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Bean leaf beetles one more concern

Soybean growers must be prepared.

November 13, 2007  By Ralph Pearce

6aFor soybean growers in Ontario, the 2005 growing season presented some relatively
new challenges. First, there was the threat of Asian soybean rust, which never
actually occurred. Then it was the realization that soybean aphids would be
more of a problem than just every other year, thanks to their overwintering
capability. And while soybean cyst nematode (SCN) is not new, it did appear
in soil tests in newer regions, namely Bruce and Brant counties.

Bean leaf beetles also became an issue for some growers at the end of the season
in 2005, making it one more insect pest which growers had to deal with. The
unfortunate reality is that in any given year, a number of different conditions
can arise in a field, weakening plant health and robbing yield.

With bean leaf beetles, the complication goes beyond their prime objectives
which are feeding on early-season seedlings and defoliating maturing soybeans.
The beetles, which look similar to ladybird beetles (ladybugs), are also a vector
for been pod mottle virus. Like soybean aphids, bean leaf beetles are known
to overwinter in Ontario, and maintain the virus in their system. "Obviously,
they overwinter better in Essex and Kent, and although the transmission efficiency
is greatly reduced coming out of the winter, even low infection levels early
in the spring increase the risk of further spread within season," says
Tracey Baute, field crop entomologist with the Ontario Ministry of Agriculture,
Food and Rural Affairs, in Ridgetown. In terms of economic damage to the plant,
she notes it is usually the Kent and Essex region where been leaf beetles attack
the very young seedling crops, inflicting the most damage at the earliest stage.
"But once it's beyond the seedling stage, you need a lot more before the
bean leaf beetle itself can feed enough on soybean plants to hurt yields."


Even so, Baute concedes there is a potential any year, and initial forecasts
for the winter of 2005/06 are for milder conditions, presenting a higher risk
going into spring. "The milder the winter, the more potential the bean
leaf beetles can overwinter farther north and be there early in the spring to
get those early-planted crops," explains Baute. "They tend to go directly
to soybeans as soon as they get out of their winter sites, and they're starving,
so they'll wreak havoc on those very young plants. If in high enough numbers
in late summer, they can also damage the soybean pods, exposing the seed to
pod diseases."

Appearances can be deceiving
Compared to the ladybird beetle, bean leaf beetles appear to be similar in size,
three to five millimetres in length, but are stouter and feed directly on the
leaf, whereas ladybird beetles tend to favour insect pests. But the telltale
sign of the bean leaf beetle is the small inverted triangle just behind the
head. All bean leaf beetles have this marking.

Other variations to the bean leaf beetle is in their colouring, varying from
red to yellow-green or tan. Bean leaf beetles produce only one generation per
year with adults emerging from the soil in late April. Overwintered adults will
feed on available forage crops until soybeans are planted. Females lay their
eggs in soybean soils in June with hatched larvae doing little damage to soybean

On the treatment side, Baute agrees that scouting is one of the best tools
for dealing with bean leaf beetles. At the seedling stage, it is important to
establish the number of pests per foot of row. Beyond that, the key determinant
is the percentage of defoliation, and growers are urged not to overestimate.
As for control options, Baute says Gaucho, Poncho and Cruiser provide some protection
against defoliation in the younger soybean plants, but not against bean leaf
beetles that are vectoring bean pod mottle virus. Although dimethoate is registered,
economic loss due to defoliation is rare.

No clear direction on where it starts
Another notion that can be deceiving is that like other disease and pest problems,
been leaf beetles are a threat in the extreme southwest first, before migrating
east and north. Are the beetles another factor associated with continuous soybeans?
They are not, according to Albert Tenuta, field crops pathologist, also with
the Ontario Ministry of Agriculture, Food and Rural Affairs, in Ridgetown. "When
we had the first few incidents of overwintering of bean leaf beetles in 2000,
one of the first fields was in Oxford County, near Woodstock," says Tenuta.

The geographic location in the province is not the issue. Geography comes into
play more as a function of the environment. "In many cases, there are more
of the environmental aspects in terms of winter survivability, the impact of
the winter conditions, as well as the microclimate within those fields or off
those fields that actually provide them with extra protection."

Taken one step further, adds Tenuta, concerns of global warming may cause an
increase in these diseases and insect pests, particularly with overwintering
and early season planting. "If the projections are correct, we'll be seeing
more of these, and it won't be just Essex, Kent, Lambton or Niagara," says
Tenuta, noting the cyclical nature of things. "If it gets warmer, we'll
be significantly impacted and you'll see it farther east." 


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