Are brightly colored insects poisonous?

No,In the world of insects, bright colors are used to warn predators. Many insects that sport bright colors feed on plants with toxic chemicals that then make the insects toxic to birds and other predators. Predators know to avoid the bright colors of these insects because they taste bad, and ingesting them has unpleasant consequences. Interestingly, many insects mimic the patterns of these toxic insects without being toxic themselves in an effort to fool predators. Both toxic and non-toxic insects rely on bright patterns with lots of contrast to warn against potential predators. 

 The best examples of insects with bright colors that are either poisonous to eat or carry venom glands include butterflies and stinging insects such as bees and wasps. Butterflies often lay their eggs on toxic plants so that their larvae can feed on the plants and ingest the toxins. 

These toxins cause both the larvae and the adults to be bitter tasting to predators. While the amount of toxin in the insects is usually not enough to kill a predator, it is enough to make predators ill and thus unlikely to attempt to eat brightly-colored insects in the future. 

Monarch caterpillars, Danaus plexippus, feed on milkweed and sport black, yellow, and white stripes. Milkweed contains toxic compounds that are not only bitter for predators but can also cause serious harm. Monarch butterflies have evolved to tolerate milkweed toxins.  Adults are bright orange with black and white patterning. Monarchs seek out stands of milkweed to lay their eggs to ensure the larvae are close to milkweed because it is their only food source. 

Pipevine swallowtail butterflies, Battus philenor, lay their eggs on toxic pipevines, and Atala butterflies lay their eggs on the cycad plant, both of which cause the larvae and adults to be toxic. The Atala butterflies, in particular, are known for causing liver damage to their predators, thanks to the toxic compounds found in the Coontie plant, the only native cycad in North America.

Other than butterflies, bees and wasps are known for poison sacs associated with their stingers. One of the most painful stings comes from the “Cowkiller,” also known as the “Velvet Ant.” Various species of velvet ants exist and are members of the Mutillidae Family. This species is not actually an ant, but a wingless wasp with bright red and black stripes on its body. The sting of a velvet ant is known to be remarkably painful and is even rated 3 of 4 on the Schmidt’s sting pain index. 

Because predators have evolved to avoid brightly colored insects in case they are toxic or venomous, a number of other harmless insects mimic the color patterns and combinations of these poisonous species. Scientists call this Batesian mimicry. For example, the Viceroy butterfly has a pattern almost identical to the Monarch butterfly, However, the Viceroy does not feed on milkweed and is not actually toxic. The pattern of the Viceroy is just slightly different than the Monarch, but the colors and pattern are close enough to fool potential predators. 

Another striking example of Batesian mimicry is the hoverfly. These small fly species mimic the colors and pattern of bees to avoid predators. Not only do their colors and patterns mimic bees, their body movements and flight habits also mimic bees. 

These examples of both toxic or venomous insects and those that mimic them are proof that not all brightly-colored insects are poisonous.