Nasturtium Pests: Blackfly, Caterpillars and Using It as a Trap Plant

Blackfly will find it, and that is the point

Black bean aphid arrives in early summer and finds nasturtium more attractive than almost anything else you grow. Colonies build on the leaf stalks and the undersides of leaves, thick enough to turn a green stem black, and the plant carries on flowering underneath them. That combination, being irresistible to the pest and largely unbothered by it, is what makes nasturtium the classic sacrificial trap plant next to broad beans and runner beans.

So the first decision is whether a given plant is a crop or a decoy. On a decoy you leave the blackfly alone, because a clean decoy attracts nothing. On plants you intend to eat you keep them clear. Grow both, several feet apart, and do not try to make one plant do both jobs.

1. Plant decoys upwind and slightly ahead Put a nasturtium every three feet (90 cm) along the windward side of a bean row, sown at the same time as the beans so it is established when the first aphids fly in.
2. Leave the decoy colonies alone Resist squashing them. A decoy carrying aphids is drawing ladybirds, hoverflies and parasitic wasps into the bed, and those predators then work on your beans.
3. Keep eating plants well away Six feet (2 m) or more between the decoys and the plants you pick from, and inspect the eating plants twice a week regardless.
4. Pull a decoy that is collapsing If a decoy goes yellow and sticky and stops growing, pull it out and bin it rather than composting it, and put in a fresh one if there is still season left.

The trap works by preference, not by repelling anything. Do not expect nasturtium to keep aphids off beans on its own: it concentrates them, and the predators do the rest.

Clearing blackfly off plants you eat

On eating plants, deal with aphids by hand and with water. A hard jet from a hose knocks off the great majority and few of them climb back up, and a repeat two days later usually settles it. Squashing a colony between finger and thumb works on a small plant and takes seconds. Neither method leaves anything on the leaves you are about to pick.

Do not spray anything you would not eat. This is a crop where you harvest leaves, flowers and pods continuously through the season, so there is no useful interval between spraying and picking. If a plant is beyond hand control, cut it back hard, which removes most of the colony with the growth, and let it regrow clean.

Ants farm the aphids and defend them from predators. If you see a steady line of ants running up a stem, deal with the ant trail as well or the colony keeps being restocked.

Cabbage white caterpillars strip it fast

Nasturtium is in the same chemical family as brassicas, so the large and small cabbage white butterflies lay on it readily. Large white lays in yellow clusters of thirty or more on the leaf undersides, and the caterpillars can reduce a plant to bare stalks in under a week. Small white lays single pale eggs and the green caterpillars sit along the leaf veins where they are hard to see.

Because of that, nasturtium near a brassica bed is a mixed blessing: it draws egg laying away from the cabbages, but it also builds up the local population if you let every batch mature. Check the undersides twice a week from early summer and rub off the egg clusters with a thumb. That single job, done regularly, prevents almost all the damage.

1. Turn leaves over twice a week Look at the underside of a dozen leaves each time, concentrating on the largest ones near the center. Yellow clusters and single white eggs both rub off easily with a thumb.
2. Pick caterpillars off in the evening They feed openly at dusk. A five minute pass with a pot at dusk clears more than an hour of daytime searching.
3. Look for the frass, not the caterpillar Dark green pellets on a lower leaf mean something is feeding directly above. Follow the trail upwards and you will find them.
4. Accept some damage on a decoy On a trap plant near brassicas, holed leaves are the plant doing its job. Only intervene when the plant itself is being killed.

Slugs, mildew and the problems of a dense mound

The other troubles all come from the same source: a nasturtium mound is dark, damp and airless underneath. Slugs and snails shelter there through the day and shred the seedlings and the lowest leaves at night. Powdery mildew shows as a gray bloom on older leaves in late summer, and stem rot appears where a runner sits on constantly wet ground.

None of these needs a treatment so much as a change in how the plant is grown. Thin the growth, lift the runners off wet soil, water in the morning and at the root, and space the plants properly at sowing. A plant with air under it rarely gets any of this.

Yellow mottling that spreads from plant to plant is usually virus carried by aphids, and there is no cure. Remove those plants and do not save seed from them.

Keep a record of when the pests arrive

Blackfly and cabbage whites both appear within a fairly narrow window each year, and that window shifts by only a few days between seasons in the same garden. Note the date you first see aphids on a nasturtium, the date the first yellow egg cluster appears, and which plants were hit worst. Next year that tells you when to have the decoys already established rather than just sown.

Also record which nasturtium stations attracted the heaviest infestation. Aphids favor soft, sappy growth, so the worst affected plants are usually the ones on the richest soil, which is the same reason they were not flowering. The pest map and the fertility map end up being the same map.