European Paper Wasps: The No. 1 Caterpillar Killer

Today as we were out picking thistle leaves, our Painted Lady caterpillar (Vanessa cardui) was killed by a European Paper Wasp (Polistes dominula)!

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Ever since the European Paper Wasp (Polistes dominula) population exploded sometime last month when summer started, we have been having a huge problem with them killing our insects. This is not the first post that we have made about them (see here).  They actively search all suspect host plants from sun up to sun down and any living arthropod on the plants they search are doomed, large or small, unless they have some sort of defense such as a nest. The wasps seem to rely heavily on smell and are incredibly adept at discerning which plants tend to have defenseless caterpillars (milkweed [Asclepias] and fennel [Foeniculum vulgare] are favorites; on another note, they rarely go for trees).

They are very clever and will find their way into a rearing sleeve or bag if they are determined to attack what is inside. This year they have decimated our Monarch caterpillars (Danaus plexippus) and Anise Swallowtails (Papilio zelicaon) in rearing sleeves, attacked our Anise Swallowtail butterflies in an ovipositing enclosure, and even injured one of our Mediterranean Katydids (Phaneroptera nana) in a screened up enclosure. With this kind of killing power, they are undoubtedly the number one cause of mortality in caterpillars by predation. 

And today they found another victim to add to their list: our fifth instar Painted Lady caterpillar (Vanessa cardui) that we had found at Tilden Regional Park on 7/9. The older of the two we had found already pupated on 7/18 but the other one still had another day or so. But when we were picking some Milk Thistle (Silybum marianum) in our backyard for it, we made the bad mistake of leaving the caterpillar out unprotected. During the two minutes we turned our backs to pick, a European Paper Wasp came by and attacked. By the time we saw, the wasp had already bitten a whole in the caterpillar and within seconds, a huge pool of internal fluids were spewing out of the wound. Of course, we got the wasp away to see if it was still saveable but from the looks of it, the damage had already been done and the caterpillar would soon bleed to death. Within a few hours after we took it back in, it died. 

To conclude, NEVER leave caterpillars (or other insects) unprotected outside if you have wasps roaming around your area, even for a few moments. 

- Brian