Why do social insects die facefirst in the nest?

Written by Joe Ballenger

First and foremost, I’m really glad we got this question:

From: Katharine

I’ve been keeping an eye on a wasp nest in my grandmother’s yard for a while. Since we just had a cold snap, I went to check on it to see if the wasps were dead so I could harvest the nest. The bottom half was gone (we’ve had some strong storms here, which may be the cause) so when I took it down, I could see the cells of the nest. Some of the cells appear to still have caps (I’m assuming they were dud eggs), but in a lot of the open cells, there are the empty exoskeletons of the adult hornets, all with their abdomens sticking out of the cells and most with the end of their abdomen broken off. There are other adults in the same position but with complete abdomens that I’m guessing are also dead. What’s going on here? Do wasps return to the cells they grew up in? Do they sleep there?

Dead Dolichovespula

Picture credit: Katharine, from the AaE inbox. Picture modified from original by Joe Ballenger

By now, the bugs are in the process of dying down for winter. If you’re in The North, the frost has killed them off already. If you’re in The South, that might not have happened yet. Insect season is almost over, for the most part.

If you’re really lucky, you can find some really neat artifacts the bugs left behind. The nests leftover from baldfaced hornets are really popular, because they’re pretty cool looking. I’ve never been lucky enough to find one myself, but that doesn’t mean I don’t look.

If you bring one inside, you should freeze it immediately. Otherwise, you might get an unpleasant surprise. Sometimes pupae take a little bit longer to develop due to the cold-but-not-lethal temperature, and that can translate into a living room full of confused wasps.

Sometimes, you’re lucky enough to find a nest…but it’s been broken apart first. If you look inside, you might see some of the previous inhabitants laying headfirst in the combs:

So why do wasps die that way?

There’s actually a number of reasons for this, but first we need to explore how hornet nests work.

Hornets: Forgotten Socialites

Wasps have social structures that are really interesting, because they’re similar to bees…but very different in ways that aren’t well understood.

The social structure of these wasps is similar enough to that of your standard honeybee, although they don’t reproduce by swarming. There’s a single queen which starts the colony, and workers generally don’t reproduce. Throughout the season, the colony grows as the wasps hunt for food and gather resources.

The main difference happens in the fall, when the colonies die. Honeybees overwinter, and honeybee queens live multiple years. Hornets, however, only live a single year. In the fall, after the males are produced, new queens (called gynes from here on out) leave the nest to mate, and things just kind of fall apart.

In the fall, the social structure completely disintegrates. The queen is sometimes killed by the workers, but since they can’t reproduce, they keep caring for the larvae. That is, if they don’t cannibalize the larvae after prey gets scarce.

The general patterns are well known, but why this happens isn’t very well understood. Wasps are really hard to study, and you have to get very lucky to make good observations.

So what’s going on with this colony?

Why are they still in the nest?

Leaving the nest is important at the end of the year, but these guys didn’t make it out. A number of things can explain this:

1.) Sometimes, the gynes don’t make it into a good hibernation place. They’ll just hang out in the nest until they die, usually from starvation or cold.

2.) Males sometimes do the same thing.

3.) Workers will care for larvae, until resources disappear. After there are no more insects for them to eat, they’ll begin to cannibalize their larval siblings.

Why are they head down?

It’s hard to know exactly what’s going on because there’s no way to know if they’re males, workers or gynes. However, it’s likely that they’re wasps who returned to their nests to find a warm spot or to find food.

See, wasps store food similar to bees. The genus Polistes stores a honey-like substance in its nest with its larvae, and the adults consume this in a manner very similar to the way that honeybees store their honey. Unfortunately I couldn’t find any literature on whether this type of wasp, Dolichovespula, does the same thing. However, I believe they do store food in their nests in a similar manner.

What are those caps?

Wasps start out as an egg, hatch into a legless larva, and then turn into a pupa.  It’s very similar to bees, but the larvae are fed a solid diet by the workers instead of a liquid diet.

When the larva begins to pupate, it spins a silken cover over itself to let the workers know that it doesn’t need food. It looks like a giant dome from up top, but the wasps underneath are busy turning into adults like the bees in the video above.

If you look closely at the first video, you can see these caps pretty clearly.

The Bottom Line

These wasps are caught out in the cold-both figuratively and literally-after their society collapsed. After the annual collapse of their society, they died looking for food.

This sort of behavior is actually very common in social insects. Honeybees do the same thing. After winter, it’s very common to see dead bees headfirst in honeycomb. They died the same way…looking for food.

While these wasps didn’t make it, some of their sisters survived the winter by finding a warm place to sleep. They’ll start nests of their own next year, and the cycle will continue anew.

It’s sad, but that’s nature for you. It’s both beautiful and cruel…all at the same time.

Works Cited

Greene, A. (1991). Dolichovespula and Vespula. The social biology of wasps, 263-305.

A special thanks to Dr. Amy Toth for her input on this post! If you’re interested in learning about the biology of Vespids like Yellowjackets, Hornets, and Paper Wasps, her graduate students have taken over #WaspLove on Twitter, and regularly talk about wasp biology!

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