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Integrated root maggot control

Root maggot flies can be diverted from laying their eggs on and around canola by leaving some weeds in the field and intercropping.


November 29, 2007
By Helen McMenamin

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The best way to control root maggots in canola may be confusing adult flies
so they lay fewer eggs. Lloyd Dosdall, associate professor of Agricultural Entomology
at the University of Alberta, has found less damage from root maggot feeding
in canola crops with some weeds.

Root maggots are the larvae of several species of Delia: flies that look like
small ash-grey house flies. The females lay their eggs on or around the base
of crucifer plants, but they prefer canola, especially juncea-type and Polish
varieties, over yellow mustard or cruciferous weeds.

Before laying her eggs, a Delia fly makes several spiral flights through a
crop canopy and lands near the ground. When four landings in a row are onto
host plants, she lays eggs. When she lands on a plant that is not a host, she
starts her test flights over again. It seems that in a weedy crop, she spends
more time and energy on test flights and less on egg-laying.

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When the eggs hatch, the larvae feed first on root hairs and lateral roots,
then on the main tap root. Plants might be able to compensate for the damage,
but it allows root rot fungi to enter the root, causing greater damage.

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The benefit of weeds in cutting down root maggot feeding was similar
no matter what species dominated the weed spectrum.

Dosdall and Neil Harker, a Lacombe Agriculture and Agri-Food Canada weed scientist,
tried delaying herbicide spraying from the recommended two leaf stage to the
six leaf stage. The hope was that weeds would reduce the impact of the root
maggots early in the season, when they do most damage. Also, Delia are most
active in the cool, moist weather of early summer in west-central Alberta and
northern areas.

The plan succeeded in reducing root maggot damage, but this benefit was not
as great as the loss in yield due to delaying weed removal, and so yields were
lower. The scientists speculate that spraying weeds once, early, rather than
twice, might reduce root maggots without allowing weeds to lower yields. "That
second flush of weeds doesn't compete effectively with the crop," says
Harker. "It doesn't even contribute to the soil's weed seedbank. A second
spray is really just cosmetic."

Also, the research crops were grown in a four year rotation. When canola is
more frequent in the rotation, root maggots tend to be worse. In that situation,
reducing their impact could have more benefit.

The benefit of weeds in cutting down root maggot feeding was similar no matter
what species dominated the weed spectrum. Fields in the research trials had
a variety of main weeds: wild oats, buckwheat, wild mustard and chickweed. The
weed species did not seem to make a difference to Delia.

This finding has led Jeremy Hummel, a graduate student with Dosdall, to research
whether an intercrop of barley and canola grown together suffers less damage
from insect pests such as root maggots. He is growing barley and canola alone
and mixed in various proportions. So far, he has found the two crops do not
compete as much as you might expect. The intercrops seem to produce more than
either crop alone. Barley's fibrous root system and canola's tap root take moisture
and nutrients from different parts of the soil.

"There's a lot of exciting advantages to this approach," says Dosdall.
"Weeds may be too competitive, but people have been growing collards and
tomatoes with clover to reduce the impact of flea beetles and diamondback moth.
In cole crops like cabbage, cabbage aphid, whitefly, butterfly and moth, as
well as Delia cause less damage with a companion crop, usually clover. A pure
crop stand concentrates resources for insect pests."

Scientists are not sure how companion crops work against pests. They may act
as a physical barrier, produce volatile compounds that repel pests or support
the pests' natural enemies. This could be by providing alternate shelter and
food sources for predators and parasites or by altering the microclimate to
favour pathogens. This may also have implications for controlling crop diseases.

Differences in susceptibility of canola varieties to root maggots suggest breeding
resistant varieties might also be possible. Scientists are also testing a European
beetle that preys on root maggots to find whether it can be safely imported
and released in this country. Dosdall thinks including other plant species in
a crop holds promise, too.