Okay, I admit that was a little unfair. I’m not sure it would be possible to identify the host plant from that photo. So here’s a better picture of the plant.
a) What are these?
At first glance, one might look at these and wonder if they’re eggs. But they’re not separate from the leaf, but an outgrowth of it (difficult as that may be to tell from the original photo).
These are plant galls. A gall is just an abnormal growth of plant tissues, which can be caused by fungi, bacteria, insects, mites, and even parasitic plants.
b) Which plant are they on?
This is a chokecherry (Prunus virginiana), a widespread shrub in North America. It produces abundant very astringent berries which are an important food for animals (and with lots of sugar, make very tasty jelly for humans).
We’re two-thirds of the way there now. Many gall-producing arthropods are very host-plant specific, and we now know that these are a) galls and b) very likely specific to chokecherries and perhaps related plants.
c) Who made them?
A search for “chokecherry gall” brings up the Chokecherry Gall Midge (Contarinia virginianae ). Sounds promising, but it looks like this tiny fly’s larve induce galls on the fruit. But these galls are on the leaves, and look very different. So adding “leaf” to the search term, we get…
The Chokecherry Finger Gall Mite (Eriophyes emarginatae), which creates finger-shaped galls on the leaves of various plums and cherries, including chokecherries! Looks right to me:
A mite is a tiny arachnid related to ticks. Gall mites of the family Eriophyidae are microscopic plant parasites. Some are major crop pests, while others can be used as biocontrol for weeds and invasive plant species.
So, what’s the take-home message here?
If you’re interested in identifying insects and other arthropods, a working knowledge of botany can be invaluable. Many groups–from gall mites to butterfly and moth caterpillars–are adapted to use only specific plants, and if you can identify the plant they’re living on or eating, it can vastly narrow your options. There are about 3,600 known species of gall mites–and there may be 10 times that number in reality–but identifying the plant as a chokecherry helped me to an identification pretty quickly.
And if you’re planning to collect and raise caterpillars at home, knowing how to identify their appropriate food plants is the difference between successful rearing and a dead caterpillar.
In the future, I’ll be writing a few more posts about different aspects of arthropod (mostly insect) identification from the point of view of a non-specialist who’s relatively new to this. It’s great to be able to use field guides–when they exist–or complicated taxonomic keys, but those can be pretty overwhelming initially, and I hope my less technical methods will be helpful to someone. My methods won’t work with all groups–species in some groups of insects can only be identified by close examination in hand, dissection of genitals, or other fiddly techniques that are out of reach to many people. But they have provided enough to keep me pretty busy, and they’re a start.