Biology and Integrated Pest Management of the Sunflower
Stem Weevils in the Great Plains
E-821 (Revised), October 2002
Janet J. Knodel, Crop Protection Specialist
Laurence D. Charlet, USDA, ARS Research Entomologist
Introduction
Distribution
Identification
Hosts
Life Cycle - Sunflower Stem Weevil
Life Cycle - Black Sunflower Stem Weevil
Damage
Integrated Pest Management of the Sunflower
Stem Weevil
Cultural Control
Plant Resistance
Biological Control
Monitoring
Economic Threshold
Chemical Control
Insecticide Recommendations
for Sunflower Stem Weevil
The sunflower
stem weevil, Cylindrocopturus adspersus (LeConte), is an insect pest
that has caused economic damage to sunflower in the northern and southern Great
Plains of the USA and into Canada. It belongs in the order Coleoptera (beetles)
and family Curculionidae (weevils), and has also been called the spotted sunflower
stem weevil. It is native to North America and has adapted to wild and cultivated
sunflowers feeding on the stem and leaves. The sunflower stem weevil was first
reported as a pest in 1921 from severely wilted plants in fields grown for silage
in Colorado. In North Dakota, the first sunflower stem weevil infestation was
recorded in 1973, causing 80% yield loss due to lodging (Figure 1). Populations
of sunflower stem weevil have fluctuated over the years with high numbers in
some areas from the 1980s to early 1990s in North Dakota.
Another stem feeding weevil called the black sunflower
stem weevil, Apion occidentale Fall, also occurs throughout the Great
Plains, and attacks sunflower as a host. In recent years, severe damage to seedling
sunflowers has been reported from southern North Dakota and South Dakota.
Figure 1. Damage caused by sunflower stem weevil
-- sunflower lodging and stalk breakage. (Click
here to link to a 206KB color photo.)
Distribution
The sunflower stem weevil has been reported from most
states west of the Mississippi River and into Canada. Economically damaging
populations have been recorded in Colorado, Kansas, Nebraska, North Dakota,
Minnesota, South Dakota, and Texas.
The black sunflower stem weevil can be found in most
sunflower production areas with the greatest concentrations in southern North
Dakota and South Dakota.
Identification
Sunflower Stem Weevil -- Cylindrocopturus
adspersus (LeConte)
ADULT: Adult sunflower stem weevils are about
3/16 inch (4 to 5 mm) long and grayish-brown with varying-shaped white spots
on the wing covers and thorax (Figure 2). The snout, eyes, and antennae are
black. The snout is narrow and protrudes down and backward from the head.
EGG: Eggs are deposited inside the epidermis
of sunflower stems (Figure 3), and are very tiny (0.51 mm long x 0.33 mm wide),
oval in shape, and yellow in color making them difficult to see.
LARVAE: The larvae are 1/4 inch (5 to 6 mm)
long at maturity, legless, and creamy-white in color with a small, brown head
capsule (Figure 4). They are normally in a curled or C-shaped position within
the sunflower stalk.
PUPAE: Pupae are similar to the adult in size
and yellow in color (Figure 5).
Figure 2. Adult sunflower stem weevil. (Click
here to link to an 80KB color photo of an adult sunflower stem weevil.)
Figure 3. Egg site of sunflower stem weevil.
(Click here to link to a 51KB color photo of
an egg site of sunflower stem weevil.)
Figure 4. Larvae of sunflower stem weevil. (Click
here to link to a 34KB color photo of sunflower stem weevil larvae.)
Figure 5. Pupae of sunflower stem weevil. (Click
here to link to a 14KB color photo of a sunflower stem weevil pupae.)
Black Sunflower Stem Weevil -- Apion occidentale Fall
ADULT: Adults are 3/32 inch (2.5 mm) long from
the tip of the snout to the tip of the abdomen. The snout is very narrow and
protrudes forward from the head, which is smaller than the large, oval-shaped
body (Figure 6).
EGG: Eggs are deposited inside the epidermis
of sunflower stems and leaf petioles, and are similar to C. adspersus.
LARVAE: Larvae are similar to the larvae of
C. adspersus except they are yellowish in color and only 3/32 to 1/8 inch
(2.5 to 3 mm) long at maturity (Figure 7).
PUPAE: Pupae are similar to the adult in size
and yellow in color.
Figure 6. Adult of black sunflower stem weevil.
(Click here to link to a 23KB color photo of
an adult black sunflower stem weevil.)
Figure 7. Larva of black sunflower stem weevil.
(Click here to link to a 19KB color photo of
a black sunflower stem weevil larva.)
Hosts
Both species of stem weevils have a wide host range including
cultivated sunflowers, native sunflowers, and many weeds. Native sunflower hosts
include Helianthus annuus, H. pauciflorus, H. petiolaris, H. tuberosus,
and H. maximiliani. Accession and hybrids of cultivated sunflower are
screened for germplasm with reduced levels of weevil larvae (see Plant Resistance
section). Weed records also list ragweed, pigweed, Russian knapweed, lambsquarter,
golden ragwort, perennial sowthistle, red clover, cocklebur, kochia, and sugarbeets.
Life Cycle
Sunflower Stem Weevil -- Cylindrocopturus
adspersus (LeConte) (Figure 8)
There is only one generation per year. Adult sunflower
stem weevils emerge from overwintered stalks and root crowns (Figure 9) from
early April to early May in the southern Plains and mid-June to late July in
the northern Plains. Degree-day (DD) models have been developed to predict adult
emergence. Using a base temperature of 5°C (41°F), the first emergence
occurs at 379-420 DD, and by 651-865 DD 90% of adults had emerged depending
on location (southern plains-northern plains).
Adults first feed on stem and leaf tissue of volunteer
sunflowers, and then mate. Weevils migrate to cultivated sunflower in the eight
to 14-leaf stage. Females descend to the lower portion of the plant to deposit
eggs under the epidermis near the base of sunflower stalks. Adults are present
in the fields until late July in the southern plains and until late August in
the northern Plains, with peak densities in late June to mid-July. Eggs are
initially deposited around the first node (cotyledon) moving higher up the stalk
over time. In North Dakota, half of the eggs are deposited by mid-July. Under
laboratory conditions, females lay 0.5 to five eggs per day for a total production
of 24-195 eggs, depending on temperature. The greatest number of eggs was deposited
at 86°F (30°C ). Females survived up to 75 days when held at 68°F
(20°C) or 73.4°F (23°C).
Larvae begin to hatch in early July. Early instars (first
and second) feed in the subepidermal and vascular tissue and move to the pith
as larvae mature into 3rd to 5th instars (Figure 10). Larvae feed in the upper
stalk until early August and then descend to the lower portion of the stalk
or root crown by late August to overwinter. Larvae chew cavities into the stem
cortex or crown root and pupate within the chamber (Figure 9). In the spring,
adults emerge from the pupae and chew their way out of the stalk. The overwintering
survival of the larvae varies with micro-habitat. Mortality of larvae increases
when larvae are exposed in the soil.
Figure 8. Life cycle of sunflower stem weevil
(development time is about 2-4 weeks earlier in southern states). (Click
here to link to a 33KB color photo of the life cycle of sunflower stem weevil.)
Figure 9. Overwintering chamber inside stem cortex.
(Click here to link to a 51KB color photo of an
overwintering chamber.)
Figure 10. Cross-section of stalk showing larvae.
(Click here to link to a 55KB color photo of a
cross-section of larvae in stalk.)
Life Cycle
Black Sunflower Stem Weevil -- Apion occidentale
Fall
Apion occidentale overwinters as an adult in soil,
plant trash, sod and weed clusters. It emerges and feeds on volunteer seedling
sunflowers, and later moves to cultivated sunflowers in the two to four-leaf
stage. Mated females deposit eggs under the epidermis in the leaf petioles or
near axils. Hatching larvae tunnel in the pith area of the stem, pupate and
emerge as adults in early August. There is a period of about two weeks in late
July and early August when little or no Apion adult activity is observed.
Adults emerging in August feed on the leaves and stems of the plant, and as
the plant matures and leaves die Apion moves to the bract of the sunflower
heads. It can then be observed feeding on the bracts until sunflower harvest.
Damage
Adult feeding from both species of stem weevils cause
minor damage to the stem and leaf tissue of the plant. More importantly, both
weevils have been implicated in the epidemiology of the sunflower pathogen Phoma
black stem (Phoma macdonaldii Boerma). Phoma is thought to cause premature
ripening syndrome (early dry down) of sunflower in North Dakota. Although premature
ripening is probably caused by a combination of both abiotic and biotic factors,
evidence shows that stem-infesting insects may transmit disease organisms or
encourage the disease by stressing the plant. The sunflower stem weevil also
has been implicated in the transmission of Macrophomina phaseolina (Tassi)
Goid, which causes charcoal stem rot in sunflower in the southern Plains.
Damage by larvae of sunflower stem weevil causes serious
stalk breakage when larval populations are large. The stem is weakened by larval
tunneling, pith destruction, (Figure 11) and/or the overwintering chambers (Figure
9), causing stalk breakage and a loss of the entire capitula (head) prior to
harvest. Populations of over 80 larvae/stalk in irrigated sunflower in the southern
Plains were required to cause a yield loss from larval feeding. In North Dakota,
a mean infestation of 38 larvae resulted in 28% lodging. Stalk breakage due
to the sunflower stem weevil is most severe during drought stress or when high
winds occur as plants are drying prior to harvest. Lodging is a good indicator
of larval densities; however, lodging is also influenced by other factors, including
stalk diameter, cortex and pith thickness of the stem, weight of sunflower heads,
wind velocity and direction, position of larvae in overwintering chambers in
the stalk, and incidence of disease. Typically, breakage occurs at or slightly
above the soil line, in contrast to breakage attributed to a stalk disease,
which normally occurs farther up on the stalks.
Damage from the black stem weevil has not been well documented,
but severe pitting on the cotyledons or seedling sunflowers by adult feeding
has been observed (Figure 12). In situations of extremely high populations,
this has resulted in stand loss. However, in most cases populations are too
low to cause economic damage to sunflower. Stalk tunneling causes only minor
injury to the plant.
Figure 11. Larval of sunflower stem weevil tunneling
inside stem. (Click here to link to a 74KB color
photo of sunflower stem weevil larval tunneling inside stem.)
Figure 12. Pitting damage on seedlings caused
by black sunflower stem weevil. (Click here to
link to a 52KB color photo of sunflower stem weevil pitting damage.)
Integrated Pest Management of the Sunflower Stem Weevil
Cylindrocopturus adspersus (LeConte)
Effective Integrated Pest Management (IPM) requires a
broad approach that incorporates knowledge of the insect's biology and population
dynamics, determination of economic injury levels and the use of resistant cultivars,
as well as biological, cultural, and chemical controls. The ideal control strategy
utilizes techniques that require low input costs, are cost effective, and avoid
negative impacts on the environment. Management techniques that reduce weevil
densities in the stalks or improve the plant's ability to tolerate weevil attack
are recommended for C. adspersus. For example, anything that promotes
thick healthy stems helps reduce losses, since damage results from lodging of
larval infested stalks. Even with the same number of larvae, a plant that has
stems of increased diameter and greater stem density will be less likely to
break. Therefore, lower plant populations, adequate fertilization, and proper
soil moisture should help decrease sunflower lodging.
Cultural Control
Cultural control tactics use standard farming practices.
They are effective when they make the environment less favorable to the pest
or more favorable for the plant. Delayed planting, plant population, and tillage
are useful for managing the sunflower stem weevil.
Delayed planting is
effective at lowering larval densities in stalks in both the northern and southern
Plains. Stem diameter is an important factor affecting lodging of weevil infested
stalks. Stem diameter decreases with later planting dates, but larval density
also decreases significantly reducing the potential for lodging. Larval numbers
decreased from 21 to two and from 23 to two larvae per stalk when planting dates
were delayed from mid-May to early June in trials at Carrington and Prosper,
respectively. There is an indication that more mature plants (earlier planted)
may be preferred by ovipositing weevils.
Plant population impacts both the diameter of
sunflower stalks and percentage of lodged plants. In a field study, larval stalk
population averaging 12 larvae per stalk was not affected by plant density.
However, stalk diameter was significantly different among three different plant
populations, with the thinnest stalks in the most dense plantings. Lodging was
low at both 9,000 and 18,000 plants per acre (22,000 and 45,000 plants per hectare,
respectively). In contrast, almost 25% of the plants were lodged when the stalk
density increased to 36,000 plants per acre (89,000 plants per hectare). These
results from North Dakota demonstrate that with no change in insect levels in
the stalk, reducing the plant population can result in decreased damage from
lodging.
Cultivation of crop residues may provide some
control of the overwintering larvae/pupae. A combination of disking to break
up stalks and moldboard plowing to bury them at a depth of 6 inches (15 cm)
can cause larval/pupal mortality and severely impact the emergence of adult
stem weevils. Otherwise, larvae/pupae are physically protected in the woody
stalks. Survival is affected only by performing both operations. The greatest
impact is when the practice occurs as an area-wide program. The value of standing
stalks in holding snowfall to insure adequate moisture in the field and impact
of plowing on soil erosion must also be considered.
Plant Resistance
Host-plant resistance utilizes the plant's own defense
mechanisms against the insect to either avoid attack, destroy the insect, or
tolerate the injury. Greenhouse and field experiments have shown resistance
to feeding, oviposition, and larval development in many native species of sunflower.
Further development of resistant sunflower has been hampered by populations
too low for screening useful germplasm, which can only be conducted when field
densities are high. Some examples of sunflower accessions with low density of
weevil larvae in stalks are PI 386230, Ames 3391, and PI 431542.
Biological Control
Conservation of natural enemies in a crop agroecosystem
is important to keep pests below levels that cause economic damage. The naturally
occurring natural enemies prevent many plant-feeding insects from achieving
pest status. The conservation of these natural enemies allows them to operate
at their full potential. Manipulating the environment to eliminate adverse factors,
such as pesticides, can effectively protect the natural enemies present.
Parasitoids
The sunflower stem weevil is attacked by both egg and
larval parasitoids. The eggs of the weevil are attacked by Anaphes pallipes
(Ashmead) (Hymenoptera: Mymaridae). Eleven species of Hymenoptera have been
recovered from overwintering larvae: Nealiolus curculionis (Fitch), N.
collaris (Brues), Bracon sp. (Braconidae); Neocatolaccus tylodermae
(Ashmead), Chlorocytus sp., Mesopolobus sp., Pteromalus
sp. (Pteromalidae); Quadrastichus ainsliei Gahan, Tetrastichus ainsliei
Gahan (Eulophidae); Eurytoma tylodermatis Ashmead (Eurytomidae); and
Eupelmus sp. (Eupelmidae).
Parasitoid species are more diverse in the central Plains.
The reduced number of parasitoid species found attacking C. adspersus
in the northern Plains may be related to factors including low host populations,
slower migration by parasitoids into the region, or incompatibility with climatic
conditions.
Nealiolus curculionis (Figure 13) is the most
prevalent parasitoid attacking C. adspersus. It parasitizes larvae of
the sunflower stem weevil in both cultivated and native sunflower. Nealiolus
curculionis also attacks the red and gray sunflower seed weevils (Smicronyx
fulvus LeConte and S. sordidus LeConte) in native sunflowers (Helianthus
spp.) throughout the central and northern Plains. Adult parasitoids are active
in the field from late June to late August. Eggs are deposited in early instar
weevils feeding within the sunflower stalk. The immature parasitoids overwinter
within diapausing weevil larvae in the sunflower stalk. Studies have shown that
overall parasitization has increased from levels reported in the late 1970s
and early 1980s from averages of 5% to 27%. The consistent rates of parasitism
compared with the variable field densities of adult parasitoids suggest that
N. curculionis effectively attacks hosts under varying host population
densities. This parasitoid appears to be a consistent mortality factor in the
population dynamics of the sunflower stem weevil in cultivated sunflower.
Quadrastichus ainsliei (Figure 14) also was frequently
recovered representing 3% to 73% of the parasitoid species attacking weevil
larvae. Q. ainsliei deposits multiple eggs inside the weevil resulting
in many parasitoids from one host.
Neocatolaccus tylodermae is more commonly found
in weevils from Colorado and Kansas. It was rare in Nebraska and has not been
found in North and South Dakota or Minnesota. It has also been reported as a
parasitoid of C. adspersus attacking sunflower in Texas.
Additional research is needed to understand the sunflower
stem weevil parasitoid complex and their population dynamics in order to manipulate
these natural enemies to improve biological control of sunflower stem weevil
in cultivated sunflowers. The impact of pesticides on natural enemies must be
considered in developing management programs, since insecticides that are toxic
to the weevil also kill parasitoids.
Figure 13. Larval parasitoid of the sunflower
stem weevil, Nealiolus curculionis (Braconidae). Size=4-5 mm. (Click
here to link to a 36KB color photo of a larval parasitoid, Nealiolus
curculionis (Braconidae).)
Figure 14. Larval parasitoid of the sunflower
stem weevil, Quadrastichus ainsliei (Eulophidae). Size=4-5 mm. (Click
here to link to a 78KB color photo of a larval parasitoid, Quadrastichus
ainsliei (Eulophidae).)
Monitoring
Field monitoring for sunflower stem weevils to estimate
population is important. However, adults are difficult to see on the plants
due to their small size, cryptic color, and "play dead" behavior.
They are inactive on the plant or fall to the ground when disturbed and remain
motionless. Adults can be found on both surfaces of the leaves, the lower portions
of the stem, in leaf axils, within the dried cotyledons, or in soil cracks at
the base of the sunflower plant. Yellow sticky traps were unsuccessful in relating
captured adult numbers to larval infestations.
Sampling for the larval stage is difficult since they
completely develop within the sunflower plant. Thus, the only method for detecting
the presence of larvae is to split the sunflower stem, a time-consuming process.
Field scouting for adults should begin when plants are
in the eight to 14 leaf stage (Figure 15), developmental stage V-8 to R1, or
late June to early July, and continue until mid-July. Select sampling sites
70 to 100 feet in from the field margin. Count the number of adults on five
plants at five randomly selected sampling sites throughout the field for a total
of 25 plants. Calculate the average number of weevils per plant. Use an "X"
pattern (or "W" pattern) to space sample sites over the entire field.
When scouting for stem weevils, approach plants carefully and slowly to avoid
disturbing the adults.
Figure 15. Proper crop development stage, 8-14
leaf, for scouting for sunflower stem weevil. (Click
here to link to a 66KB color photo of 8-14 leaf crop development stage.)
Economic Threshold
1 adult sunflower stem weevil per 3 plants
Average field counts of one adult sunflower stem weevil
per three plants can result in damaging larval densities of over 40 larvae
per stalk at the end of the season (based on sampling during the two-week
period between 24 June and 7 July in North Dakota.) Larval populations of
25-30 or more per stalk can weaken the stem tissues and cause breakage.
Chemical Control
Chemical control strategies are directed at adults and
must be initiated prior to late June or early July to prevent significant egg
laying and to reduce the risk of economic damage from larval induced lodging.
By mid-July, 50% of egg deposition has occurred.
The use of both foliar and systemically applied insecticides
has been shown to be effective in reducing larval populations and percentage
of stalks lodged. Application of a foliar insecticide is recommended only when
adult weevil populations have reached the economic threshold level in a field.
Keep in mind that insecticides destroy natural enemies of the weevil and are
more adverse to the environment.
Insecticides registered for the sunflower stem weevil
in North Dakota as of 2002 are listed in the following table. Please check with
the current Field Crop Insecticide Management Guide -- E-1143, NDSU Extension
Service, North Dakota State University, Fargo, ND 58105 -- for updated insecticide
registrations. It is important that insecticide users READ, UNDERSTAND, and
FOLLOW ALL LABEL DIRECTIONS.
Insecticide Recommendations for Sunflower Stem Weevil
---------------------------------------------------------------------------------------------
Dosage Product
Insecticide lb ai/ acre Per Acre Restrictions on Use
---------------------------------------------------------------------------------------------
FOLIAR APPLICATIONS
---------------------------------------------------------------------------------------------
Asana XL* RUP 0.03-0.05 5.8-9.6 fl oz Do not apply within 28 days of harvest.
esfenvalerate
Baythroid* RUP 0.025-0.0375 1.6-2.4 fl oz Do not apply within 30 days of harvest.
cyfluthrin
Sevin 1-2 rate varies by Do not apply within 60 days of harvest.
carbaryl formulation Do not allow livestock to graze on treated forage.
Furadan 4F* RUP 0.5 1 pt Do not apply within 28 days of harvest.
carbofuran
Lorsban 4E* RUP 0.5 1 pt Treat about 5 to 7 days after adult stem weevils
chlorpyrifos begin to appear. Do not apply within 42 days of
harvest. Do not allow livestock to graze in treated
areas. Do not apply more than 9 pints per acre per
season.
Scout X-TRA* RUP 0.014-0.0164 2-2.33 fl oz Do not apply within 21 days of harvest.
tralomethrin
Warrior* RUP 0.02-0.03 2.56-3.84 fl oz Do not apply within 45 days of harvest.
lambda cyhalothrin
---------------------------------------------------------------------------------------------
AT PLANTING APPLICATIONS
---------------------------------------------------------------------------------------------
Furadan 4F* RUP 0.08 2.5 fl oz per Apply directly into the seed furrow. May be mixed
carbofuran 1000 linear feet with water or liquid fertilizer.
of row (or 1.4
quarts per acre
with 30 inch
row spacing.
---------------------------------------------------------------------------------------------
*RUP - Restricted use pesticide
For more information on this and other
topics, see: www.ag.ndsu.nodak.edu
E-821 (Revised), October 2002
|
