Red and Black Bug Nymphs – White Margined Burrower Bugs
We found these bug nymphs on a piece of landscaping timber on September 15, 2019. Given the time of year, I think this is a hibernation aggregation. They were looking for a protected spot to overwinter.
They all appear to be the same species, with a jet black head and thorax, and a cherry red abdomen with three black lines across the back and little black marks all around the edges.
They clearly aren’t the same age, though. If we look closely, the largest ones are several times bigger than the smallest ones. The very largest ones have the beginnings of black wing buds extending partially down over the abdomen, indicating that these were probably only one molt away from being adults.
I originally thought they were some kind of stink bugs, in the family Pentatomidae. A lot of stink bugs have nymphs with that characteristic chunky shape, and many of them are also some shade of red, although these seem to be redder than most. It was really hard for me to pin them down to species, because stink bugs frequently have a fairly radical color change when they become adults.
When I first posted this, I hadn’t found anything online showing an overwintering aggregation of any bugs that look quite like these. The closest I had seen are certain species in the predatory subfamily Asopinae. Several of these look close, although none look quite right. Still, what they don’t look like, is one of the notorious pest stink bugs like the infamous Brown Marmorated Stink Bug.
But then, it occurred to me that maybe my original assumption was flawed. Maybe they aren’t stink bugs at all. What if I backed up the taxonomic tree a bit further, and looked at things that are only related to Pentatomidae? So, I moved up to the superfamily Pentatomoidea and searched again . . .
And promptly found pictures of the nymphs of the White-Margined Burrower Bug, Sehirus cinctus. These look identical, and are well known for making this kind of overwintering aggregation. And, of course, we know that this species lives around here, because I have photographed an adult previously. And in fact, while I didn’t get pictures, I have seen aggregations of the adult coming out of hibernation in the spring! It really looks like the immature nymphs aggregate together in the fall, and then mature while hibernating to make the springtime adults. Although, some of these look so tiny that I don’t see how they would be able to go through several molts to mature over the winter, so probably some of them stay nymphs until at least the end of May.
Anyway, these beetles mainly feed on developing seeds of plants in the mint family. And since mint is not cultivated for its seed, they don’t actually harm any part of the plant that humans care about, and they are not agricultural pests in spite of their large numbers.
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Are they going after the developing seeds because they contain more protein that the juices of the plants that the aphids suck?
I expect so, yes. Plant juices don’t have a very high nutritional value, while the fluids in a developing seed are full of all the nutrients that a seedling plant will need to grow big enough to begin photosynthesizing. It’s like the difference between drinking cow’s blood, and drinking milk.