In early September, the agricultural landscape in the central U.S. awakens from its monotonous cloak of summer green and turns ablaze with a hundred shades of yellow, gold, and tawny. The “fall composites” as they are commonly called, a dazzling diversity of mostly yellow-flowered herbaceous plants in the family Asteraceae, are one of the chief contributors to this explosion of color, and among them none contribute more than goldenrod (Solidago sp.). Occupying nearly every fence row, drainage ditch, and fallow field, the bright yellow fronds of tiny flowers are not only pleasing to the eyes of humans, but a smorgasbord of pollen for all manner of flower-feeding insects. Bees, flies, wasps, beetles, and moths all flock to the bounty in numbers rarely seen during the dog days of summer. Spiders, ambush bugs, mantids, and other predators take up residence amongst the flowers as well—not to feed on the flowers, but rather the abundance of flower-feeding insects upon which they will prey. It is rare to find a goldenrod plant without at least a few insects upon it.
My favorite goldenrod insects are, of course, the longhorned beetles of the genus Megacyllene, and at least here in Missouri there are none finer than Megacyllene decora (see A classic fall ‘bycid). However, I keep an eye out for other insects as well, and when I first saw this “wasp” sitting on a flower head I had to do a bit of a double take—”something” just didn’t seem quite right about it. A little lean forward was all that was needed to confirm that this was indeed no wasp, but rather a clearwing moth (order Lepidoptera, family Sesiidae)¹. To my mind, of the many insects that try to mimic wasps, none do so more effectively than members of this family. From the elongate body to the yellow abdominal banding and narrow transparent wings, everything about this moth says “wasp”—well, almost everything or I wouldn’t have done a double take to begin with.
I actually did a fair bit of work with this group in my early years with the Department of Agriculture. Females of most (all?) species emit species-specific pheromone blends that males can detect at incredibly low volumes (only a few molecules are needed to elicit a response by the male antenna). Components of these pheromones have been synthesized, and since a number of species have economic importance in landscape and nursery settings (larvae of most species are borers of woody or perennial plants), these synthetic pheromone blends are commonly employed in traps for survey and detection (e.g., Snow et al. 1985). I conducted surveys for several years during the mid 1980’s in east-central Missouri using these traps and, thus, developed a good eye for distinguishing these moths in flight from the wasps that they so effectively mimic. In fact, I used to keep a pheromone tag on my waist bag, which resulted in male moths frequently flying up to me and “searching” for the female. I never tired of seeing the faces of nursery growers—first showing concern as they were convinced I was under attack by a wasp, and then shock as I calmly reached out and grabbed the “wasp” in mid-flight with my bare hand! (And to be perfectly honest, it took me a while before I could bring myself to start grabbing them out of the air!) I even had one nursery grower continue insisting it was a wasp and could sting even after I had caught it (“Naw, them things sting—I seen ’em!)
The moth in these photos seems to best match Synanthdedon decipiens, widely distributed east of the Rockies and inhabiting the woody galls of cynipid wasps on oaks. In Georgia adults of this species exhibit a bimodal pattern of seasonal occurrence suggestive of two generations per year (Snow et al. 1985), so this September-occurring male might represent a second Missouri generation as well.
Snow, J. W., T. D. Eichlin & J. H. Tumlinson. 1985. Seasonal captures of clearwing moths (Sesiidae) in traps baited with various formulations of 3,13-0ctadecadienyl acetate and alcohol. Journal of Agricultural Research 2(1):73–84.
Copyright © Ted C. MacRae 2012