Visibly different, part 34

This week we have another current set of images, all taken within a few days and since last week’s offering.

One of the many things that we planted this year was tomatoes, and started a serious number of tomato vines in various locations. As always, the hornworms eventually discovered these and started doing their damage – in this case it was the tobacco hornworms (Manduca sexta, actually the larva of the Carolina sphinx moth.) Tomato and tobacco plants are closely related, and the tomato and tobacco hornworms will feed freely on either, though I’ve only ever seen the tobacco hornworms in the past several years; they’re distinguished by the white markings on the sides, diagonal lines for tobacco, chevrons for tomato.

I had significant help in controlling them this year, though, because the braconid wasps were active early on, as determined by the number of hornworms that I found already sprouting cocoons.

tobacco hornworm Manduca sexta larva showing cocoons of braconid wasp
Braconid wasps (there are 1700 subspecies, so no I’m not going to be specific) lay their eggs in the bodies of numerous species of caterpillar, where they hatch and the larva begin eating the caterpillar from the inside. As they reach pupal age, they burrow out of the skin and create cocoons on the outer surface of the caterpillar’s skin, where they will eventually emerge as adults. This is, as you might imagine, a bit hard on the hornworms, and though they’ll remain alive throughout the process, they expire soon afterward. I found quite a few examples of hornworms so adorned, which made my job easier.

The other evening, I found the one above, and another within a handful of centimeters but lacking any evidence of braconid attentions.

tobacco hornworm Manduca sexta larva without cocoons
Given their proximity, I was a bit skeptical that this one had actually escaped the wasps, and suspected that it would soon erupt in cocoons, so I left it alone on the tomato plant alongside the other (which I knew would be doing minimal damage, if any, beyond this point.) And I made sure to check from time to time.

Notably, it wasn’t moving or feeding much, which I considered a good sign (good for me and/or the braconids, not so much for the caterpillar.) I wasn’t checking it frequently, but I did make it a point to examine the back for any signs of egress, never spotting anything. Three days later, however, my suspicions were confirmed.

same tobacco hornworm Manduca sexta larva now showing new braconid wasp cocoons
Yep, that’s a serious litter right there, but wait – what’s that in the center (that I didn’t fully notice until examining the photos after unloading)? Yeah, that’s a braconid larva/pupa, in the process of spinning its cocoon. This made me go back out to examine it closely, with the possible intention of doing some video of the process.

Alas, it was barely moving, so video was pointless, but I did collect the branch with the hornworm and bring it in for more controlled ‘studio’ work – this allowed me to go for the serious magnification without worrying about my own steadiness, the wind, getting the flash past the nearby leaves, and so on.

braconid wasp larva/pupa newly emerged from body of tobacco hornworm Manduca sexta larva
There’s a wad of silk barely visible behind the head on the left, but the larva’s movement was sparse – certainly not seeming to make a cocoon. I couldn’t vouch for what was happening here, but another discovery on the opposite side of the hornworm might be a clue.

tobacco hornworm Manduca sexta larva showing exit wound and started cocoon from braconid wasp
It would be easy to believe this was a cocoon that had already hatched the adult, but a) it’s way too soon, and b) I’ve seen them hatch for one species at least, and that one popped little caps from the top of the cocoon. Was this one started by my naked subject up there before it got disoriented or something? I can only speculate, but I will point out the exit wound at the base of the cocoon, while that orange spot is the hornworm’s spiracle, what they breathe through.

I may hang on to this guy and see if I can capture the adults emerging this time; the endeavor really needs some kind of motion-sensing camera that can run for days and triggers the video when the action starts, and come to think of it, this might not be that hard to do (though I doubt I’m going to go to those lengths.) You’ll know if I’m successful, of course.

As for the tomatoes, we’ve just about written them off. Despite a better start than any previous year and numerous plants, they yielded almost nothing, due far more to the heat than predation, and it’s late enough now that I’m no longer bothering to water anything not showing active fruit. The peppers have just started coming ripe, and the basil did exceptionally well – there’s a couple jars of homemade pesto in the fridge now. We just have to select the stuff that can handle these conditions.

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