Posted February 16, 2005: Trap crops
are used to lure pests away from a commercial crop by
presenting the pest with a more attractive alternative.
Trap crops are planted near the commercial crop with
the hope the damaging insect will feed on the trap crop
leaving the commercial crop alone. Although the concept
makes rational sense, it is not as simple as it sounds,
according to research being done in Finland there are
several criteria that must be considered before deciding
on a trap crop.
First, researcher Heikki Hokkanen -- University of
Helsinki, Laboratory of Applied Zoology who has found
encouraging trap cropping results for carrot psyllid
(Trioza apicalis) and the cabbage maggot (Belia radicum)
-- looked at characteristics of pests including flying
ability, colonization mode, mobility pattern and movement
orientation but concluded that few patterns emerged
in regards to pest susceptibility to a trap. He did
find that insects specializing in one or a few hosts
were more likely to be oriented to the trap crop than
were generalized pests. He also found trap cropping
to be more effective if only one generation spends a
short time, rather than multiple generations over a
sustained period, attacking the main crop.
In 2003 Hokkanen established a residence index (RI)
for how long an insect remained on the plant. The RI
can be used to predict the effectiveness of a plant
as a trap crop. Using the index a graph can be drawn
to calculate the percentage of trap crop needed to keep
a determined percentage of the pests off the main crop.
Hokkanen then used the RI to set up what he called the
“perfect” trap cropping experiment.
Every August, cauliflower, canola and other crucifers
in Finland and England are hit hard by the pollen beetle
(Meligethes aeneus). Hokkanen hypothesized this was
because there were few good smelling crucifers with
pollen available during late summer. He further reasoned
that if the beetles were presented with a more appealing
offer they would leave the cauliflower, canola and other
To test his hypothesis Hokkanen planted a row of Brussels
sprout trap crops next to a field of cauliflower. He
found that the trap crop reduced losses from 35% to
3% during the first year.
While Hokkanen remains an advocate of trap cropping
he also warns it is not economical or effective in all
situations. He suggests trap crops are best used when
a non-random pest is terrorizing a high-value crop on
a rather large acreage.