Celebrating Silky Willow

Simply not mowing the lawn, and welcoming whatever plants decide to grow in its place, has done wonders for the biodiversity of our yard. But we have also welcomed gifts of native plants from friends, and today I’d like to shine a light on a silky willow (Salicaceae: Salix sericea) shrub that Adam Kohl gave us a couple of years ago. We planted it in a moist spot at the edge of the woods, and last year it was discovered by several insects I hadn’t seen in the yard before. The first I noticed was this sawfly larva, which appeared on May 27:

Although I’ve made some good progress on my guide to sawfly larvae this winter, I haven’t yet tried to make a key to the willow-feeding species—and I think there are more sawflies on willows than on any other plant genus—so for now all I can say is that this is something in the subfamily Nematinae (Tenthredinidae). I of course collected it to see if I could rear it to an adult, and the next day it turned purple, indicating that it had entered the non-feeding prepupal stage.

I put it in a jar of soil, since most sawfly larvae burrow into the ground to pupate, but after wandering around for two days it spun a cocoon between two leaves.

So far, no adult has emerged; maybe one will when I take all my overwintering bugs out of the fridge in a few days.

When I visited the silky willow on June 2, I noticed evidence of a leafminer I hadn’t seen in my yard before:

A larva of Caloptilia stigmatella (Gracillariidae) had made a little underside tentiform mine in one leaf, then moved to a fresh leaf and used silk to roll the tip into a cone, where it was now feeding on the leaf externally, but within the shelter of the cone (as is typical of Caloptilia species). Both the mine and the cone were more conspicuous from below:

Caloptilia stigmatella has been reared from a variety of willows and poplars, but I have seen no records of it from silky willow, so I collected the leaf with the cone to make absolutely sure it was that species. Six days later the larva finished feeding and spun a cocoon:

Alas, before I discovered the Caloptilia larva, an ichneumon wasp had already laid an egg on (or in) it, and a week and a half after the cocoon was spun, her daughter emerged from it:

Meanwhile, on June 12 I noticed some more underside tentiform mines on the silky willow, but these were larger and more “tented,” indicating they were made by some species of Phyllonorycter (also Gracillariidae) instead of Caloptilia. Phyllonorycter species complete development in their mines instead of exiting to feed in leaf rolls.

If you tried to identify these mines using my key to willow leafminers, you’d get to Phyllonorycter and would then find that you need a reared adult to get further, because the leaf mines of the willow-feeding Phyllonorycter species all look pretty much the same. So I collected this leaf, and adults emerged from both of the mines five days later, leaving their pupal skins poking out.

Now, if the antenna weren’t obscuring the white streak along the costal margin at the base of the wing, the key I made would correctly identify this as Phyllonorycter scudderella (which also has not been reported from silky willow before). However, the first step in Davis & Deschka’s (2001) key to Salicaceae-feeding Phyllonorycter species ignores all external features and asks whether or not the male genitalia have symmetrical valvae. Here’s the answer:

The left valva is big and broad and the right one is little and shrimpy, just as in Davis & Deschka’s illustration for P. scudderella, and from there their key easily takes my moth to that species based on wing pattern. (For whatever reason, I am unable to get photos in sharp focus using my compound microscope’s lowest magnification, so I took three photos at higher magnification and quickly spliced them together here.) A year ago I posted photos of the genitalia of P. nipigon, which has symmetrial, slender valvae; see that post and the previous one for some explanation of what dissecting these tiny moths involves.

Anyway, on June 12 I noticed an adult sawfly on the silky willow, and I thought it might be the same species as the larva I’d collected two weeks earlier…

…but it turned out to be Thrinax dubitata (Tenthredinidae: Selandriinae), whose larvae eat sensitive fern (Onocleaceae: Onoclea sensibilis). Which makes sense, because there is lots of sensitive fern all around that willow. But this adult sawfly was more than casually interested in the willow, and for quite a while after that, every time I walked by, I noticed that numerous very tiny wasps (~1 mm long) were also very interested in it, scurrying all over its leaves. On July 7 I finally got around to bringing my camera over and investigating what they were up to—as well as confirming my suspicion that they were playgastrids. They scattered each time I got the lens near them, but I managed to get five of them in the frame in one shot:

All platygastrids (as the family is currently circumscribed) are parasitoids of gall midges (Cecidomyiidae), and there were no midge galls in sight. So what were these little wasps up to? It only took a few moments of watching them to realize that they were drinking from extrafloral nectaries along the leaf margin. In the photo below, note that each of the leaf serrations has a round appendage on the left side, and each of these appendages has a little droplet of delicious honeydew bubbling out of it.

Here’s a closer view of three of these nectaries from the underside of the leaf, with the droplets coming out on the right side:

I had previously only noticed extrafloral nectaries on the petioles of leaves (like cherry), so this was all news to me.

Getting an in-focus photo of one of these wasps, as it briefly sipped from one nectary before moving on, was pretty challenging, but I at least managed to document them doing it.

As I explained in my previous post about extrafloral nectaries, the idea is that they attract predators such as ants, whose presence will result in fewer herbivorous insects munching on the leaves. I suppose it’s possible that these platygastrids could be helping the plant by defending it against gall midges, but galls don’t really impact the health of the plant and I think these wasps are pretty much freeloaders. There was, however, a big ol’ ant that came along to partake of the nectaries while I was trying to get pictures of the platygastrids.

And, in retrospect, that’s what the adult sawfly had been up to as well.

Obviously attracting sawflies isn’t necessarily what the plant is going for, since larvae of many species eat willow leaves, but adults of some species are predatory—in fact, here is one chomping on a platygastrid—and the yellow dusting on this one’s head and thorax indicate that it has also been visiting flowers for nectar, so it is a potential pollinator as well. As is so often the case, whether an insect is “beneficial” or “detrimental” is not so clear-cut here, and it makes no difference to me. The bugs that come to eat the plants, and the bugs that come to eat those bugs, are all equally welcome in my yard.