Bean-seed flies GWF235
Updated January 2009
The maggots of this pest attack seedlings, especially those with large seeds. Introduced by
accident from America, it is now established in this country.
Q What are bean-seed flies? growing tip leads to distorted same way as the bean-seed fly.
growth called snake head, which Bean-seed fly typically attacks a
A They are two species of fly describes the look of the emerging week or two before onion fly.
which are common, but usually plants. In severe cases, the plants Millepedes often take up
occur in local hot spots, so many die. However, even minor attacks residence where bean-seed flies
areas will escape them. Bean- cause a slowing or halt of growth, have been and continue the
seed flies (Delia platura and reducing yields and delaying damage – so control the bean-
Delia floriga) are mainly harvest. Attacks can open the way seed fly and the millepedes are
scavengers but often attack to rots and, if the weather is dry, unlikely to be a problem.
vegetables and a few ornamental the plants can wilt.
plants. Unfortunately, they are Occasionally this pest will Q When should I expect them?
common where there is lots of
organic matter in the soil, which
attack beetroot, broad beans, A Bean-seed flies spend the
bulbs (eg freesia and gladioli),
is what all gardeners aim for. winter in the soil as pupae. In
cabbage family crops (especially
late spring the adults hatch and
cauliflowers and radish), lettuce,
Q How do I recognise them? potatoes (if cut-up seed is used),
the female goes on a six-week
egg-laying spree. She will lay eggs
A The adult is a small (5mm) sweetcorn and spinach.
in the soil, especially where it
grey fly which is very hard to spot Onion damage is at its worst in
has been freshly disturbed, by
and identify. The maggot is the late summer when overwintered
hoeing for example. Freshly
cause of garden problems. It is onions are sown. This is normally
planted courgettes, marrows,
white and about 7mm long, very around the third week in August.
squashes and pumpkins are
like the fisherman’s maggot, but The seedlings can often fail to
particuarly at risk, as the bean-
smaller. Look for them in emerge, and those that do
seed fly maggots can get into the
affected plants inside the seeds, germinate often fall victim while
rootball, especially if the
stems or seed leaves. they are still in the ‘bent’ or
compost is a soil-free kind, and
‘crook’ stages, soon after
do severe damage to the roots.
Q What plants do they attack? emergence. Often, after feeding
The eggs hatch in a few days
on one seedling or young plant,
A As well as runner and French the maggot will travel to the next,
and the maggots enter the seed,
beans, courgettes, marrows, stem or leaves of vulnerable
so a series of plants in the row
squashes and pumpkins are plants. After 1-3 weeks they are
will be killed. Older plants wilt
commonly attacked. fully grown and pupate in the
and collapse very quickly when
The bean-seed fly gets inside soil. After 2-3 weeks as pupae,
attacked by bean-seed fly.
the young plants or seeds and the the adults are on the wing again
germinating seeds may not reach Q Could I confuse them with and the cycle begins anew.
the surface. Where seedlings do As many as four generations
emerge, the fleshy seed leaves may occur during the summer if
(cotyledons) and tips are A The damage caused by the the conditions are suitable. These
damaged by holes and surface onion fly is identical to that generations overlap so you can
marks as well by tunnels into the caused by bean seed flies. expect attacks at any time.
seeds and stems. Loss of the Fortunately, it is dealt with in the However, the flies seem to prefer
to feed on dead plant material,
and usually only the first
Q What can organic gardeners Q What should I do with
use? affected plants?
generation is troublesome to
growing plants. Late-sown onions A Go for coverings of fleece and A They should be lifted and
are the exception to this. mesh. Not only will bean-seed fly burnt or binned to prevent the
be controlled, but greenfly, larvae adding to the next
Q Can the damage be avoided? blackfly and onion fly will also be season’s problems.
A Getting plants through the excluded. Both seeds and
vulnerable germination stage as transplants will benefit from a Q Can I reduce the risk to next
quickly as possible will limit fleece covering. This will boost year’s crop?
growth by warming and sheltering
losses. Ideally, you should A There is some evidence that
prepare a firm, moist seedbed in the seedlings. Make sure it is free
digging in the autumn, rather
advance, because bean-seed fly of holes and well-secured around
than the spring, will expose the
favours recently cultivated soil. the edges by burying them at least
pupae to the pest’s natural
Make the seed bed at least a 5cm deep to exclude the bean-
enemies and the effects of the
month before sowing, and remove seed fly females and they will
weeds with a contact weedkiller, have to lay their eggs elsewhere.
hand-weeding carefully, or The downside is that you will Q Are there any resistant
keeping the seed bed covered have to remove the fleece to
with black polythene or old weed, and replace it quickly
carpet to avoid weeds altogether. before any flies can get in. A We know of no resistant kinds.
Not only will bean-seed fly be As fleece is very efficient at
trapping warmth, it might cook Q Are there any biological
discouraged, but the need for
seedlings after late May. The controls?
weeding will be reduced.
Nevertheless, if attacks in the better-ventilated insect-proof A There are some trials which
past have been heavy, preparing mesh is a cooler choice to protect
involve putting the seeds in a
a seed bed in this way is unlikely late sowings.
freezer to kill the larvae. This has
to be enough on its own to had some good results, but is still
protect the plants fully.
Q Will crop rotation help?
in the trial stages.
Also gardeners in wet districts A You should always practice
or those with clay soils may find rotation where onions and beans Suppliers of insect-proof
this early preparation impossible. are grown as there are several mesh
If this is the case, go for soil-borne diseases and nematodes
transplants raised in pots or cell that can attack these vegetables. Agralan
trays indoors or in a coldframe. However, since these flies are 01285 860015
Avoid incorporating organic so mobile it is unlikely that www.agralan.co.uk
matter shortly before sowing or rotation alone will control them;
planting. Put old plant material they can fly to other crops within DT Brown
on the compost heap and dig in a wide radius. 0845 1662275
manure and compost during the www.dtbrownseeds.co.uk
winter. Q Is companion planting
effective? Ferndale Lodge
Q Are there any effective 0870 444 1342
A We know of no cases where www.ferndale-lodge.co.uk
companion planting has been
A There are no insecticides effective.
The Organic Gardening
available to gardeners. Catalogue
0845 130 1304
What else does Which? Gardening offer?
Which? Gardening is the only truly independent gardening magazine. We don’t accept advertising, free
product samples from manufacturers or free plants from nurseries. This gives us the freedom we need to
report the facts about the products, plants and companies we put to the test.
In each issue you’ll find:
News stories to keep you up to date with developments in gardening and horticulture
Our thoroughly researched testing reports which reveal the Best Buy tools, chemicals and equipment
Details of the best new plants and products
Plant trials which highlight the best value plants for your garden
Design ideas to inspire you
Plant profiles packed with detailed information about popular plants
New growing techniques to save you time, money and effort
Membership benefits – including offers, competitions, give-aways and much more.
No-one works harder than Which? Gardening to find the best plants, products and techniques for your garden.
We carry out tests and trials throughout Britain, including government test sites, universities and the state-of-
the-art Consumer Research and Testing Centre at Milton Keynes. Many trials take place behind closed doors,
but you can visit our Trial Gardens at Capel Manor College in north London.
Member benefits and services
Free expert advice during our regular phone-ins
Over 180 factsheets available online and on request
Members’ trials that give thousands of our members the opportunity to test new plants and techniques
A soil analysis service to help you get the most from your plot
Much more on our Which? Gardening website www.which.co.uk/gardening