Gregarious feeding

Gregarious
–adjective
1. fond of the company of others; sociable.
2. living in flocks or herds, as animals.

It is an interesting adaptation of certain breeds of butterflies and moths that their larvae will feed gregariously.

Gregarious animals are ones that move in packs, herds or groups like elephants, cows, geese and even sometimes insect larvae. Gregarious caterpillars tend to be ones emerging from the same clutch of eggs that continue to feed together in groups for a number of possible reasons.

In the case of butterflies and moths, lets look at the larvae of the Polydamas Swallowtail (Battus polydamas). Adult female Polydamas Swallowatails (also known as Goldrim Swallowtails for the yellow edging on their lower wings) lay clutches of eggs on their host plant of Pipevine (aristolochias). Egg clutches are usually laid on some of the freshest growth or newest leaves of the vine to provide tender food for the young caterpillars.

Upon emerging, the young caterpillars from the clutch will travel over the vine in groups and
feed together on the same leaf. As they eat, the young caterpillars consume and store aristolochic acids that are contained in their host plant (1). These acids make the caterpillars distasteful to predators.

The young caterpillars continue to feed together through the first few instars (developmental stages between periods of molting, usually 5 instars) but then become solitary in their feeding habits as they approach the stage of pupation and need more food.

What are the possible benefits of this behaviour?

• Laying the eggs in groups could decrease risk to the female butterfly when ovipositing by allowing her to lay her eggs more quickly and without as much exposure as could be had from flying greater distances. (2)
• Caterpillars in groups would likely be easier to see by predators, but any predator that tastes one of the individuals would likely leave the rest of the clutch alone possibly increasing survival chances for the rest of the larvae. Other groups of larva would also likely gain protection from this behavior reminding the predator of a previous bad experience with caterpillars in groups. (2)
• Warning colors, such as the red/orange tubercles (fleshy bumps) of Polydamas Swallowtails are better advertised by visual repetition.
• Caterpillars use silk threads to help them navigate and hold onto their host plant when they could otherwise be knocked off the plant. Gregarious feeding larvae could share the silken threads of their brothers and sisters which would reduce the need for individuals to produce silk allowing them to focus on eating. (3)

Possible disadvantages of gregarious feeding?

• An increased risk for the spreading of diseases between individual caterpillars
• Greater competition for food

It is a matter of debate wether gregarious feeding habits help to protect caterpillars from predators or not, but I certainly do notice a huge population of Polydamas Swallowtail larva and adults buzzing about the garden. Clearly, whatever they are doing is working well for their population.

Sources:

1.) The Genetical Theory of Natural Selection By Ronald Aylmer Fisher, J. H. Bennett

2.) IFAS document EENY-062 (IN219) Polydamas Swallowtail, Gold Rim, Tailless Swallowtail, Battus polydamas lucayus (Rothschild and Jordan) (Insecta: Lepidoptera: Papilionidae: Troidini)

3.) How Caterpillars Work by Tracy V. Wilson