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Palestriped Flea Beetles Defoliating Sunflowers--
Economic Thresholds Available

            
                                             (Click on thumbnail for a bigger picture)

Palestriped flea beetles, thistle caterpillars, and black stem weevils have all been feeding
on sunflower leaves in the past few weeks in central SD where most of the state’s sunflowers
are grown.  These insects are not considered common pests of sunflowers so economic
thresholds may not be readily available to growers.

Fortunately, because these insects feed on leaves, sunflower defoliation data due to another
serious sunflower defoliator - hail - can be used to estimate economic thresholds of the insects.
Hail and insect defoliators both destroy leaves and affect sunflower photosynthesis.

According to established sunflower defoliation data due to hail, sunflowers may still produce
78-85% of their potential seed yields even at 100% defoliation during the early vegetative stages
from VE through V8 (emergence through eight-leaf vegetative stages).  These data assume that
the growing points of the sunflower plants were not destroyed by hail.

However, sunflowers are less tolerant of defoliation once they start producing buds and flowers
later in the season.

Based on the hail-defoliation data, economic thresholds may be calculated for insects. And
the percent defoliation at which intervention via insecticide treatment should be considered
can be determined and applied to the combined effects of all sunflower insect defoliators such
as palestriped flea beetles, thistle caterpillars, black stem weevils, and even grasshoppers.

From VE through V8 (emergence through eight-leaf stages), there has to be an average defoliation
of 50% for spraying with an insecticide to pay for itself. This assumes a chemical-plus-application
cost of $8 per acre, sunflower oilseed market value of $9 per 100 pounds, and an expected yield
of 1,500 pounds per acre.

I calculated this threshold using the quadratic relationship between % defoliation and % yield loss.
Call me (688-4603) for details.

Percent defoliation can be easily and accurately measured using a 5-by-5-to-an-inch graphing paper
copied onto a transparency.  We have used this method successfully in measuring defoliation in
soybeans and should also be applicable to sunflowers.

Most insecticides labeled for use on oilseed sunflowers can also be used to control insects that
may not be listed on the label.  This is according to Section 2ee of the Federal Insecticide,
Fungicide, and Rodenticide Act (FIFRA).  Since the effective rates may not be known, manufacturers
may not be held liable if the said insecticide does not perform well in controlling the target insect.
The palestriped flea beetle and the black stem weevil are not specifically listed in any of the sunflower
insecticide available to SD growers. 

Common insecticides used to control insects on sunflowers in SD include Asana XL, Baythroid 2,
Furadan 4F, Lorsban 4E, Sevin XLR Plus, Stryker, and Warrior.  Asana XL has been recommended
to control the palestriped flea beetle on carrots in California at the rate of 5.8 to 9.6 fluid ounces per
acre, and Sevin XLR Plus at 1 to 2 pints per acre.

In 2000, the palestriped flea beetle (scientific name: Systena blanda) caused leaf injuries in
sunflower seedlings in seven central SD counties.  Reports of damaging numbers of the flea beetle
came from Hughes, Sully, Walworth, Faulk, Hyde, Potter, and Tripp counties.  Sully County has
again reported significant palestriped flea beetle numbers this year.

The palestriped flea beetle is a known pest of carrot and tomato in California and North Carolina,
respectively.  It is also known to feed on numerous plants including potato, grapes, beans, strawberry,
and corn.  In SD, it is mainly found feeding on weeds.  The reason for the switch to sunflower feeding
is currently unknown.

Records show that the palestriped flea beetle has a widespread presence both in South Dakota
and the entire United States.  Adults are about 3/16 of an inch long, black, with 2 longitudinal stripes
on the back.  Like any flea beetle species, the palestriped flea beetle has enlarged hind legs enabling
them to jump from plant to plant.

Adults overwinter on the field under the soil and plant debris.  They then resume feeding in the spring
of the following year.  These overwintered beetles will then lay eggs in the soil near the base of host plants.
Grubs feed on plant roots then transform into pupae, then adult flea beetles in the summer.  There is only
one generation per year.

Inspect Knee-High Corn for Corn Borers in Bivoltine Areas Now

Our European corn borer moth flight tracking project indicates that the first flight has passed in the
bivoltine areas (along I-90 south of Dell Rapids).  Moth flights in the univoltine areas (along I-29 north of
Dell Rapids) are currently in full swing.

We inspected our plots in Beresford last week (June 26) and saw an average of 10-15% "shothole" infestation.
Moths were still laying eggs then so this may have changed already.  Larry Sax, Crop Consultant in Minnehaha
and Union counties also observed similar infestation rates last week (June 28).

All knee-high corn may now be scouted.  Calculate economic thresholds online using the University of
Nebraska worksheet linked to my corn borer Web page.

Happy scouting!  Let me know what you see out there.

Web posted on July 2, 2001 by Mike Catangui

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