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Late Season Sunflower Insects
Compliments of
the Agronomy Services Department
There are several species
of late season sunflower insects requiring field monitoring at this time of year. Not all
sunflower areas harbor all of these species and even for those areas that do, wide swings
in populations occur from year to year. Fortunately, most of the insects that are found in
any given sunflower field are not harmful. We will discuss four of the most damaging
species.
Sunflower Head Moth
The sunflower head moth is a major threat to sunflower production in
the southern third of South Dakota and areas further south. The adult moth is cigar-shaped
(see picture below) and is attracted to the yellow ray flower petals, laying eggs among
the flowering florets of the sunflower head. Upon hatching, larvae feed on pollen and
tender flower parts until large enough to enter the developing seed to feed. Individual
larva can destroy 12 to 14 seeds. Damage to the head also promotes the development of
Rhizopus head rot. When a head becomes severely damaged because of head moth larvae, there
is a trashy appearance on the sunflower head, and untreated infestations can result in
severe loss.
Scouting for larvae feeding on the surface of the flower is
impractical. Control must be directed toward the adult moth rather than the larvae,
because once in the seed, they are protected from any insecticide application.
Look for head moths from the time plants are just beginning to
exhibit the yellow of the ray flowers (the yellow petals around the edge of the head)
until pollination is over. Examine several heads with visible yellow petals in the calm of
the morning or evening. Some producers treat with an insecticide if any moths are found.
However, the economic threshold is figured at 1 to 2 sunflower moths per 5 plants.
Producers who are hit regularly with head moths automatically begin pesticide application
at 10% bloom with close attention paid to early planted fields.
Sunflower Head Moth
Banded Sunflower Moth
Banded sunflower moth, so called because the adult has a dark band
across its back, is common throughout sunflower production areas. However, they are more
numerous in the northern regions of production. Banded sunflower moths, like the head
moth, need to be controlled as adults. The larvae tunnel into the developing seeds and
cannot be effectively treated at that stage. They begin to emerge in late July and into
August, congregating in field margins, grassy areas or in adjacent crops. About 1 week
after emergence, the adult begins to lay eggs on the outside of the bract area of buds.
Scouting should be done at least 75 feet in from field margins and should be started when
fields are in the R4 stage (coffee cup size buds) to the R5.1 stage (beginning bloom).
(One moth per two plants is the recommended threshold when sunflower prices are at
$10/cwt.)
The larvae (pictured below) will tunnel into developing seeds and
each will destroy 5 to 7 seeds. Damage is generally much worse on field margins, although
crop loss is seldom field wide and is usually less than 30% of total yield.
Adult Banded Sunflower Moth
Banded Sunflower Moth Larvae
Gray Seed Weevil
The gray seed weevil (pictured below) needs to be scouted before
sunflower bloom occurs. Scouting should begin at the R3 to R4 stage or about 7 to 10 days
before the onset of bloom. The gray seed weevil begins egg laying prior to bloom and
deposits eggs into several developing ovules. The larvae that grow inside the developing
seed consume the meat of that seed almost entirely. Populations of gray seed weevils are
generally much lower than the red seed weevil and most often will not need to be
controlled in oil type sunflowers. When confections are grown, infestations can become
serious enough to warrant control.
Scouting should be conducted at least 100 feet in from field margins
and each head for ten plants in a row should be checked in several different spots in the
field. When an average of 1 per plant is found in confections, a pesticide should be used.
The threshold in oil type sunflowers would be 8 or more weevils per plant.
Gray Seed Weevil
Red Seed Weevil
The red seed weevil (pictured below) is generally a much more
serious pest in sunflowers. The adult will emerge starting in late June and into July.
They begin to collect on the sunflower buds, but will cause no damage in the field until
pollination starts. The female red seed weevil must consume pollen for several days before
laying her eggs. Scouting for the red seed weevil should begin at the first showing of
yellow ray flowers and continue until 100% bloom occurs. Because the eggs are deposited
into the developing ovules, adult control is the only means possible. Each female lays 7
to 10 eggs and the developing larvae will consume about half of the meat of that seed.
Monitoring for red seed weevil in confection sunflowers is
absolutely necessary. When the larvae reach maturity, they tunnel out of the seed. As a
result damaged seed with holes in them remain in the harvested sample and contribute to
discounts or rejection of the seed. Economic thresholds are 1 weevil per head on
confections or 12 to 14 per head on oil type sunflowers. Application of an insecticide
should take place prior to 10% pollination in confections and 20% pollination in oils.
Delayed treatment results in poorer damage control.
Red Seed Weevil
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