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| Are foraging bees vulnerable to nionicotinoid seed dressings ? |
Phil Jarvis; What
are the pests that oil seed rape plants are vulnerable to?
Dr Alastair Leake;
Spring and autumn sown rape crops are attacked by a range of pests. These
include the peach potato aphid, which spreads beet yellows and turnip yellows
virus, pollen beetles, cabbage seed weevil, brassica pod midge, cabbage stem
flea beetle, mealy cabbage aphid, cabbage stem weevil and rape winter stem
weevil.
Phil ; Are there
practices farmers can employ that don’t involve seed dressings or spraying?
Alastair ; There
are a number of cultural techniques farmers can employ to help control pests,
but many are not practical or have other consequences. Delaying drilling after
ploughing by 5 weeks reduces the “green bridge” effect, but results in backward
crops; converting to a min-till establishment system increases the number of
beneficial predators but exacerbates the ”green bridge”.
Phil ; So why do
growers prefer to use nionicotinoid seed dressings ?
Alastair; The
most effective means of protecting the emerging seed is to use a nionicotinoid
seed dressing. This removes the need for an insecticide spray for 6 weeks post
drilling. In a crop of autumn rape, this would provide protection for August
and September when many other non-target insects would still be active in the
field. After 6 weeks the crop should be monitored regularly and where
thresholds are exceeded a spray applied. Trials have indicated this approach
can add an extra 0.3 t/ha to yield worth approximately £150/ha to the farmer.
Phil; Farmers are
concerned that any ban on these seed dressings will affect yield and reduce
food production. What are the alternatives to using nionicotinoid seed
dressings?
Alastair; Rather
than treat the seed, many growers will be forced to spray emerging crops. The
return to the widespread spraying of these non-selective insecticides would be
considered by many to be a retrograde step.
There are two other groups of insecticides available and
approved for use on OSR. These are the synthetic pyrethriods (e.g.
cypermetherin, deltametherin) and the carbamates (e.g. pirimicarb). The former
group are highly toxic to bees, the later moderately toxic. A high proportion
of the of the peach potato aphid population is resistant to pyrethroids and the
vast majority resistant to pirimicarb, so there is little left in the armoury
for dealing with virus vectors. The problem of resistance is becoming greater
and greater as active ingredients are withdrawn leading to the more widespread
and greater frequency of use of the remaining few.
Phil; How should
growers deal with pollen beetles later in the season when oilseed rape is
flowering?
Alastair; Dealing with pollen beetles is particularly tricky as they
infest the crop just pre-flowering, at the green/yellow bud stage. HGCA run a
migration forecast scheme and there is a well defined system for determining
threshold levels, above which sprays should only be applied. Some work has been
done growing turnip rape around the headlands which flowers a little earlier
than OSR and acts as a trap crop. Where a threshold is exceeded pyrethroids can
be sprayed, or Thiacloprid, which is a neonicotinoid and has escaped the
proposed ban. Spraying for pollen beetle should be limited to the green/yellow
bud period when economic damage can be done, but ceased by flowering time, as
the pollen beetles may be helpful with pollination at this point. There is a
code of practice which sets out how spray operators can minimise the effect
their activities may have on bees.
Phil; When will this
ban on nionicotinoids take place?
Alastair; The ban
on the three nionicotinoid actives affected by the EU decision is likely to
come into force by the end of the year, which means that this autumn’s crops
can still be treated with a seed dressing.
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