I’ve been reading Ron Russo’s Field Guide to Plant Galls of California and Other Western States. I first heard about the book several years ago from Andrea Adams-Morden, the head of the docent program at the Carpinteria Salt Marsh. It didn’t really excite my curiosity at first; I’d been drawn to the marsh by my interest in birds, and didn’t know much about plants and insects. Over time, though, as my appreciation of the marsh ecosystem grew, galls began to seem more interesting, and eventually I went ahead and bought Russo’s book.
I’m glad I did. Field Guide to Plant Galls is a book that combines lots of detailed technical information with an infectious sense of wonder. And it turns out that galls really are quite interesting.
Galls are tumorlike grows of plant tissue produced by the host plants in response to the chemical and/or mechanical stimuli of invading organisms (fungi, mites, insects), resulting in accelerated production of plant growth hormones…
Since the galls of many insects (especially wasps and many flies) are specific to their species in size, shape, and color, there is most likely some genetic programming relationship between the compounds provided by the adults or larvae and the manifestations of plant cellular tissue as influenced by the host’s own hormones. Something in the chemicals provided by gall organisms directs the expression of normal plant genes in the development and expansion of the host plant’s tissues. Scientists have been looking for a long time for this “blueprint” that seems to control gall characteristics. This has become the “Holy Grail” of gall research.
The complex relationships between gall inducers, host plants, and the communities of inquiline and parasitic species that associate with them are fascinating — and mysterious. Again and again in the book, Russo calls attention to the limits of current knowledge. Science increases the amount we know, but it also increases the amount we don’t know, in the sense that the larger an island becomes, the more extensive its shoreline. Russo’s enthusiasm for splashing around at the water’s edge is contagious.
One interesting aspect of galls is that they can persist for a long time. In this post I’m going to look at three old, dried galls that I discovered on my botanical crush, Baccharis pilularis, long after the original inhabitants were gone.
Gall #1: Rhopalomyia californica
One of the most common galls on coyote brush is the one created by the coyote brush bud gall midge, Rhopalomyia californica. I wrote previously about finding this old, dried-out R. californica gall:
I had read that R. californica was used successfully to curb an invasive weed in Australia (not coyote brush, but a related species, groundsel bush, Baccharis halimifolia). Recently I googled up some interesting material about that.
The host specificity of Rhopalomyia californica Felt (Diptera: Cecidomyiidae) and its importation into Australia as a biological control agent for Baccharis halimifolia L. is a journal article by W. A. Palmer, G. Diatloff, and J. Melksham. Originally appearing in Proceedings of the Entomological Society of Washington in 1993, it includes this description of R. californica’s life cycle:
Briefly, adults emerge from the gall in the first few hours after daybreak and mate soon after their emergence. Males are particularly short lived (2-4 hours) while females live 12-14 hours and occasionally longer. Females can be recognized by their orange abdomen, the coloration of which is caused by the mass of vermillion colored eggs. They oviposit approximately 100-150 eggs on the surface of meristematic tissue at stem terminals and occasionally in leaf axils. Neonate larvae enter the stem between bud scales and produce a gall on the stem. A number of larvae form a community gall and these galls can contain more than 50 chambers each housing one larva. The larvae complete their development and pupate within the gall. Flies emerge within two months after oviposition.
I also found a Google Books link for Pests of field crops and pastures: identification and control by Peter T. Bailey. The book includes this photo of a female R. californica laying its eggs on B. halimifolia:
I still haven’t seen an adult midge on coyote brush, but I’ll have a better idea what to look for now. I also really want to see a gall with the insects’ pupae protruding from its surface, or with the spent exuviae left behind after the adults emerge. Chuck Baughman has some beautiful pictures of what I think are exuviae (though he commented previously that he thought they might be the bodies of emerging adults killed in a cold snap); they’re definitely worth checking out at BugGuide: Coyote Brush Bud Gall Midge – Rhopalomyia californica.
Gall #2: Rhopalomyia baccharis
In many cases it is easier to identify gall-inducing insects by their galls, rather than by the insects themselves. That’s the case with another Rhopalomyia midge that forms galls on coyote brush, Rhopalomyia baccharis, the coyote brush stem gall midge. The insect itself is more or less indistinguishable from R. californica. But the gall it produces is very different.
I’d been looking for these galls (unsuccessfully) for the past few months, and then last week I noticed an odd thickening on a dead stem. It looked smaller to me than the galls I’d been looking for, but in hindsight that was my mistake; after removing the object, bringing it home and measuring it, I realized that it perfectly matches Russo’s description of “50 to 90 mm long by 5 to 8 mm in diameter”:
Once I examined it up close I was also able to see what I believe are the odd, elliptical openings that form at the outer ends of the gall’s individual larval chambers:
According to Russo:
The rounded-edged, elliptical holes are not typical of the normal sharp-edged, round exit holes created by insects, and they do not appear to have been chewed open. While we have no clear answer at this point, the convenient exit holes appear to be created by the plant after larvae have stopped feeding and stimulation from the larvae has ceased. These holes develop before the larvae (at the bottom of the chambers) change into pupae. Shortly after the holes are created, pupae develop and remain at the bottom of the chambers. Pupation occurs inside the galls, and the adults emerge through the openings in November and December. These unique circumstances make this species stand out from all of its relatives in which the pupae partially push their way out of the gall before the adult emerges. Much more needs to be learned about the habits of this mysterious and interesting gall midge.
Gall #3: Gnorimoschema baccharisella
My favorite line from the movie Men in Black is when Frank the pug observes:
You humans. When’re you gonna learn that size doesn’t matter? Just ’cause something’s important, doesn’t mean it’s not very, very small.
My final gall illustrates this point nicely.
I’ve written previously about the gall of the Baccharis stem gall moth, Gnorimoschema baccharisella. Here’s an image I previously posted of an old, dried G. baccharisella gall at the bluffs:
These galls are monothalamous, meaning that each gall contains a single larval chamber. Russo describes how the larvae chew exit holes, then drop to the ground to pupate. But that’s not the end of the story for the galls:
The frass that accumulates inside the gall serves as a culture medium for various fungi, which are eaten by several fungus insects after departure of the moth. Tilden (1951) found at least 10 parasites associated with this moth in addition to 17 other insects that were connected to the gall or the moth in some manner. The intricacies of these complex relationships stagger the mind for such a common, yet so disregarded, shrub.
The other day at the bluffs William and I came across an old G. baccharisella gall. I removed it and handed it to him, pointing out the emergence hole. He took his scientific curiosity a step farther than I had: He broke it in half.
Looking inside it, he made a discovery. “What are these yellow things? They look like eggs.”
They did. But after bringing the gall home and examining it more closely, I think they may be the fruiting bodies of some kind of fungus. Here are some closeups:
While examining these through the handheld microscope Linda gave me for Christmas, I noticed something crawling around. Here’s the video I took of it:
I’m not sure, but I think that’s a fungus mite of the order Mesostigmata (see, for example, this similar-looking mite at BugGuide).
I few years ago I would have overlooked all three of these galls. But with helpful people like Ron Russo showing me the way, I’m learning to look closer.