Last year I posted a question about mysterious black bugs on my tomato plants, and we never got to the bottom of it. The bugs are back - this year even worse than before. I found them on my potato plant too. The damage looks like many small holes.
Here are pictures of the plants and damage:
Potato Plant (this first one I think they're mating):
You told me to capture one of the bugs, put it in the freezer, and then take close up pictures of its mouth parts. As I tapped a few of the bugs into a tupperware for freezing, I noticed the bugs move almost like fleas. They hop super fast. You see the bug, then it vanishes and reappears a few centimeters or inches away. Even so, it wasn't difficult to capture a few of them.
Unfortunately, I didn't get any good pictures from the frozen bugs. My camera takes pretty good closeups, but the bugs are so small I couldn't get pictures with any resolution of their body parts. Instead, I tried to get a few more pictures showing them on the leaf:
That's the best my camera can do. What do you think?
So, this is a different kind of "little black bug" than the one from last year. I'm still not sure what those things were, but they definitely weren't beetles. These are clearly Flea Beetles. These little guys are kind of cute, but they can do a lot of damage to seedlings.
You can tell these are beetles because of the hard shell that covers their wings. It's called a carapace. If the insect is at rest, and you can still see clear wings, then it probably isn't a beetle. If you see a hard shell, it's likely a beetle. I was already thinking flea beetle because they are so small, but you also discovered how they got their name: They jump like fleas! The damage also fits. Flea beetles are one of the relatively rare pests in which the adult causes the most damage. They chew the leaves and create holes we call shotholes, because it looks like buck shot (rabbit shot? tiny thing shot? I don't know much about shotguns).
They're a common pest in spring and early summer. The adults lay eggs in cracks in the ground around the plant. The grubs feed on plant root hairs, but they rarely do much damage. It's the adults who cause the most problem, chewing through seedling leaves. They don't usually hurt an established plant, which can handle a surprising amount of damage without dying. Well, if they chew holes in a beautiful lettuce plant, it probably still sucks. With a tomato plant's leaves, though, you're probably ok if the plant is established. If it looks like they are really hurting the plant, there are a few options.
These guys don't respond very well to pesticides, and they're also pretty good at evading natural enemies (the jumping thing). It can make it difficult to control them. Some people put out sticky traps (those yellow cards covered in resin), but I've heard they don't work very well. Diatomaceous Earth works really well on beetles. It scratches the wax layer on their exoskeletons and makes them lose water. You can apply it to the leaves. It shouldn't hurt the plant. The only problem is that you have to apply it after every rain or watering that gets on the leaves. Row covers or any kind of mesh cover will work to exclude them in the first place. If you ever have them on your seedlings, then this is a great way to control them. You can try it on established plants too; you just need to remove them sometimes for pollinators. To be honest, though, a lot of people vacuum them. Do you have a handheld DirtDevil or anything like it? They work really well, because they won't suck up the plant leaves. You can use a regular vacuum attachment, but you'll need to be really careful that you don't damage the plant leaves. Just hold it at an angle pointing to the sky above the plant. When you do this, make sure you don't block out the sunlight. A good shadow will make these guys scatter. Part of the reason they are difficult to control comes from their ability to disperse quickly. So, try to sneak up on them. I've never tried it myself, but I've always wanted to give this particular piece of advice. (Mostly because it makes me giggle to think of stalking tiny beetles with a vacuum cleaner). They actually make bug vacuums, which are mostly for kids or people scared of bugs in the house. These are probably over-priced for what you get. We used a keyboard vacuum (you know, the kind that picks up dust) to remove moths from the wind tunnel in grad school. It worked really well.
Now I'm going to go back to the previous black bug post to look at the damage again. I think some of the damage may look like flea beetle damage. If they hid when you approached, it's possible that your picture was of the wrong culprit.