Western grapeleaf skeletonizer: ‘Voracious,’ invasive moth found in Napa vineyard

NAPA – This week, Napa County agriculture officials put out a warning that they have discovered an animal that they describe as a “voracious feeder” that sounds like something from a monster movie.

Napa County Insect Trapper Jesse Guidi was checking traps along Dollarhide Road in the Pope Valley when he discovered a moth never seen in the area that officials say has the potential to decimate entire vineyards. 

While his discovery wasn’t a monster, it does have a pretty scary name: the western grapeleaf skeletonizer.

The black metallic-looking moth has a 1 1/2-inch wingspan and UC Davis Entomologist Frank Zalom said it gets its name from the way it eats all the green material leaving only the ribs–or skeleton–of the leaf.

“It can be a pretty impressive insect when it occurs in high population because their larvae feed in large numbers and they feed across the leaf surface and can defoliate grape vines,” said Zalom.

The larvae have one other creepy feature–tiny spines that contain hydrogen cyanide.

“So, yeah, handling it is…they’re not good insects to handle because of that,” he said.

The moths are rare, but not unheard of, in the Napa Valley.

County Ag Commissioner Tracy Cleveland said one shows up every five years or so.  But they’re not sure how one ended up in Pope Valley since they don’t fly far and usually just hitch a ride into an area.

“We’ve never found a producing population,” Cleveland told KPIX 5. “So, we’re hoping that’s the same thing again–a hitchhiker, a one-off, and we’re good to go.  But we do take these things seriously.”

No one is overly worried about it because even if there is an outbreak, nature provides its own control, a granulosis virus that targets only that one species of moth.

“When you have an outbreak of them in these natural areas, these granulosis virus just increases and eventually controls the population,” said Zalom. 

Chemical pesticides work as well, but, still the county isn’t taking any chances. They’ve installed 25 new traps within a mile of the discovery and are asking locals to keep their eyes peeled for the hungry little worms on the bottom of grape leaves.

“You always want to be looking, be on the lookout for anything that looks out of the ordinary or is kind of funky,” said Cleveland.

So, it looks like it might not be so scary after all.  Which is good news for grape growers, but bad news for those who like monster movies.