Bean Leaf Beetle
Bean leaf beetle populations
have been increasing in Iowa, following a trend for warmer winters that
create favorable conditions for overwinter survival. High numbers of bean
leaf beetles have created a concern about direct losses from beetle feeding
injury plus losses related to bean pod mottle virus, which they can transmit.
The bean leaf beetle
normally has three populations throughout the year: overwintered, first
generation, and second generation. The abundance of the previous beetle
generation and environmental factors can influence the timing and duration
of the following generation.
The most critical
time for the transmission of bean pod mottle virus, is during the emergence
of overwintered and first-generation beetles. These early (May) and mid-season
(late June and July) populations cause the initial introduction and spread
of this pathogen into soybean fields.
The second generation
of beetles is not important for transmission of bean pod mottle virus.
However, these beetles will cause feeding injury to the pods. Both bean
pod mottle virus and pod feeding cause economic damage to soybean yield
and seed quality.
Early season management
to reduce risk of virus transmission
Treatment thresholds
based on beetle numbers are not considered critical for virus control
as even small numbers of beetles can transmit and spread the virus. Treatment
is only recommended if the presence of bean
pod mottle virus has been documented the previous season. Signs of virus infection include significant yield reductions from previous years, "green stem" symptoms are harvest, and soybeans with "bleeding" or stained hilum. Occasionally leaf symptoms are present.
Researchers at ISU have constructed a flowchart to help growers with insect/virus management decisions. The flowchart is a dynamic, two-pronged decision guide based on field history. If bean pod mottle virus has not been present in your fields, then the only reason to consider early-season bean leaf beetle management is if populations reach extremely high levels (more than 2.5 beetles per plant). Insecticide treatment will not be cost-effective if the virus is not present.
Early season management
to reduce injury from defoliation
If early-season
bean leaf beetles are being managed to reduce defoliation, much higher
densities of the beetle can be tolerated. Economic damage will not result
until cotyledons are destroyed and regrowth is suppressed by continued
feeding.
See Management
of bean leaf beetle to reduce defoliation (Integrated Crop Management,
May 2005) for threshold levels needed to cause economic
injury to soybean.
Management of
second-generation beetles
Later in the season,
feeding by second-generation beetles poses the greatest injury risk. In
the past, the second-generation beetles were sampled and controls applied
as necessary. Substantial damage may occur during the time it takes the
beetle numbers to increase to treatable levels.
The current recommended
approach bases the control of second-generation beetles on first-generation
beetle numbers. If first generation numbers reach densities that indicate
the second generation will cause economic losses, treatments are made
in August as soon as the second-generation beetles are found. Sampling
of the first-generation beetles should begin when peak densities are forecast,
based on the heat units accumulated from soybean emergence (degree days).
Therefore, it is important to record crop emergence to be used later to
time beetle sampling.
See Predicting first-generation bean leaf beetles (Integrated Crop Management 7/12/2004) for details on how to predict populations based on degree days.
More resources
Bean pod mottle virus is back with a vengeance in 2006.
Bean
leaf beetles and others: correctly identifying the pest (Integrated Crop Management, May 2003).
ISU Entomology
Image Gallery This site has good photographs of the bean leaf beetle and other beetles
that may be mistaken for it.
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