Flea beetles

lots of tiny round holes and shallow pits peppered across the young leaves, so the leaf looks like a sieve or like it was hit by birdshot ("shot-holes"). On a badly-hit seedling the little holes join up, the leaf dries brown at the edges, and the whole plant wilts and can die.

look closely in the warm morning and you'll see very small, shiny beetles — about the size of a pinhead or a sesame seed (2-3 mm). Most are shiny black, some are metallic blue-green or have pale stripes down the back. The dead giveaway: when you touch the plant or wave your hand over it, they jump away like fleas on their fat back legs.

they feed on the top of the leaves in warm, sunny mornings, then drop to the soil, under leaves, or into cracks and crop trash when it is hot at midday or cool in the evening. They arrive from weeds (especially wild cabbage-family weeds) and the stubble of a recent crop. On sweet potato the pale grubs also scar the roots underground, so a seedling can wilt with nothing showing above the soil.

caterpillars and grasshoppers leave big ragged bites from the leaf edge, not many small round holes. Slugs and snails leave shiny slime trails and feed at night. Diamondback moth grubs scrape thin see-through windows on the underside of brassica leaves. Only flea beetles make many small round shot-holes AND jump like fleas when disturbed — those two signs together are your certain answer.

check every few days from the moment seedlings come up and through the first few weeks after transplanting — the seedling and early-growth stage, while plants are small and soft. This is the only window that matters; once plants are big and leafy they shrug the damage off, so don't waste effort scouting a mature crop. Look extra hard during hot, dry spells, when beetle numbers suddenly climb.

scout in the warm morning, when the beetles are up on the leaves and easy to see. Walk quietly — if you thump around they jump off and hide before you spot them.

walk a big "W" shape across the seedbed or field so you cover all four corners and the middle, not just the row by the path where you always look. Make about 5 stops along the W.

look at 10 seedlings — count the fresh shot-holes on the leaves and watch for the tiny jumping beetles. Check the growing tip and the newest leaves of young transplants, where feeding shows first.

act now if seedlings are averaging more than 2-3 fresh shot-holes per leaf, or if you see any wilting of very young seedlings. At that point the beetles are ahead of the crop and you must step in the same day — waiting even a few days in hot, dry weather can cost you the seedlings.

as soon as you hit the threshold, treat the seedling leaves with a neem spray or a dusting of wood ash and kaolin (fine white clay). This grits and coats the leaves so the beetles won't feed, and it buys the seedlings time to grow past the danger. Apply in the cool of the morning or evening, cover the tops and undersides of the leaves, and re-apply after rain because it washes off.

use a pyrethroid (a common contact insecticide — "contact" means it kills only the beetle it lands on, so you must hit them directly). You can treat the seed before sowing or spray the seedlings at threshold. Safety — every time: cover your skin, eyes, nose and mouth; mix and spray in the cool of the day with the wind behind you so the spray blows away from you; keep it off food, off open water, and away from children, animals and bees; wash yourself and your clothes afterwards; and respect the pre-harvest interval — the number of days on the label you must wait after spraying before it is safe to eat or sell the crop — which matters a lot for leafy vegetables you eat soon.

if you must spray more than once, rotate to a different group of chemical each time (not just a different brand of the same pyrethroid), so the beetles don't get used to it and stop dying.

re-cover the seedbed with netting, widen your weed and stubble clean-up into the neighbouring plots the beetles are coming from, and repeat the botanical dust every few days. If the seedlings are already killed or badly stunted, cut your losses: pull out the ruined seedlings and re-sow into a clean, netted bed away from the infested ground rather than nursing plants that will never catch up — and put that patch under a non-host crop next season to break the cycle.