree, they’re almost never a problem. They may chew through some leaves, but healthy established trees handle the damage without any noticeable effect on fruit production. The University of California’s pest management program notes that orangedog caterpillars rarely cause economic damage in mature orchards.
Young citrus trees are a different story. A few caterpillars can strip the limited foliage on a small tree, slowing its growth. If you only have a handful, picking them off by hand is the simplest approach. They’ll flash that orange osmeterium at you and it smells unpleasant, but they can’t bite or sting. For heavier infestations on young trees, a spray containing Bacillus thuringiensis (Bt), a naturally occurring soil bacterium, targets caterpillars without harming beneficial insects. Parasitic wasps also help keep populations in check naturally.
Many gardeners choose to leave them alone entirely. The Giant Swallowtail is a spectacular pollinator, and tolerating a few chewed leaves on a citrus tree is a small trade for watching one of North America’s largest butterflies emerge in your yard. Some citrus growers even plant sweet fennel nearby as a trap crop, giving the caterpillars an alternative food source and drawing them away from the citrus.