Top Photo: Mystery objects on redbud leaf.
Rangers Becca and Robert discovered an unusual leaf mixed in with the regulars in the leaf liter at the base of a redbud tree next to the path near the Cafe Plaza. The leaf had dozens of objects attached which appeared to the two rangers as some sort of insect eggs or larvae. A radio call and several minutes later, I was staring at the objects myself.
My first impression was of some sort of cocoon, pupal casing, or exuviae of insects that have pupated, emerged as adults and departed. They varied from 7 mm – 10 mm in length. Each object had a hole chewed along the back where some living creature had exited.
Close-up photos revealed a similarity of the objects to caterpillars, though much faded and hardened. They appeared mummified. They had been eaten from within and hollowed out. The only thing I could think of as perpetrator was a wasp, probably braconid or ichneumon, but certainly a small parasitic wasp.
Braconids typically lay dozens of eggs within each caterpillar host. The wasp larvae eat the caterpillar within, chew their way out through the caterpillar’s skin then spin and attach a cocoon to the outside of the host.
There can be anywhere from two to over a hundred cocoons hanging from the host caterpillar. Some species crawl off after chewing their way out of the host and form individual cocoons housed under a silken tent on the same leaf as their host. Subsequently, they pupate and emerge as adult braconid wasps.
It was probably not a braconid. There were no cocoons hanging from the caterpillars nor in a silk tent nearby.
The evidence points to an ichneumon wasp. They typically lay one egg per victim. The problem is, there are over 3,300 known species of ichneumon wasps north of Mexico.
The mummified caterpillars had a superficial resemblance to a familiar caterpillar in the area, though the colors were far from what the live animal would display. The red-humped caterpillar fit the bill, and they’re gregarious in their feeding behavior, at least in the early larval stages. A lone parasitic wasp could very efficiently lay eggs in dozens of caterpillars on a single leaf.
Luckily I found a photo on the internet of the exact same subject matter, hardened and faded caterpillars parasitized by a wasp. Everything fit, red-humped caterpillar and, suggested by an expert right there on the same web site, an ichneumon wasp called Hyposoter fugitivus. I’m convinced. The images were exact matches for my photos.
Another web site I visited gave me a little more in the way of life history of the wasp in question, but actually ended up serving to confuse. The two quotes below from the University of California Agriculture & Natural Resources’ Statewide Integrated Pest Management Program seem to contradict one another.
“Hyposoter fugitivus larvae feed singly and pupate inside the skin of the caterpillar, which becomes swollen and mottled gray with black.”
“A female Hyposoter wasp injects an egg into an early instar caterpillar; the egg hatches into a larva that feeds on the body fluids and tissues of the host. The host dies in the third or fourth instar, and the Hyposoter larva emerges from the host’s body, spins a cocoon, and pupates. The adult wasp emerges from the cocoon to mate and seek new hosts; each female may destroy up to 100 host caterpillars. The life cycle takes as few as 15 days, depending on temperature.”
In one quote, the wasp pupates inside the caterpillar, the second states they pupate outside of the host.
There are hundreds of species of Hyposoter wasp, perhaps some pupate within the hosts while others pupate after exiting the hosts as larvae. I’m satisfied knowing what genus the wasp in question was (Hyposoter) but if there are any experts out there who can confirm as to species by the behavior described, please chime in.
In either case, it seems it was an adult Hyposoter wasp which started the process by coming upon the group of red-humped caterpillars on the redbud leaf and laying a single egg into each caterpillar. It’s always a good day when you learn something new. Had that leaf not fallen when and where it did, I may never have been introduced to this particular wasp and the potential fate of red-humped caterpillars.