Gadenin日本語Get the appFlea beetle is the defining problem
Arugula is a brassica, and flea beetles find it faster than they find cabbage. The damage is unmistakable: dozens of small round holes, an eighth of an inch (3 mm) or less across, punched through the leaf blade so that a bad case looks like lace held up to the light. Gardeners call it shot-holing.
The beetles themselves are tiny, shiny, dark, and they jump like fleas when you disturb the row, which is the quickest way to confirm what you are dealing with. Look on the undersides of the leaves in the middle of a warm afternoon and you will see them.
A mature plant tolerates a surprising amount of this. A seedling does not. Beetles hitting a stand at the seed leaf stage can kill the row outright in three or four days, which is why the timing of your defense matters far more than its strength.
| Conditions | Pressure | What happens to the crop |
|---|---|---|
| Warm dry spring, first flush emerging | Very high | Seedlings destroyed within days |
| Hot dry summer | High | Constant damage, leaves unsaleable but plants survive |
| Cool wet weather | Low | Beetles inactive, clean leaf |
| Cool autumn, shortening days | Low to moderate | Minor damage, the cleanest crop of the year |
| Under cover in winter | Very low | Effectively none |
The single most effective non-chemical control is choosing the season. Late summer and autumn sowings routinely come through with clean leaf while a spring sowing in the same bed is shredded.
Covering the crop, done properly
Fine insect mesh is the only method that reliably delivers unblemished arugula. It works because it is a physical barrier, and it fails for exactly one reason: it went on too late, or it has a gap.
Use mesh with a hole size of about 0.8 mm. Standard row cover works too and adds a little warmth, but it holds humidity in summer and can bring on mildew, so mesh is the better choice once the weather warms.
- Lay it on the day you sow Not when the seedlings appear, and certainly not when you see the first holes. Beetles find an emerging brassica within hours, and a cover put on afterwards simply traps them inside with the crop.
- Seal every edge with soil Bury the perimeter under an inch or two of soil or weigh it down the whole way round with timber. A six-inch gap at one corner makes the entire cover pointless.
- Give it height with hoops Mesh resting on the leaves lets beetles feed straight through it, and in sun the contact points scorch. Wire hoops holding it four to six inches (10 to 15 cm) clear solve both.
- Do not cover ground that grew brassicas last year Flea beetles overwinter in soil and debris. Covering a bed that already holds them seals the pest in. Rotate to a bed that has not carried brassicas for three years.
- Lift it only to harvest, and close it again Cut in the early morning while the beetles are sluggish, and put the mesh straight back. An afternoon left open in warm weather undoes a fortnight of protection.
What to do when you have no cover
If the crop is already up and already being eaten, the aim shifts from prevention to keeping the plants growing faster than the beetles can damage them. A vigorous well-watered plant outgrows moderate shot-holing; a checked one does not.
Flea beetles dislike moisture and shade. Watering the row in the middle of the day, which is normally poor practice, genuinely suppresses feeding in a dry spell, and a mulch stops the next generation emerging from the soil beneath the plants. Some gardeners sow a strip of a coarser brassica along the sunny edge of the bed as a trap crop, and it does draw a share of the beetles away, though it is a partial measure rather than a solution.
Sticky yellow traps set just above the foliage catch adults and, more usefully, tell you when a flush has started so you can act before the damage shows.
The rest of the cast
Flea beetle takes the attention, but three or four other things regularly damage an arugula bed, and they are easy to tell apart once you know what to look at. The distinction that matters most is between holes with clean edges, holes with slime, and holes surrounded by discolored tissue.
| Problem | What you see | When | What to do |
|---|---|---|---|
| Flea beetle | Many tiny round holes across the whole blade | Warm dry weather | Mesh from sowing, water and mulch |
| Slugs and snails | Large ragged holes, slime trails, damage from the edge in | Wet weather, overnight | Clear debris, trap at night, keep the row clean |
| Cabbage caterpillars | Big irregular holes with green droppings below | Mid to late summer | Mesh keeps the butterflies off, otherwise pick them off |
| Aphids | Clusters on the undersides, sticky curled leaves | Warm spells, soft overfed growth | Wash off with water, reduce nitrogen |
| Cabbage root fly | Sudden wilting, roots eaten away below ground | Spring and early summer | Collars round transplants, rotate the bed |
| Leaf miner | Pale winding tunnels inside the leaf | Late spring onward | Remove affected leaves, mesh excludes the fly |
Diseases, and telling them apart
Arugula is a short-lived crop, so it usually gets harvested before disease matters. The exceptions are a dense unthinned stand in wet weather, and any crop grown under cover where the air does not move.
Almost all of it comes down to leaf wetness. Water the soil rather than the foliage, thin so air can pass between the plants, and ventilate a covered bed on every day that is not freezing, and you will avoid most of what follows.
| Disease | Symptom | Conditions that bring it | Response |
|---|---|---|---|
| Downy mildew | Yellow angular patches above, gray fuzz on the underside | Cool damp nights, crowded stand | Remove affected leaves, thin, stop overhead watering |
| White rust | Raised chalky white blisters on the underside | Cool wet spells | Pull affected plants, do not compost them, rotate |
| Damping off | Seedlings collapse at the collar and topple | Cold wet soil, sowing too thick | Sow thinner and shallower, improve drainage |
| Gray mold | Fuzzy gray growth on cut stubs and old leaves | Under cover, still humid air | Clear cut debris after harvest, ventilate daily |
| Bacterial leaf spot | Small dark water-soaked spots, often angular | Splashing rain on a wet crop | Remove leaves, mulch to stop splash, avoid working the bed wet |
Keep a note of the date you first see each problem and which bed it was in. Flea beetle in particular arrives at almost the same point each year in a given garden, and once you have two seasons of dates written down you can have the mesh on before the beetles arrive rather than a week after.