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

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

A few weeks ago we collected a few Monarch (Danaus plexippus) eggs since the species is so common this year, but now the caterpillars have all been killed by European Paper Wasps (Polistes dominula).

Back on 6/24 we decided to collect a few Monarch (Danaus plexippus) eggs just for the heck of it even though we were (and still are) bogged with tons and tons of various rearing projects this summer. The butterflies are so extremely abundant this year (a record high in over a decade according to some sources!) and finding eggs (or catching butterflies to lay eggs for you) is not a hard task. In just a half an hour we had collected 18 eggs without really even trying at all and brought them home to raise casually (more detailed in this post). But since then, we have gotten a bit lazy with them and decided to just stick them outside on our Tropical Milkweed (Asclepias curassavica) with in rearing sleeves. Big mistake. . .

Now, don't get me wrong here. Rearing sleeves for raising caterpillars outdoors on live plants is an excellent way to do it and is probably the best way that exists when looking for quality. It provides a lot of space and natural environment abd doesn't mess with their circadian rhythm. Most importantly, it provides much better sanitation and ventilation than most indoor rearing enclosures since caterpillars get to eat leaves fresh off the plant and don't have to swim in their own frass all day, decreasing the risk for disease outbreaks by a huge margin. So what exactly went wrong with our Monarch caterpillars? Well, as background information, the rearing sleeves were home-made out of window screening and clipped around the plant using clothespins. Not to say they were poorly designed, but they weren't terribly effective at keeping out small invertebrate predators.

The protective mechanisms of the sleeve is actually two-fold. The first line of defense is to conceal the area so that predators have a harder time detecting the caterpillars through vision. The second line of defense (if the first fails) is to create a physical barrier between the caterpillars and the predator. But in this case, the rearing sleeve failed to do either because we discovered that a whole bunch of European Paper Wasps (Polistes dominula) somehow managed to get their way into the rearing sleeve (don't ask us how they did it because the sleeve was shut tight on all sides with no openings at all that we knew of!). We caught them red-handed ripping apart the caterpillars and by today, only one Monarch caterpillar was left. Not to mention, a whole bunch of our Anise Swallowtail caterpillars (Papilio zelicaon) in rearing sleeves and some of our ovipositing females were also killed and eaten by the same wasps. 

The European Paper Wasps are incredibly intelligent. In fact, Hymenopterans (ants, bees, wasps, and sawflies) in general are arguably the most intelligent of all insects. The adult European Paper Wasp population exploded overnight one of these days when summer hit and now they are everywhere, searching on popular caterpillar host plants such as milkweed (Monarchs) and fennel (Anise Swallowtails). They clearly recognize that in our particular region, these plants are the most likely to have caterpillars and they search these almost exclusively over others; we also currently have Cecropia (Hyalophora cecropia) and Ceanothus (Hyalophora euryalus) outside in rearing sleeves on their respective host plants, and they have so far been completely untouched by the wasps who show no interest in them whatsoever. Once they hone in on their prey, they rip it up with their monstrous looking jaws and carry bits of the flesh away to their nest. The things are some of the hardiest insects out there and are extremely hard to kill. Today we tried crushing one with a rock but it was still alive and flew away even after being brutally smashed due to the fact that its head is so incredibly tough and unbreakable. 

Clearly, the best time to raise caterpillars outdoors is during the spring when the wasps are virtually nonexistent. Like stated earlier, they all of sudden came out of nowhere in huge numbers when summer started. Back in the spring when we put about fifty Anise Swallowtail caterpillars outside on the fennel without any rearing sleeve at all and not one was killed by predation. If we were to do that now, not one would be remaining within two days! 

Anyway, that is it for those Monarchs. It is not a terribly big loss since we weren't terribly into raising them in the first place when we first caught them. Even now we can always go back and collect more eggs since there are a whole bunch on our milkweed as of today. Plus, the butterflies still haven't disappeared, though they do seem to be a little less common than before. 

- Brian