A few weeks ago I harvested and rescued what I could from my onion and garlic crop due to an attack from the onion fly. I always tend to plant the majority of my onions and garlic in October to allow them to overwinter and develop a good root network, but I also plant a few rows in March too. Luckily, by the time the onion fly was in full swing I was able to save some of the crops that had overwintered before the damage from the grubs was too bad. As the overwintered crops had been in the ground for a longer period of time and had managed to get established into decent sized plants, they were able to ward off a more serious attack rather than the newly planted crops that hadn’t yet had time to get established and therefore were more susceptible.
One of the telltale signs to look for is that the foliage will suddenly start to curl and fold rather than standing upright. When you lift a bulb, you should also see damage to the layers of skin where the grub burrows its way into the centre of the bulb. The grub looks like a tiny brown grain of rice and is very clearly seen by the human eye. Normally, there are more than one grub in each plant. Unfortunately, when the onion fly attacks there is very little that can be done to save the plants unless you spot the signs quickly and remove the plants from the ground.
All of my March sown onions were unusable and had to be removed from the plot and disposed of in the bin. I had no intentions of adding these to the compost bin as this would have been the ideal place for the grubs to turn into the adult fly and therefore continue the destructive cycle. Most of my garlic had to be thoroughly inspected and separated into individual cloves to expose and reveal the grubs that had burrowed into the bulbs.
My advice would be to plant alliums in October and to keep them covered with some fine mesh to prevent the fly from landing and laying their eggs. Don’t plant them in the same place as any alliums have been grown in the previous year. This includes leeks too. Crop rotation is also very important as the grubs can survive in the soil for a long period of time.
The link below will give more information on how to deal with the onion fly.
Here’s hoping that you are never attacked by these and have a great harvest.