During a post-vacation check-up of my heat-challenged, newly planted Japanese maple this morning I spied this pixel-patterned egg mass.

Aha! I exclaimed, summoning my vast store of entomological knowledge, Those are definitely the eggs of some kind of insect. No, I’m not an entomologist. But I can be a fairly good detective when it comes to mysteries like these. So I whipped off the leaf to ID this swarm-in-the-making.

The eggs are small, white, smooth (not fluffy) and laid in large numbers (not singly), narrowing the probable ID to Datana ministra (yellownecked caterpillar), a moth larva which is a common pest on hardwood trees in Ontario.

Bugs can’t read, or the mum of these eggs would have known that she’s supposed to lay them on the underside of the leaf, and she’s supposed to do it in June and July. Luckily for me and my tree, she mistakenly deposited her brood on the top of the leaf, making it easy for me to spot.

One info sheet described these guys as “gregarious” because they chow down in numbers – though not in as large a mass as tent caterpillars.

While not a major danger in forests, where natural predators help with control, they can do significant harm to a small ornamental tree like my baby Acer palmatum. Newly hatched caterpillars skeletonize the leaf; older ones chomp up the entire leaf tissue, leaving only larger veins and stems. If the specimen is small, or if the attacks recur over subsequent seasons, this can seriously weaken the tree.

Other members of the Datana family feed on shade and fruit trees such as oak, chestnut, linden, elm, serviceberry and apple.

Once hatched, the caterpillars feed for about a month, shedding their skins between instars or nymph stages, before dropping to the earth to pupate, and emerging next year to start the cycle over. They only produce one generation per year.

If you find the black and yellow or orange striped caterpillars feeding on your leaves, simply pick them off and drown or squish them. Use garden gloves, however, as some use a defensive secretion that’s irritating to the skin. Even better: keep a close watch and you might be able to catch them, as I did, at egg-sactly the best time.