Wild Ideas: The case of the disappearing stink bug

By Pam OwenBy Pam Owen
Usually sticking to smaller, softer prey, this fragile-looking cellar spider in Pam Owen’s house takes on a brown marmorated stink bug, but did the spider actually eat the bug?

Although, with the relatively warm winter, I’ve been seeing more invertebrate activity outside than I usually do this time of year, the most interesting encounter with them has been inside.

As I was watching TV one night a few weeks ago, a brown marmorated stink bug (BMSB) landed on me. The house had gotten warm enough during the day that the bug must have come out of whatever overwintering spot it had found earlier in the fall. Too lazy to catch it, I just flicked it off, thinking I’d track it down later.

In a few minutes, I noticed some movement up near the ceiling. Even from where I was sitting, I could see a stink bug — probably the same one — trying to escape from the web of a cellar spider. These spiders are common in houses and harmless, so I mostly ignore them, knowing they prey on unwanted insects.

Despite their long legs, cellar spiders have bodies that are small (a quarter to a third of an inch long), thin, lozenge-shaped and fragile. These spiders look far from able to consume, or even wrap up, a stink bug, as this one was doing.

Also known as “skull spiders” because of the markings on their bodies, cellar spiders are among the estimated 1,340 species of the Pholcidae family, which is primarily tropical. The most common of these here is a European immigrant, the long-bellied cellar spider (Pholcus phalangioides), which is probably the species I saw dealing with the stink bug.

Pholcidae (in the spider order, Araneae) and harvestmen (in another arachnid order, Opiliones) are both often referred to as “daddy longlegs” because of their long, spindly gams. Cellar spiders have webs that are messy and three-dimensional (unlike the classic orb-weavers’ two-dimensional circular ones).

By Sven Siegmund via WikimediaBy Sven Siegmund via Wikimedia
Although the long-bodied cellar spider, also known as a “skull spider,” originally is from Europe but has long been common in American homes.

On and off, I watched the cellar spider try to wrap up the much-bigger bug, noticing that another cellar spider, a bit smaller, had apparently been attracted by the commotion. The two spiders seemed to tussle briefly, with the smaller one eventually backing off. At that point the larger one returned to wrapping up the bug, and was busy with it all evening. I thought the spider might actually be consuming the bug, but I couldn’t tell from where I sat and was too lazy to get up.

This is not the first time I’ve seen a cellar spider try to deal with a BMSB. A couple of years ago, I wrote a column (Feb. 9, 2013) about a similar situation. The spider I observed that time kept touching the bug with its back two legs. I thought the spider might be trying to determine what it had trapped or was intrigued by the scent of this other, recently arrived invader. Or maybe the spider was trying to figure out how to get through the bug’s hard carapace to the feast inside.

In that case, the stink bug was gone by the next day. There’s no way the cellar spider could have consumed the stink bug overnight, so I assumed the bug escaped on its own or was shaken loose from the web by the spider. Cellar spiders often shake their webs violently to ward off predators and to shake loose anything unwanted in the web.

Cellar spiders also can move rapidly when disturbed, “flexing their legs so that the body gyrates in a circular motion,” according to the Encyclopedia of Life (eol.org; search on “pholcidae”). The shaking can be so hard that the spiders become a blur. The flexing can also make the cellar spider appear larger to potential predators. Cellar spiders can deliver venom, but it’s mild even to their prey, and harmless to larger species, including humans.

The recent spider-bug encounter ended similarly to the previous one. The next day I dragged out my stepstool and climbed up to the web for a closer look, flashlight in hand. Other than missing a few legs, I couldn’t see much damage to the bug, although it obviously was still quite dead. But the next day it was gone.

Although cellar spiders are effective predators and, according to Spiders.us (spiders.us; search on “pholcus phalangioides”), will eat “any kind of flying, jumping, or walking insect that enters the web,” they prefer small, softer prey, including mosquitos, small moths, flies, gnats and woodlice (pillbugs).

They’ll also eat other spiders, including species much larger than themselves. Cellar spiders will sometimes shake other spider’s webs to mimic caught prey, trying to lure in the web’s owner. When food is scarce, they can turn cannibalistic.  

So had this little spider merely been wrapping up the bug to immobilize it, removing any threat it could pose? It seems unlikely the spider would spend that much time and silk on something that wasn’t food. That would also go against what is known about how they treat predators. But I also couldn’t find any information about whether cellar spiders are known to eat stink bugs, or if they even have the equipment to do it.

My guess is that the spider was treating the BMSB as prey, but where did the bug go? I can’t see how such a tiny spider could consume, and digest, such a large bug overnight. And no detritus from the inedible portions of the bug was on the floor. Even if it had been alive, the bug couldn’t have broken through the silk that was tightly wrapped around it.

Whether cellar spiders are eating BMSB or not, if they’re killing them, they’re helping to keep down the bug’s numbers. And if cellar spiders are indeed adding stink bugs to their menu, they’ll be joining an increasing list of predators in our area, native and otherwise, that are warming up to this annoying (and costly) Asian invader.

© 2015 Pam Owen


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