November 13, 2007
By Ralph Pearce
Bean leaf beetles weather dependent.
It seems as though no matter what the weather, there is always some disease
or insect pest waiting to show itself as the scourge of the growing season.
In 2006, the bean leaf beetle, with its distinctive black triangle behind its
head, appeared in soybean fields in Huron, Perth and north Middlesex counties,
a region where it is not normally found. But just as growers there, and areas
farther to the north and east have found with soybean cyst nematode and aphids,
bean leaf beetles are just one more pest or disease to challenge management
practices and patience.
As in the case of any disease or insect pest, bean leaf beetles are manageable,
but must be monitored diligently. And that could be part of a growing trend
across the region. "We can't just plant the seeds and walk away from soybean
fields anymore," insists Tracey Baute, field crop entomologist with the
Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) in Ridgetown.
Given the relatively warm winter leading into the 2006 growing season, the
fact that bean leaf beetles migrated farther north is of little surprise to
Baute. "We're probably going to see more of an increase as climate change
becomes a true issue. If there were populations that successfully over-wintered
because we had mild temperatures or some snow cover to insulate them, that allows
those populations to multiply in the spring and spread farther."
Cannot over-estimate damage
Understanding when to spray with dimethoate (Cygon or Lagon) is one of the trickier
components to controlling bean leaf beetles, primarily because growers tend
to over-estimate the damage to the leaf surface. Yet as Baute pointed out in
a ministry pamphlet in late August 2006, the level of feeding has to reach about
25 percent throughout the entire plant as the soybean crop reaches the late
reproductive stages to warrant spraying.
"It certainly takes a lot of holes for them to impact yield, but we have
to also be aware that they can do injury on the pod," explains Baute. In
a year like 2006, where cool, wet conditions continued and crops remained standing
in the fields, any pod damage could have increased the potential for other diseases
that might have affected the quality of the beans. In food grade or IP soybeans,
10 percent pod feeding is sufficient to warrant spraying.
Baute adds that scouting for bean leaf beetle can be difficult, since beetles
can sense a person's approach and drop off the plants before being spotted,
making threshold counts a real challenge. "This is a good example of how
a $20 to $25 investment in a sweep net can pay off," says Baute. "It's
a cheap tool and it's not hard to use, and it is the best way to really get
a good assessment."
Quality and quantity an issue
Bean leaf beetles are also the vector for bean pod mottle virus which, according
to Albert Tenuta, is becoming a greater problem if climatic conditions really
are warming. "It's a particular virus that has been closely associated
with bean leaf beetle, but has also been closely associated with increased distribution
incidents throughout much of the soybean producing regions in the mid-western
US," says Tenuta, field crop pathologist, also with OMAFRA in Ridgetown.
"The majority of fields throughout the primary soybean producing states
have significant bean pod mottle virus incidence, and the concern is that you
could potentially have some injury and yield loss, but the main factor becomes
the marketability and the quality issues of those beans."
Of course, if the winter of 2006-07 returns to more traditionally normal conditions,
much of the concern regarding bean leaf beetles might vanish. But as Tenuta
and Baute remind growers, if it is not bean leaf beetles and bean pod mottle
virus, then it will be something else like aphids, SCN or Asian soybean rust
that could become a problem: something to show growers that they cannot let
their guard down, at least not any more. -30-