Your brassicas were doing so well. But suddenly they’re struggling, especially the seedlings. The leaves look blueish or yellow and they’ve wilted, perhaps on a sunny day. Heads of cauliflowers blow prematurely, and cabbages have no hearts. Watering and fertilising doesn’t restore them. Young plants may have even died off altogether. So what’s the problem?
If any of your cabbage family plants are displaying the symptoms above, there’s a good chance that there’s a specific pest at work: cabbage root fly, also known as cabbage root maggot (Delia radicum). Unfortunately it’s hard to know for sure why your plants have sickened until you dig them up and find their roots munched to a stump or, in the case of root crops such as swedes and radishes, riddled with tunnels.
We talk about this a lot at GrowVeg, but when it comes to pests, prevention really is better than a cure. Cabbage root maggots are no exception. So let’s find out more about these gluttonous grubs, and what can be done about them.
Cabbage Root Maggot Life Cycle
Cabbage root flies, which are pretty much identical to houseflies in appearance, lay tiny white eggs on the soil surface near the stems of cabbage family plants. There are two generations each year – in late spring and early to midsummer – and sometimes a third generation follows in late summer or early autumn.
The eggs take up to seven days to hatch into legless white maggots that look like grains of rice. They burrow into the soil in search of tasty brassica roots and may munch their way into turnips, swedes and radishes, leaving tunnels similar to those created by carrot root fly larvae in carrots.
Seedlings are very susceptible to this feeding and can be killed by it. Older plants are often strong enough to pull through, but the size of the final crop will probably be affected. With few roots left to anchor them into the soil, badly affected plants will be easy to pull up. You might even spot the larvae amongst the remaining roots.
Once the maggots have eaten their fill, they pupate in the soil. They’ll then emerge as adult flies within a few weeks or overwinter in the soil.
“Don’t put that cardboard in the recycling,” I said. “I need to make some cabbage collars.”
My partner looked surprised. “Are you taking them for a walk?” he asked. Nope – just protecting them from cabbage root fly!
Cabbage collars are placed on the soil around the stems of brassica plants, the idea being that any root fly eggs laid on top of it will shrivel up and die before the larvae have a chance to hatch.
You can buy cabbage collars but they’re a bit pricey, so I decided to make my own. Longer-lasting alternatives to cardboard would be roofing felt or carpet underlay, but if you have cardboard to hand by all means use it – just be prepared to renew it later on.
Cabbage collars are very easy to make. Simply cut circles or squares 10-15cm (4-6in) in diameter, make a slit to the middle then cut a Y-shape. Slip the collar around the stem of the plant. You might need to drench it with water so that it lies flat on the soil surface. And that’s it!
Cabbage collars are not foolproof. Strong winds or growing stems can dislodge them, they can become buried in the soil when hoeing around the plants and, of course, cardboard collars will inevitably rot down – but they are worth a try! If care is taken to keep them in place they will help suppress weeds and keep the roots cool and moist too.
Other Ways to Prevent Cabbage Root Fly
If you’ve had cabbage root flies before, there’s every chance you’ll see them again. The first step to minimising problems is to dig up any plants you believe to be affected and bury them deep in your compost heap, where the larvae will almost certainly expire.
Rotate crops to help reduce problems in subsequent seasons. The adults are flies however, so they will, unsurprisingly, fly to another bed or another garden to locate a promising host plant. Protect brassicas with fine mesh netting or horticultural fleece supported on hoops or frames and well-secured at ground level. Alternatively, use vertical barriers made of the same stuff. Cabbage root flies tend to fly low as they search for promising egg-laying spots and are unlikely to peek over the top to check what’s inside!
Mulch during the growing season to encourage grub-hungry beetles. Try spreading a layer of wood ash or diatomaceous earth around the stems to deter the flies from laying their eggs. For plants that you suspect are already affected, pile compost around the stems. The plants will sprout new roots into the compost, which may give them just enough energy to keep going despite the initial damage.
From time to time during the winter, hoe and fork the soil around overwintering host plants and anywhere brassicas have been grown in the past season. This brings the pupae to the surface where insect-hungry birds and beetles can dispatch them. Help nature to help itself!