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Bean-seed flies

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					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
                                      anything else?
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
                                                                            weather.
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
                                                                            varieties?
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
sprays?
                                      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
                                                                            www.organiccatalog.com
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