Saturday, September 06, 2008
The Sharpshooter Phenomenon
You might interpret the title of this post as a shameless attempt to cash in on the Palin buzz and trick unaware completely-not-interested-in-bugs web surfers into reading my blog, but you'd be wrong. This post has nothing to do with moose, the NRA, teen pregnancy, earmarks or anything at all (known or more likely unknown) about that female candidate for vice president of the United States. I am a woman, though, hoping you'll just continue reading out of some sense of misplaced enthusiasm!
This post is about that pesky bug Homalodisca vitripennis referred to by the common folk as the glassy-winged sharpshooter, is a leafhopper in the leafhopper (Cicadellidae) family with strong leafhopper family values. In the ornamental garden these bugs usually do little damage and they are sort of cute in a homely way. Their non-threatening appearance disguises their potential to wreak great damage, since they are a carrier of a bacterial disease of plants. The sharpshooter sucks the juice out of a broad range of plants by inserting its mouthparts into the fluid-conducting xylem. While it's sucking, if it is harboring xylella fastidiosa bacteria on its mouthparts the disease organism can be transmitted to a new host plant. This organism causes Pierce's disease which could wipe out life as we know it here in California: that is, it kills grape vines at the base of our wine industry. It also kills nerium oleander, a nice evergreen shrub that we have learned to live without but miss dearly.
This summer I've seen more sharpshooters than has been typical, but I've also noticed them on more plant hosts than before. These opportunistic little buggers have been feeding on sunflower, abutilon, privet, citrus, artemisia, texas ranger, pittosporum, dodonaea, asclepias, elm, broom corn, senecio, acacia, pseuderanthemum, and even opuntia. Cindy has quite a gathering on a fern in her yard. They are expanding their feeding host base.
What about their egg-laying hosts? I've found egg masses on privet, dodonaea, the kumquat and broom corn. The eggs are inserted beneath the epidermis on the underside of leaves, and so can be hard to spot on thicker leaves especially. So, of course there could be egg-laying on other species of plants that I just haven't seen.
Some parasites are beginning to take hold within the sharpshooter range, including my yard. Tiny wasps in the genus gonatocerus lay their own eggs on those of the sharpshooters, which then hatch, parasitize and kill the host. I've photographed some egg masses to show the difference between a parasitized mass and one that hatched sharpshooter nymphs. The first photo is a parasitized egg mass, and shows the characteristic pinholes which are the exit holes of the emerging wasps. Lots of the egg masses I am seeing had pinholes, and this is the first year I've seen them. The second photo down is a dried up egg mass that hatched. You can see there are no pinholes. The third photo is a dodonaea leaf that was heavily used for egg laying, where the dead portions have dried up and fallen out. No way to tell if they had or had not been parasitized. I haven't actually seen the wasps; the photos and information at UC IPM are consistent with my observations and so I believe these are indeed the wasps at work.
So the sharpshooter population has increased and spread out, but their parasites has also begun to take hold in the little domain of my yard. Evidence of pest-controlling parasites such as these gonatocerus wasps is a compelling reason NOT to use insecticides in my garden and yours since the sprays kill the wasps as well as they might the target pest. By the way, there are also lots of leafhopper assassin bugs hatching recently, which prey on the sharpshooters. Its been my experience that parasites and predators are more effective at pest control than spray over the longer term, and it will be interesting to see how the glassy-winged sharpshooter population moves next summer. Perhaps homalodisca vitripennis is on the verge of control and the IPMers will be able to chalk up another biocontrol victory.
And why are they called sharpshooters? After they insert their mouthparts into the xylem stream the volume of fluid is such that they must excrete in order to allow more plant juice in. So they shoot the shit so to speak in order to keep feeding. You'll be standing unawares in the yard enjoying the day or looking for bugs or thinking about which presidential candidate to vote for when all of a sudden a droplet or perhaps several droplets of excreta hit you in the face. I don't know about you but I tend to go for the guy with the most science and the least excreta.