On a late spring walk in Mission Trails Regional Park, I had my first encounter with a skeletonizing leaf beetle. While examining the leaves of a goldenbush plant (Isocoma menzieii), I found several dull greenish-yellow beetles that were cruising along the stems and leaves. I tend to notice goldenbush most often in the fall when it graces us with spots of color in the brown landscape. This flowerless plant was almost unrecognizable to me with its gray hairy leaves. An unexpected surprise, the beetles were a decorative surrogate for the flowers that were lacking at this time of year. As I continued along the trail, I found the same kind of beetles on other goldenbush plants.
Although the name “skeletonizing leaf beetle” may send shivers up the spine of any plant lover, it is apt since the larvae consume leaf blades, leaving behind the skeleton-like veins and midribs. The genus Trirhabda has 26 species in the U.S. and Canada, 16 of which have been reported in California. The Latin name probably refers to the three spots on the pronotum (tri=three, rhabdus=rod). Trirhabda are “stenophagous,” meaning they eat a limited variety of plants. Their preferred plants are in the Asteraceae and Hydrophyllaceae families. Different species of Trirhabda prefer specific plant species and have adapted to tolerate secondary compounds found in their host plants. Since each species may feed or rest on plants other than the host plant, you cannot identify the species of Trirhabda solely by the plant it is on. This is unfortunate, since the species look similar. I decided not to let that deter me. After reading about Trirhabda species and their preferred plants, including several species of plants which are common in MTRP, I decided to go on the hunt for Trirhabda.
Proving the idiom “seek and ye shall find” took only a few visits to the trails of MTRP. In addition to Isocoma menziesii, adult Trirhabda beetles were feasting on coastal sagebrush (Artemisia californica), California encelia (Encelia californica), and yerba santa (Eriodictyon crassifolium). How many different species of Trirhabda did I find? I'm not sure, although some morphological differences were apparent – the overall size, shape of spots on the pronotum, and color and hairiness of the elytra—suggesting 4 different species.
In spite of my efforts, I did not find Trirhabda beetles on coyote brush (Baccharis pilularis) – not too surprising since there is only one historical record of the species that prefers this plant in San Diego County. And now, the season has passed and there are no Trirhabda beetles to be found in MTRP. Adult Trirhabda beetles may spend their entire 90-day lifespan on one plant. Females lay multiple clutches of eggs at the base of the host plant, where the eggs spend the winter. Come February, the eggs will begin to hatch and the larvae will ascend the stem of the host plant and munch away for several months before returning to the soil at the base of the plant to pupate. After that, my window of opportunity: I will resume my hunt for Trirhabda on coyote brush in MTRP.
Sources:
http://www.sbnature.org/collections/invert/entom/cbphomepage.php
https://www.zin.ru/animalia/coleoptera/pdf/clark_ledoux_et_al_2004.pdf
http://bugguide.net/node/view/35037/tree
