The vegetable garden can be a tranquil place where gardeners and nature work in harmony. However, it can also be a war zone! In times of war intelligence is everything and no less so with the allotment. Only by understanding our enemy, even sympathizing with its cause to some extent, can we become more inventive and effective in protecting our crops. Take the onion fly for example. This insignificant fly would go unnoticed and certainly unidentified by all but the most avid Dipterist and yet in areas where it is prevalent it can result in crop losses upwards of 33 per cent,
Delia antiqua, as the onion fly is known to science is only 5-7mm (0,2in) in length. It is a fairly non-descript ash grey and while males do have a 'go-faster' stripe along their abdomen, females do not so a seriously expert eye is needed to identify one of these guys. Although it travels incognito, the onion fly is a cosmopolitan beast, found across the world from North America, across Europe and China to Africa; it is found from forest tundra to tropical Ghana and up to 3600m (11,800ft) above sea level. A quick breeder, this hardy little creature can roll through as many as four generations a year.
In the wild our fly feeds on wild onions and other plants in the lily and iris families. However, it is only the larval stage of the fly that eats these plants as the adults live off nectar from flowers such as dandelion and cherry to meet their high energy requirements for mating and egg laying. In fact the adult flies mean us no harm whatsoever. They are merely looking for a suitable place for their children to grow up, safe from predators inside the nutritionally complete environment of a growing bulb.
The onions, chives, garlic, leeks that we plant in our gardens look just right for the job. When a female fly finds a nice looking onion, she gently alights on it and lays between five and 20 tiny elongated white eggs on shoots, leaves, bulb or even in the ground adjacent. Once her eggs are safe she flies off, ready to repeat the cycle every 15 days. She must be expert at finding her host plant as she will only live for two months at the outside, during which she will lay up to 200 eggs.
After three to eight days the eggs hatch into white larvae which burrow into the onion bulb. These siblings will stick together in a group, eating their fill until they grow to around 10mm (0,3in) long. Other unrelated larvae may also live in the same bulb, and more than 50 maggots from different mothers have been found in one bulb! The fat little larvae feed for 15-25 days, shedding their skins three times to allow them to grow, after which they leave the onion and head for the soil. At a depth of 10-20cm (4-8in) they become pupae, miraculously rearranging their entire body structure during the next two to three weeks.
Then, if they are the spring or summer generations, they will emerge into the light as adult flies. But, for autumn generations, cooler temperatures mean they will stay, suspended inside their pupa throughout the winter in a state of 'diapause' until springtime.
The damage caused to the onion bulb by accommodating even one unwanted visitor is usually terminal as fungi and bacteria take hold, rendering the onion inedible. Infected onions show yellowing, floppy outside leaves at first and then, when closely inspected, a rotten bulb.
Prevention is the only real option as there is no treatment for infected onions other than to lift and destroy by burning before the maggots leave to pupate in the soil. For this reason it is also important to clear any onion debris away at the end of the season and dig over the ground to expose any hiding pupae to bird's beaks and frosts. Regular crop rotation will also inhibit the onion fly lifecycle and regular checks to your plot will allow you to remove any infected plants quickly. Nemasys produce a biological control called 'Grow Your Own', a mixed nematode treatment that can be watered on to an infected onion patch to stop the maggots spreading to further onions.
So what is the best way to keep the greedy fly babies from taking up residence in the first place? Scientific studies have demonstrated that the hardworking lady onion fly is very adept at finding the best place for her children using a number of host-selection mechanisms. She uses a sophisticated sense of smell in her long range searching and then a combination of visual, scent, and even tactile cues once she gets up close. It is surprising how many people have dedicated thousands of man hours to finding out what sorts of things onion flies do or don't like the smell of! To stop our intrepid ladies securing an onion, the simplest thing is to install a covering of fine fleece or mesh as soon as you plant out your onions. Onion sets are also less vulnerable than those grown from seed.