Orthotydeus lambi!

One of the most interesting talks I saw at APS was David Gadoury's "Very small sheep." He began by describing how his group failed to detect significant epidemics of powdery mildew (a plant-infecting fungus) on a wild species of grape (a plant that is very susceptible in conventional agriculture). In greenhouse experiments, these wild grapes were just as susceptible as agricultural grapes to powdery mildew when inoculated. They discovered that a species of wild mite, that occurs throughout the United States, control powdery mildew of wild grapes by roaming their leaves and grazing heavily on the spores and mycelium of the parasitic fungus.

They then asked themselves if these mites lived in commercial grape fields, and if not, if something could be done to encourage their presence. They first suspected that the mites were absent from commercial grape varieties because these varieties have many fewer "hairs" than the wild species - and these mites are thought to rely on these hairs in order to hide from predatory mites. But despite having fewer "hairs," known as domatia, mites seemed just as happy to live on commercial varieties - in fields that weren't sprayed with pesticides...

Agricultural grapes are routinely treated with pesticides to control damaging insects and fungi, so they ran an experiment comparing mite populations in grape fields treated with different pesticides. Ironically the "organic" fungicide, sulfur, killed the mites, but some of advanced synthetic fungicides didn't hurt the mites, at least not at lower application rates. One member of the audience, possibly hoping for an organic solution, asked if mites might be found that were resistant to sulfur. David replied that it was not possible, and that you might as likely find an organism that's resistant to "fire" - reflecting the extreme toxicity of this "organic" biocide.

They then tried to see if mites could control powdery mildew in the absence of pesticides. It turns out they could - but not to economically acceptable levels. In the end, the best treatment was a low level of fungicide that controlled powdery mildew partially, yet allowed mites to exist to control the rest. Not a pesticide-free solution but certainly better than the original!