How to Catch a Leafroller Caterpillar (Video)

Solitary wasps are amazing creatures, and their hunting behavior is particularly interesting. While my wife and I were in western Massachusetts last month, I was privileged to witness a female mason wasp, Ancistrocerus unifasciatus, successfully capture a leafroller caterpillar. I also got parts of the drama on video.

The scene: A large suburban residential backyard in Athol, Massachusetts, specifically planted to attract all manner of pollinating insects. Natural habitat is mixed hardwood and conifer forest, that abuts the property where this episode took place. Western Massachusetts is an odd mix of suburban and rural, actually, and it is difficult not to characterize this particular property as a "farm."

The leading actress: Ancistrocerus unifasciatus is a mid-sized mason wasp in the family Vespidae, subfamily Eumeninae. Females have a forewing length of 9.5-12 millimeters, males a forewing length of 6.5-9 mm. Both genders are basically deep brown or black with few yellow markings on the face, antennae, base of the abdomen, and legs. Males have a large yellow patch on the face, females a couple of spots.

Male, from Illinois. Note hooked antennae

Only females do hunting and nest building. The species ranges throughout the eastern U.S. west to Iowa, Kansas, and Texas. It also occurs in southern Ontario and Quebec, and Prince Edward Island.

The victim: I honestly cannot verify the identity of the caterpillar in this case, but it is certainly a leafroller in the moth family Tortricidae, and *possibly* the species Archips purpurana, which has goldenrod as a known host plant (among several others).

Is it coming out yet?

The wasp did not pursue its prey the way I thought she would. It even took me a minute to ascertain that she was hunting. I figured she would simply tear open the rolled leaf, grab the caterpillar, and go. Apparently the silk-bound fortress is more impregnable than I imagined.

Maybe at this end?

What the wasp did instead was pure genius. She took advantage of the caterpillar's instinct to "eject" when it feels like its home will be compromised. The wasp ran to one end of the leaf roll, then walked back, vibrating her abdomen to entice the moth larva to jetison itself. Back and forth she ran, as this video shows.

Unfortunately, I did not keep the camera running through the entirety of the hunt, partly due to fatigue, but when I saw the wasp suddenly had the caterpillar in her jaws, I resumed the video in time to see her fly to a nearby leaf to finish stinging her prey into paralysis.

Normally, the caterpillar, if threatened, takes a flying leap out of the leaf roll, trailing a silken line that it will then use to climb back once danger has passed. Kind of the caterpillar version of bungy jumping. The wasp obviously exploits this tendency, and very effectively.

Finishing off her victim with more stings

So, what does the wasp do with the caterpillar? Where does she take it? Adult wasps typically feed on flower nectar, aphid waste ("honeydew"), fermenting tree sap, and/or other liquids high in carbohydrates to fuel their energetic activities, like flying. So, this morsel of protein is not something the wasp would eat. The caterpillar is instead intended for her larval offspring.

And away she goes!

Ancistrocerus unifasciatus likes to set up house in abandoned mud dauber nests, especially those of Sceliphron caementarium and Trypoxylon politum. The mason wasp caches several paralyzed caterpillars in one of those cells. She lays an egg in the cell, then erects a partition and begins filling another cell along the length of the original, larger cell made by the mud dauber.

The larval wasp that hatches then has a larder of fresh food to feed it. Eventually it spins a silken cocoon, and pupates inside. An adult wasp emerges weeks later and chews through the mud wall to freedom.

Sources: Buck, M., Marshall, S.A. and Cheung D.K.B. 2008. "Identification Atlas of the Vespidae (Hymenoptera, Aculeata) of the northeastern Nearctic region," Canadian Journal of Arthropod Identification No. 5: 492 pp. (PDF version).
Krombein, Karl V., Paul D. Hurd, Jr., David R. Smith, and B.D. Burks. 1979. Catalog of Hymenoptera in America North of Mexico Vol. 2. Washington, DC: Smithsonian Institution Press. 2209 pp.

Spider Sunday: Mother Wolf Spider

Last Sunday was Mothers’ Day, but if you will forgive me I would like to pay tribute to arachnid mothers today. During my trip to South Carolina in early May, I had the occasion to momentarily observe a large female wolf spider of the genus Hogna on the afternoon of May 6, 2012. She had an egg sac and was living in a burrow on a wooded embankment adjacent to a road. I had stopped along with members of the Dragonfly Society of the Americas to look around a stream for dragonflies and damselflies. Descending the steep slope to get to the watercourse, another member of the party spotted the spider.

The arachnid was out of her burrow and standing on a network of threads just outside its entrance. The footfalls of others caused her to dash back into her refuge and I thought I’d seen the last of her.

I waited patiently next to her lair and was surprised when she returned to the lip of the burrow shortly. I was hoping she would climb back out for more photo opportunities, but instead she did something remarkable. She turned around inside her burrow and backed out to the entrance.

Why would a large wolf spider, with quite excellent vision, and perfectly capable of defending herself with her fangs, choose to present her vulnerable abdomen, and the egg cocoon attached to it, to potential lurking enemies? The only logical explanation that came to mind was that she was sunning her egg sac.

Indeed, there was filtered sunlight streaming through the forest canopy in the late afternoon: not too intense as to cook the ova within her silken package, but warm enough to help incubate the eggs. Still, had she not repeated this behavior, turning around at one point to make sure no threat was present and then backing out again, I’m not sure I would have believed what I was seeing.

It turns out this kind of behavior is not unheard of in other wolf spiders. Studies of Pardosa wolf spiders of two species revealed that at least one of them seeks open areas in which to expose the egg sacs to the sun (Buddle, 2000).

Had I been able to return to the site day in and day out, I would have eventually witnessed the mother spider cracking open her egg sac, allowing the newly-hatched spiderlings to emerge. Wolf spider babies crawl from the egg sac onto their mother’s back, completely covering her abdomen and sometimes much of her cephalothorax as well. She will transport the youngsters until their next molt, after which they disperse to live freely on their own.

Such devoted parental care is considered uncommon among spiders, but the more we learn the more the picture changes. There are even some female spiders that die immediately after their spiderlings hatch, offering their bodies as their offspring’s first meal.

A piece of long-standing advice is to “stop and smell the roses,” but it also pays to stop and observe animals, too. Science knows very little of the lives of most organisms that are not economically important, be they food sources or pests. Who knows what could come from your own casual observations of the insects, spiders, and other wildlife that share your yard, garden, and neighborhood park?

Source: Buddle, Christopher M. 2000. “Life History of Pardosa moesta and Pardosa mackenziana (Araneae, Lycosidae) in Central Alberta,” The Journal of Arachnology 28: 319-328.