The spectacular amorpha borer, Megacyllene decora, was not the only black-and-gold colored beetle that I saw on the flowers of goldenrod (Solidago sp.) a few weeks ago. In addition were several delta flower scarabs, Trigonopeltastes delta. This species is much more commonly encountered than the amorpha borer—not only geographically but also throughout the season on a greater diversity of flowers. Nevertheless, I had failed in my previous attempt to photograph the species at the very same locality just a few weeks earlier due to the much higher summer temperatures and resultant flightiness of the beetles.
In the case of this beetle, the scientific name almost completely describes the beetle—the genus name being derived from the Greek words trigon (i.e., triangle, triangular) and pelt, (i.e., a shield), referring to the triangular and shield-shaped pronotum, and the species name based on the Greek letter Δ (“Delta”) in reference to the distinctive white triangle on the pronotum that resembles it. I mentioned the diversity of flowers on which adults of this beetle can be found. Pascarella et al, (2001) found this species on 13 different plant species (including mass aggregations numbering in the thousands on inflorescences of Sabal palm, Sabal palmetto) in their study of flower-visiting insects in the Everglades National Park. In Missouri, I see these beetles most commonly on Queen Anne’s lace (Daucus carota) and several other plants with white inflorescences such as American feverfew (Parthenium integrifolium), New Jersey tea (Ceanothus americanus), wild hydrangea (Hydrangea arborescens), and—most recently—hairy mallow (Hibiscus lasiocarpos). Interestingly, on this day there was an abundance of white-flowered snakeroot (Eupatorium sp.) in bloom at the same site, but I only saw the beetles on the yellow-flowered goldenrod.
It has been suggested that the Delta pattern on the pronotum and orange-and-black coloration of the elytra combine to mimic the appearance of paper wasps in the genus Polistes. Paper wasps are frequent visitors to many of the same flowers that these beetles frequent; however, the much smaller size of the beetles might suggest mimicry of aculeate hymenopterans (stinging wasps and bees) in general rather than paper wasps specifically. A more unusual type of mimicry has also been suggested in that a rear view of the beetle with its large, white, triangular pygidium seems to resemble the head of a hornet. Supporting this idea is the habit of the beetles to raise and hold their long hind legs in the air when disturbed in a manner that makes them resemble a hornet’s antennae!
Pascarella, J. B., Waddington, K. D. & P. R. Neal. 2001. Non-apoid flower-visiting fauna of Everglades National Park, Florida. Biodiversity and Conservation 10(4):551–566 [abstract & pdf link].
© Ted C. MacRae 2014