What's Bugging You This Year?
Thursday, April 15, 2004
filed under: Insects
Most cutworm damage occurs when plants are in the early stage of development. Damage consists of young plants being chewed off slightly below or at ground level. Some cutworm feeding injury may occur on foliage. Cutworms primarily feed at night. When checking fields for cutworms during the day, dig down into soil an inch or two around recently damaged plants; there you can find the gray to gray-brown larva.
Warranted when one cutworm or more is found per square foot or there is a 25 to 30% stand reduction observed.
Wireworms join cutworms as crop pests that growers should watch for early in the growing season, particularly in a no-till system. Wireworms have a longer life cycle than cutworms, and usually feed on roots and germinating seedlings below ground. Wireworms prefer more moist, cooler soil temperatures (50-55 degrees F) and move deeper into the soil if soil gets too dry and when soil temperatures become too hot (>80 F). Currently the only insecticide registered for wireworm in sunflower that provides seedling protection is the new seed treatment Cruiser (thiamethoxam). Some producers who have applied Furadan 4F (Carbofuran) at planting for treating early-season insects have observed wireworm control. The systemic product can be mixed with water or liquid fertilizer, and producers have observed that applying it in-furrow with the seed doesn’t hurt seed germination or seedling development.
Grasshopper numbers are usually weather dependent, with the most severe infestations likely to occur during seasons when the weather is hot and dry. Scouting should begin in May and early June, and producers should be prepared to start management measures when young hopper populations reach threatening levels. Most grasshoppers emerge from eggs deposited in uncultivated ground. Sunflower growers should expect to find grasshopper feeding first along field margins adjacent to these sites. Later infestations may develop when grasshopper adults migrate from harvested small grain fields.
Grasshopper control is advised whenever 20 or more adults per square yard are found in field margins or 8 to 14 adults per square yard are occurring in the crop.
BANDED SUNFLOWER MOTH
Banded sunflower moths (BSM) begin to emerge from the soil about mid-July. Peak activity normally occurs about the last week of July or the first week of August. Moths fly from last year's field to the current year's field. At this time moths congregate around field margins. The moths move to fields in the mid-to-late stages. Eggs are laid on the back of the bud and the outside of bracts. Newly hatched larvae move from these sites to the face of the flower and begin feeding on bracts and florets. BSM damage last year was much lower in S.D. compared to 2002. Last fall’s sunflower survey indicated some increase in N.D., but still low levels of damage overall.
Consider when 1 moth for every 2 plants inspected can be found. Because moths initially congregate around field margins prior to flowering, treatment of field margins has reduced the adult population.
A migratory insect, this grayish-tan moth moves into fields in early bloom, depositing its eggs on the face of the flower. Damage is similar to that caused by the banded moth. The same monitoring strategies are recommended for sunflower moth as those for the banded moth. Last year’s sunflower survey indicated very low levels of this insect in the High Plains states, with the exception of fields surveyed in Texas, where seed samples indicated an increase of moth damage compared to the previous year.
When 1 to 2 moths are found for every 5 plants inspected, treatments should be considered.
The midge is a small tan-colored fly, 3/32 inch in length. It emerges in early July, preferring to lay eggs on developing buds, 1 to 2 inches in diameter. The cream to yellowish-orange larvae feed on bract tissue at first and later on the flowers and seeds. When feeding is confined to the bracts, damage results in little economic loss. When many larvae feed in the center of the bud, seed is not produced. This type of injury appears as twisted and gnarled flowers. Often, infestations will be limited to field margins. When populations are large, damage may extend into the field and significant field losses may be observed. Good soil moisture in the month of June appears to promote survival and emergence of midge. Sunflower midge levels were generally low last year, according to NSA field surveys.
There are no effective chemical controls currently recognized for this pest. The best management strategy is crop rotation. Rotate to crops other than sunflower in the vicinity of large infestations. Staggering planting dates to promote different budding periods between fields aids in reducing risk of damage to all fields in the same geographic areas. Some hybrids may have greater midge tolerance than others – ask your seed dealer for guidance.
SUNFLOWER SEED WEEVIL
The red sunflower seed weevil begins to emerge in early July and continues until mid-August. Peak emergence occurs in late July. Start counting adult seed weevils when the yellow ray petals are just beginning to show. Counts should continue until the economic threshold level has been reached or most plants have reached 70% pollen shed, when few seeds are still suitable for red seed weevil egg laying. Fields where most plants are at 70% pollen shed should no longer be susceptible to further significant damage. When sampling, use the X pattern and begin counting at least 70 to 100 feet into the field to avoid field margin effects. Count the number of weevils on five plants at each site for a total of 25 plants. The ideal plant stage for treatment is when three out of 10 plants are in early bloom. Last year’s NSA field surveys indicated sporadic seed weevil damage in the High Plains, less damage in S.D., but slightly higher damage in N.D.
For confection Sunflower, treatment is recommended when 1 to 2 weevils are found per plant. For oil sunflower, the economic threshold can be calculated using the following formula.
Threshold (Weevils per head) = Cost of Insecticide Treatment .
(Market Price x 21.5) (0.000022 x Plant Population + 0.18)
SUNFLOWER STEM WEEVIL
The sunflower stem weevil is 3/16 inches in length, and grayish-brown with varying shaped white spots on the wing covers. The weevils emerge in mid-to-late June. Eggs are deposited in epidermal tissue of the stem. If controls are directed at the adults in order to minimize egg laying, treatments should be initiated during the first few days in July. About 50% of the eggs will be deposited by this weevil by mid-July. When about 25 - 30 larvae are present in a stalk, it becomes weakened, with breakage most likely to occur during drought stress or high winds.
Scouting is difficult due to their size, coloration, and habit of “playing dead.” Examine 5 plants each at 5 locations and keep record of the number of weevils found. Approach plants carefully to avoid alarming the weevils, causing them to drop to the ground. Scout from late June to mid-July. Last year’s NSA field surveys indicated an increase in stem weevil activity in Colorado and N.D., but less in Kansas, S.D., and Texas.
Treat for sunflower stem weevils when scouting determines that an average of 1 adult per three plants is found.
Sunflower beetles begin feeding shortly after they emerge from overwintering. Emergence starts in mid-May. Most feeding by adults is concentrated on the true leaves. When beetles are numerous, fields may be severely defoliated. Adults quickly begin laying pale yellow eggs on stems and the underside of leaves. Eggs hatch in about 8 days. The pale green humpbacked larvae begin feeding, eating holes throughout the leaf. Larvae do not feed during the day, resting in the plant tops where they are easily observed. There was generally low activity of this insect last year.
Adults-Treatment is recommended when scouting determines that an average of 1 to 2 beetles per plant can be found throughout the field.
Larvae-When an average of 10 to 15 larvae per plant is found, defoliation levels of 25 to 30% would be expected. Treatment is suggested when damage levels reach this point and most larvae are 1/4 inch in size.
Other Insects to Watch
The palestriped flea beetle has a widespread presence in the U.S., but has been a localized problem in South Dakota the past few years, causing leaf injuries to sunflower seedlings in about seven central S.D. counties. It is also known to feed on numerous field crops including potatoes and corn. In S.D., it is mainly found feeding on weeds; the reason for its recent feeding on sunflower is currently unknown.
Adults are very small, about 3/16 of an inch long, dark brown to black in color, with two white stripes down its 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 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 transform into pupae, then adult flea beetles in the summer. There is only one generation per year.
The new insecticide seed treatment Cruiser has demonstrated control of the palestriped flea beetle. Some growers have also observed flea beetle control when using the product to treat other early-season insects. Flea beetles produce a characteristic injury known as “shot-holing.” The adults chew many small holes or pits in the leaves, which make them look as if they have been damaged by fine buckshot. Young plants and seedlings are particularly susceptible. Colorado State University has a fact sheet on flea beetles online: http://www.ext.colostate.edu/pubs/insect/05592.html as does SDSU: http://plantsci.sdstate.edu/ent/entpubs/SEE_010702.htm
Lygus (Tarnished Plant Bug)
The incidence of kernel brown spot from Lygus feeding on confection sunflower in N.D. was reduced in 2003 compared to 2002, but is still something for confection sunflower producers to monitor. Sunflower is susceptible to Lygus damage during flowering, from anthesis through seed hardening. Lygus can be treated at the same time confection sunflower is treated for other insects, such as the seed weevil and banded sunflower moth. NDSU entomologists suggest two insecticide treatments for confection sunflower to adequately protect heads from insect feeding: One application at about 10% bloom, followed by a second treatment 7 days later.
Longhorned Beetle (Dectes or Soybean Stem Borer)
Last year’s sunflower survey indicated sporadic incidence of this insect in the central Dakotas and High Plains. The bluish gray adult stem borer is about 5/8 inches long with long banded antennae. In soybeans, adults lay eggs during July and August, and larvae tunnel through stems until September. Even if an insecticide for control was available (no sunflower insecticide labels include this insect), treatment would be difficult due to prolonged emergence of the adults. Early research indicates that timely harvest will help prevent lodging due to this insect. Avoid rotating soybeans with sunflower where this insect has been a problem. See fact sheet online: http://www.planthealth.info/stemborer/stemborer.htm
Insect Info on the Internet
2004 N.D. Field Crop Insect Management Guide
NDSU Crop Insect Publications
North Dakota State University Entomology
Kansas State University Extension Entomology
South Dakota State University Extension Entomology
Colorado State University stem weevil fact sheet
University of Nebraska Insect Treatment Recommendations
Online Maps Point Out Last Year’s Sunflower Pest Problems
The following maps indicate spotted stem weevil incidence (field interior) of
sunflower fields surveyed last year in sunflower producing states. While last year’s pest problems may not necessarily carry over to this year, and are highly dependent upon weather and other factors, results from the survey, coordinated by the National Sunflower Association, can help pinpoint potential hot spots for disease, insects, and weeds.
Maps similar to these indicating pest incidence in sunflower producing states can be found online, www.sunflowernsa.com. On the home page, click on the “Growers” link, then “USA Sunflower Survey.” There, field maps by state can be viewed and compared with survey results from previous years.