Saturday, December 3, 2011

A Charismatic.......and Sometimes Drunk Weevil

Usually when I find weevils (Coleoptera: Curculionoidea), they're tiny, relatively bland, or....."otherwise occupied."

Tulip Tree Weevils (Odontopus calceatus), otherwise occupied.

While researching assassin bugs this summer at the Barbara A. Beiser Field Station, however, I came across a much more charismatic weevil, the oak timberworm (Arrhenodes minutus).

Not quite on an oak, this guy was picked up during sweep netting.

If you compare the oak timberworm with the tulip tree weevils in the previous picture, you'll probably notice quite a difference in size and body shape. The oak timberworm doesn't have elbowed antennae, either. So what gives?

The oak timberworm is a species of primitive weevil (Subfamily Brentinae), which look very different from other weevils. They're characterized by their straight snouts (their family is the straight-snouted weevils, Brentidae, after all), antennae that aren't elbowed, and the tendency of their body shape to usually be flat and elongated. The only primitive weevil you're going to find north of Florida is the oak timberworm, others have a tropical distribution.

The oak timberworm can be an economic pest of oaks, as the larvae feed on the wood during their development. Despite this, it's quite a pretty beetle, mostly red with short yellow longitudinal lines on its elytra. Males and females look very different: females have elongated snouts that look almost like a straw, while the males have relatively large jaws at the end of their snouts. These jaws aren't used for hunting prey, since the adult weevils feed on sap, however. Rather, the jaws are used for catching females, helping females drill egg holes in wood, and for fighting other males.

Why would males want to use their jaws to catch females? Well, scroll up to the other weevils which were "previously occupied."

Yep.

The males prowl around looking for some fine females to mate with, and when they find them, the females don't always want to mate. So, they'll run. The males don't appreciate this, so they give chase until they can grab the female's rostrum (the elongated mouth tube). After the male grabs the female, she stops running and he will try to mate with her.

......and the female might try running again. At this point, one of two things will happen: the male will give chase and try again, or he'll give up. If the female keeps resisting, the male is likely to take the latter choice.

The males aren't always focused on mating, of course. They also defend the females and help them with their egg-laying. The males will set up a territory around the female by walking around the females in a circle, while the female works on boring holes in the wood in which to lay an egg. If another male approaches and tries to interrupt the female (with intentions to mate with her), the other male steps in and the two males throw down. Their fights can last for ten minutes or more (30 minutes or more if they're drunk, but we'll come back to that later), and the winner achieves victory by getting his jaws underneath the other male and throwing him off the wood. Size is an important factor in this game: larger males have the advantage. When one male is substantially larger than the other, things can get crazy. Sanborne (check the citation at the end of this post, his paper provides some great observations on the oak timberworm) noticed this and thought it was hilarious: the smaller male he saw fighting with a larger one was thrown 10 centimeters away, soundly defeated. This aggressive behavior by the males is quite helpful for the females, giving them time to drill their egg holes. Each hole can take up to an hour to drill, and the last thing the females want is to be constantly accosted by male suitors--they wouldn't get anything done!

I mentioned something about drunk weevils in that last paragraph, so while we're still on the topic of aggressive behavior in males, let's go back to that. As I said earlier, adult beetles feed on sap. Knowing this, Sanborne made a sap to feed his weevils on by boiling oak leaves, twigs, and bark in water, then added sugar to the mixture. He successfully kept his weevils alive on this mixture from June to September, without problems. Eventually, he noticed something interesting about his sap and the weevils' reaction to it: the higher the degree of fermentation of the sap, the more aggressive his male beetles became.

Essentially, the weevils would become drunk on the fermented sap, and proceed to get into weevil bar fights. The weevils actively searched for fights, tumbling all over the rearing cage once they found another male to fight with. The fights lasted a half hour or more. Sanborne doesn't report why, but I suspect that the weevils didn't have the coordination to get their jaws underneath the other combatant in order to throw it in the air: after all, that's tough to do when the room is already spinning! If the sap was fermented too much, the weevils would just pass out for a while before getting back up.

Sanborne could not conclusively determine if they had a weevil hangover.

When male oak timberworms aren't getting drunk or mating, they help the female drill her egg holes. When the female gets her rostrum stuck in the wood, they will position themselves to help leverage her out of the hole--described as tool use. The female then continues drilling. When their mandibles get clogged with bits of wood, they use their antennae to clean it out.---when you don't have a toothbrush, you need some way to take care of those little pieces! Once the hole is completed, one egg is laid, then the female plugs the hole with the pieces of wood trash she excavated and secretes some sticky stuff to hold it together.

When oak timberworms need to rest, they find hiding places under bark. Males defend their spots from other males, but are chivalrous and will allow females to join them.

You may have already surmised that oak timberworms are hardy: it's sort of required with all that fighting.Their exoskeleton is quite strong ("heavily chitinized" is another way to put it) and resistant. Since they feed on tree sap, they compete with other insects to obtain this food, such as ants. Despite the attempts of ants to chase them away by biting the weevils, the weevils are protected and can continue to enjoy the delicious sap, and the ants can't do anything about it.

An alcoholic weevil that likes to fight....how's that for charismatic?

References:

Sanborne M. 1983. Some Observations on the Behaviour of Arrhenodes minutus (Drury) (Coleoptera: Brentidae). The Coleopterists Bulletin. 37(2): 106-113.

Primitive Weevils of Florida - by Michael Thomas
BugGuide - Species Arrhenodes minutus - Oak TimberwormBeetles in the Bush - Different Jaws for Different Jobs

No comments:

Post a Comment