Larry D. Charlet
Northern Crop Science Lab
Gary J. Brewer
Department of Entomology
University of Nebraska
Sunflower, Helianthus annuus L., is the only row crop in North America coexisting with its native congeners. When extensive commercial planting of sunflower began in the 1970s, many of the insects which evolved on native Helianthus, a complex of 51 species, transferred to the cultivated crop. This situation aided in the development of insect problems by providing monocultures where only isolated hosts were previously available. Over 150 phytophagous insect species have been reported from cultivated and native sunflower. However, only a few insect species have adapted to cultivated sunflower and have become economic pests. The discussion of the major insects pests associated with sunflower that follows is organized by the plant part attacked.
Stem and Root Feeding Species
The sunflower bud moth, Suleima helianthana (Riley) (Lepidoptera: Tortricidae) is found on sunflower in central North America from Mexico to Canada, occurring in all sunflower growing areas. Two generations occur in the northern Plains. Larvae tunnel into the stalk, leaf petiole, or receptacle to feed on the pith. Entrance holes, characterized by a protruding, black, sticky frass in the stalk, leaf petiole, or less commonly, the bud, are very diagnostic. Larvae of the second generation are more likely to feed in the receptacle of the capitula than the first generation. Although larval feeding sometimes produces malformed stalks, leaf petioles, and capitula, tunneling in stalks does not normally interfere with stem development. Yield loss is generally only noticeable when larvae feed and burrow into an unopened bud, preventing normal head development and thus production of seed.
The longhorned beetle or stem girdler, Dectes texanus LeConte (Coleoptera: Cerambycidae), has been noted as a pest of sunflower since the early 1970s when it was reported to have caused considerable damage in south central Texas. Fairly high populations of this pest were noted in 2002 in stalks from the central Plains extending into South Dakota. The adult is 6 – 11mm in length and pale‑gray with long gray and black banded antennae. The adult produces feeding scars, which may be apparent on the sunflower stem, but they usually do not penetrate the cortex nor encircle the stalk. The eggs are deposited singly in leaf petioles. The larvae feed and tunnel in the petioles, then into stem pith, and finally move to the base of the plant to overwinter. In late summer, the mature larvae girdle the inside of the lower stalk or root crown, move below the girdle, and pack frass into the tunnels. Stalks often break at the point of girdling, leaving the larva protected in its frass packed tunnel during the winter. The larvae are cannibalistic, thus stalks usually harbor only a single larva even though several may have originally hatched in a stalk. There is one generation per year. Host plants include sunflower, ragweed, and cocklebur. The longhorned stem girdler also attacks soybean. Plant lodging resulting in economic damage has been reported in Kansas. Scouting methods have not been developed for this pest, but adults are fairly easy to locate resting on the upper leaves of the sunflower plant. The adults have an extended emergence period and so may be present in fields for many weeks. This has made chemical treatments for adults on soybean and sunflower impractical. Earlier research in the southern Plains showed that later planting dates and fall or winter tillage have reduced sunflower infestations by this pest. When larvae are present in the stalks, plants do not always lodge. If fields are suspected to be infested, prompt harvesting may reduce losses from lodging. Also, lower plant populations producing thicker sunflower stem may reduce damage from lodging.
The sunflower stem weevil, Cylindrocopturus adspersus (LeConte) (Coleoptera: Curculionidae), is an economic pest of cultivated sunflower in both the northern and southern Great Plains of the United States and in Saskatchewan. Occasionally, western Kansas, eastern Colorado, and North and South Dakota experience severe losses due to this pest. Larvae feed, develop, and overwinter in the stem. Adults are 4 to 5 mm long and grayish‑brown. Adults emerge from overwintered stalks and root crowns in early to mid-April in the southern Plains and mid‑to‑late June in the northern Plains. Adults feed on stem and leaf tissue. Females deposit eggs under the epidermis at the base of sunflower stalks. Early instar larvae feed in the vascular tissue and, as the larvae mature, tunnel into the pith. Larvae descend to the lower portion of the stalk or root crown by late August and excavate chambers in the stem cortex where they overwinter as fifth instars, pupating the following year in late spring. Generally, feeding by adults causes minor damage to the plant. If the larval population in a plant is high, the stem, weakened by tunneling, pith destruction, or overwintering chambers, may break causing a loss of the capitula prior to harvest. In North Dakota, a mean infestation of 38 larvae per stalk 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. Populations of over 80 larvae/stalk in the southern Plains were required before there was a yield loss due to larval feeding. The sunflower stem weevil has also been implicated in the epidemiology of sunflower pathogens, such as Phoma black stem (Phoma macdonaldoii Boerma), that contribute to the premature ripening syndrome in the northern Plains and may predispose plants to infection by charcoal stem rot (Macrophomina phaseolina (Tassi) Goid) in sunflower in the southern Plains. The economic threshold for the sunflower stem weevil is one adult per three plants. However, sampling for adults is difficult because they may be on both leaf surfaces, on the stem, in leaf axils, within dried cotyledons, or in soil cracks at the base of the plant. They also fall from the plants when disturbed and remain motionless. The use of foliar insecticides at the 8- to 14-leaf stage has been effective in reducing larval populations and stalk lodging. Insecticides applied in-furrow at planting also have shown efficacy in reducing numbers of larvae. Delayed planting was valuable in both the northern and southern Plains in lowering larval densities in stalks. Tillage was unsuccessful in increasing mortality of overwintering larvae due to protection of the larvae within stalks, but burial of the stalks did result in reduced emergence of adults the following spring. Resistance to feeding, oviposition, and larval development occurs in many native sunflower species. A number of species of parasitic wasps attack the larvae.
The black sunflower stem weevil, Apion occidentale Fall (Coleoptera: Curculionidae) has been reported from North Dakota, Minnesota, and Texas. Overwintering adults emerge from late May to early June. Adults are totally black, about 2.5 mm in length, and feed from early spring to late July on leaf and stem tissue. Feeding is initially on volunteer sunflower until cultivated fields develop to the seedling stage. Eggs are deposited under the epidermis in leaf petioles or stems near axils. Larval feeding takes place in the vascular and pith tissue of the stem and petiole. Larvae pupate within the feeding tunnels and adults emerge in August. Adults feed on leaves and stems and the bracts surrounding the sunflower head, moving into the soil or plant residue to overwinter as plants senesce or are harvested. Feeding damage is seldom significant, but the weevil has been associated with the transmission of the pathogen, Phoma macdonaldii Boerma, the causal agent of Phoma black stem. In recent years, severe pitting on cotyledons or seedling sunflower has been observed and occasionally has resulted in some stand loss. In most cases, damage from the black sunflower stem weevil has been noneconomic.
The carrot beetle, Ligyrus gibbosus (DeGreer) (Coleoptera: Scarabaeidae) has a wide host range and occurs throughout the United States, southern Canada, and northern Mexico. Twenty-four species of noncultivated plants have been noted as hosts for this beetle. Damage has been reported from Kansas, but the carrot beetle has only been a serious pest of sunflower in the High and Rolling Plains of Texas and adjoining areas of Oklahoma. The carrot beetle is univoltine with mass emergence of overwintering adults from the soil in early April. Females begin oviposition by early May. The adult burrows into the soil around the plant roots at night. Eggs are deposited at the base of plants in soil high in organic matter. The larvae or grubs are detritus feeders and do not occur in sunflower fields. Adults overwinter in the soil. Damage occurs as a result of adult pruning of lateral roots and feeding on the taproot which causes sudden wilting and death of the plant. Application of insecticides has been ineffective in preventing damage from adult feeding. Adults are attacked by bacteria, fungi, five species of parasitic flies in the family Sarcophagidae, and by a number of vertebrate predators. Larval mortality is due to attack by predaceous carabid beetles and pathogens. Resistance has been reported in some of the perennial Helianthus species.
There are three species of sunflower maggots that are pests of cultivated sunflower and attack different parts of the plant. These include the sunflower receptacle maggot, Gymnocarena diffusa (Snow), the sunflower maggot, Strauzia longipennis (Wiedemann), and the sunflower seed maggot, Neotephritis finalis (Loew) (Diptera: Tephritidae). The adult forms of all three sunflower maggots have wings with a distinct brown or yellowish‑brown pattern. While all three fly species are similar in appearance, they do have distinguishing differences.
The total larval period is 14 days. The first generation pupates in thehead; the second generation overwinters in the soil as pupae. Economic damage from any of these species is rare. The magnitude of damage to sunflower seeds by sunflower seed maggot larvae largely depends on the larval stage and seed development. Seed sterility occurs when newly hatched larvae tunnel into the corolla of young blooms; a single larva feeding on young flowers will tunnel through 12 ovaries. Mature larvae feeding on older sunflower heads destroy only one to three seeds. While infestation levels of the sunflower maggot have occasionally reached nearly 100 percent, damage within sunflower stalks from larval feeding usually is light.The larvae of the sunflower maggot are creamy-white and attain a length of about 7 mm at maturity. The sunflower seed maggot is the smallest of the three species with a body length of the adult about 6 mm and a wing span of approximately 7 mm. The wings have a brown lace‑like appearance. The larvae attain a length of 4.5 mm at maturity. The small, brown pupae are found in the face of the sunflower bud, usually surrounded by a small number of damaged disk flowers. Adults of the sunflower receptacle maggot emerge in late June to early July after sunflower buds reach 5 to 10 cm in diameter. Eggs are laid on the bracts of the developing sunflower heads. Egg laying occurs from mid-July through August. The hatched larvae tunnel into the spongy tissue of the receptacle. Damage to the head is negligible. After 30 days, the mature larvae cut a small emergence hole on the underside of the receptacle and drop into the soil to pupate. Overwintering pupae are found about 19 cm deep in the soil by August or early September. Some larvae will pupate in the sunflower head. There is only one generation per year in North Dakota. The sunflower maggot has one generation per year. This species overwinters as a larva in plant debris in the soil; pupation and adult emergence are completed in early June. Females lay eggs in stem tissue of young sunflower, and larvae feed in the stem pith tissue. Unlike the other two species of sunflower maggots, two complete generations per year of N. finalis occur in North Dakota. Adults of sunflower seed maggot emerge during the first week of July andoviposition occurs on the corolla of incompletely opened sunflower inflorescences. The sunflower receptacle maggot is the largest of the three with a body about 10 mm long and a wing span of approximately 19 mm. The eyes of this species are bright green and the wings have a yellowish‑brown and somewhat mottled appearance. The larvae are yellowish‑white in color attain a length of nearly 8 mm at maturity. The adult has a wing spread of about 13 mm and a body 6 mm long. The wings bear broad, dark bands that form a fairly distinct F‑shaped mark near the tips.
The sunflower root weevil, Baris strenua (LeConte) (Coleoptera: Curculionidae), can cause severe wilting of sunflower plants, although damage is usually in localized areas and the fields typically have sandy soil profiles. Adults are robust looking weevils with a somewhat oval shaped body, are 6 mm long, have a short, almost blunt, downward projecting snout, and are dull-black in color. The larvae are similar in appearance to sunflower stem weevil larvae but much larger and are not located in the sunflower stalk. Adults emerge during the latter part of June and feed on sunflower foliage in early morning and late afternoon. About two weeks after emergence, adults begin to congregate around the root zone near the soil surface. Feeding activity during this period produces callus tissue under which the bright yellow eggs are deposited in groups of two or three. Larval hatching normally occurs during the second week in July. Larvae form circular tunnels and development to the fourth instar takes place in the same area where hatching occurs. The sunflower root weevil adults cause negligible mechanical injury to the sunflower foliage. The destruction of root tissue by the larvae causes the plants to wilt and lodge, if the infestation is severe. In late August to early September the larvae have developed to the fourth instar and form a “soil cocoon" and overwinter among the remaining roots in the soil. Overwintering larvae have been recovered from a depth of 38 cm in North Dakota.
Foliage Feeding Species
The palestriped flea beetle, Systena blanda (Melsheimer) (Coleoptera: Chrysomelidae), has recently become a serious pest of seedling sunflower in central South Dakota. The adult is about 3.2 mm long, shiny black in color, with two white stripes on the back. The hind legs are enlarged and modified for jumping. The exact life cycle of palestriped flea beetles on sunflower fields is currently unknown. However, the adults seem to overwinter in the field under soil clods, field debris and crop residues. They become active again in the spring, perhaps feeding first on alfalfa and weeds before moving to and feeding on sunflower seedlings in June. They have been observed feeding on sunflower through July. The biology of the egg, larval, and pupal stages is currently unknown. Palestriped flea beetles have a wide host range which includes various weeds, potato, tomato, carrot, peanut, corn, oat, cotton, pea, beans, strawberry, watermelon, grape, and pumpkin. Palestriped flea beetles are considered an important pest of commercially grown vegetables in some areas of the United States. Recently, palestriped flea beetles have been observed delaying regrowth of alfalfa and were also observed feeding on soybean seedlings in eastern South Dakota. Adults chew on the cotyledons, leaves, and hypocotyls of sunflower seedlings causing them to wilt and die. Injured leaves become riddled with holes giving them a "lacey" appearance. The sunflower plant is most sensitive to palestriped flea beetle injury from seedling emergence (V‑E) through the four‑leaf stage (V‑4). Significant stand losses may result from heavy feeding by the palestriped flea beetles. Control is recommended when 20% of the seedling stand is injured and at risk to loss due to palestriped flea beetle feeding. This economic threshold is a guideline based on published hail injury data that predicts potential yield loss relative to seedling stand loss. Research has shown that insecticide treatments may provide up to 75% control of adults. (information provide by Mike Catangui, South Dakota State University).
The painted lady or thistle caterpillar, Vanessa cardui (L.) (Lepidoptera: Nymphalidae) is probably the most widely distributed butterfly species in the world and has a host range of over 100 species of food plants. Larvae feed primarily on Canada thistle (Cirsium arvense (L.) Scop.) and help reduce populations of this noxious weed However, H. annuus is a common host and the painted lady has been an occasional pest of sunflower in Canada, the U. S., and Mexico. Adults migrate to the northern areas of North America from overwintering sites in Mexico, arriving in early June. There is no evidence of diapause or any stage surviving cold winters. Larvae feed on the leaves producing a loose silk webbing and, if numerous, can completely defoliate a plant. Defoliated leaves with black fecal pellets associated with the webbing are indications of thistle caterpillar infestations. Occasional outbreaks in the northern Great Plains have resulted in localized severe damage of cultivated fields. Parasitization and infections of bacterial disease generally limit damage from this pest.
The sunflower beetle, Zygogramma exclamationis (Fabricius) (Coleoptera: Chrysomelidae) occurs in most areas of the United States, but economic damage has usually been confined to the northern Plains and Manitoba. The adult is 6 to 8 mm long with a reddish‑brown head. Each elytron is creamy‑white with three dark stripes that extend the entire length. A shorter, lateral stripe ends at the middle of the wing in a small dot resembling an exclamation point. Larvae are yellowish‑green, humped‑back, and about 10 mm long at maturity. Adults overwinter in the soil and emerge during May and feed on the first available foliage of sunflower. Beetles mate shortly after emergence, and eggs are deposited singly on the stems or undersides of sunflower leaves. Mature larvae enter the soil to pupate in earthen cells. New generation adults emerge at the end of July or the beginning of August. The adults re-enter the soil after a couple weeks to overwinter. New generation adults do not mate and no eggs are developed before entering the soil. There is one generation per year in the northern Plains. Adult feeding on leaf edges can be distinguished from larval feeding which occurs over the entire leaf surface. Adults feed throughout the day, whereas the larvae feed mostly at night and spend the day hidden in the terminal growth area. Population densities of only two adults per plant plus the resulting larval feeding can reduce seed yield by over 20%. Larval populations of 25 or more per plant can completely defoliate a plant and reduce yields by as much as 30%. Control measures are advised if defoliation reaches 25 to 30% on the upper 8-12 leaves and larvae are still in the early stages of growth. Application of insecticides when larvae are actively feeding has been shown to be effective in preventing economic damage. New generation adults generally do not cause economic damage to the sunflower plants, although when populations are high, feeding injury may be present. Adult and larval populations decrease as planting date is delayed and defoliation also is lowest at the later planting dates. As a result, delayed planting is effective in preventing yield reductions caused by sunflower beetle feeding, but can make fields more attractive to later season insects, such as red sunflower seed weevil. Spring or fall cultivation does not reduce the overwintering populations of adults or influence emergence patterns from the soil during the spring and summer. Resistance to feeding or reproduction has been shown in approximately one‑half of the native sunflower species. Perennial species exhibited antibiosis to both larvae and adult in the laboratory. General predators such as ladybird beetles, carabid beetles, lacewing larvae, nabids, stink bugs, and anthocorids destroy both sunflower beetle eggs and larvae. Parasitoids also attack all stages of the sunflower beetle.
Head and Seed Feeding Species
The tarnished plant bug, Lygus lineolaris (Palisot de Beauvois) (Heteroptera: Miridae), has recently become a problem in confection sunflower. The presence of scarring on sunflower seeds, known as kernel brown spot, was determined to be caused by feeding of lygus bugs on the developing seed. The quality issue is significant because processors are allowed only 0.5% damage in the finished product and the incidence of damage in 2001and 2002 ranged between 1-7% in some areas in the northern Plains. The most common species occurring in sunflower fields is the tarnished plant bug. This species attacks at least 385 different plant species and occurs in 39 U. S. states and 5 Canadian provinces. The adults are small, cryptically colored insects with a distinctive yellow triangle or a "V" on the wings. They vary in color from pale-green to dark-brown. The immature stages or nymphs are similar in appearance to the adults, but lack wings, and are usually green in color. They are often confused with aphids. The species of Lygus feed preferentially on either the developing reproductive organs or on the apical meristematic and leaf primordial tissue causing a necrosis around the feeding site due to the injection of enzymes. This tissue destruction causes the brown spot on the sunflower kernel and can also result in a bitter taste to the seeds. Adults overwinter in leaf litter and there are probably at least two generations per year in the northern Plains. Populations probably move to sunflower from alfalfa or canola when those crops have either been harvested or senesced. Sunflower crop surveys in 2001 and 2002 showed that damage was present in cultivated confection fields in both North and South Dakota, although only a few fields were sampled in South Dakota. Damage averaged about 4% in North Dakota. Results from studies conducted on lygus bug and its impact on sunflower showed that all life stages of lygus bug are present in both confection and oilseed sunflower. Greenhouse and field studies showed that 33 to 37 seeds were damaged per adult lygus bug and that all reproductive growth stages were vulnerable to attack. The economic injury level for tarnished plant bug was one adult per 10 to 15 plants, each head averaging 500 to 600 seeds, for a damage level of 0.5%. Sticky trap catches in Minot, North Dakota, showed that lygus bugs were present throughout the reproductive growth stages of sunflower. Insecticide treatments with pyrethroids have reduced feeding damage by lygus bugs when applied at the beginning of flowering.
The sunflower moth, Homoeosoma electellum (Hulst) (Lepidoptera: Pyralidae) is the most widespread and damaging pest of sunflower in North America, except in North Dakota, South Dakota, and Minnesota where the red sunflower seed weevil, Smicronyx fulvus LeConte, and the banded sunflower moth, Cochylis hospes Walsingham, cause more yield loss. The sunflower moth occurs from Mexico to both coasts of the United States and to the Canadian Prairie Provinces. The adult is a small shiny-gray moth. The larva has alternate dark and light stripes running longitudinally on a buff-colored body. The first seasonal appearance of moths and larvae each season depends on the latitude of the location. Infestations in Texas occur in early May, but do not develop in North Dakota and the Canadian Prairie Provinces until August. Infestations are frequent and usually severe from Texas to Kansas. In the more northern limits of their range, the sunflower moth does not overwinter. Infestations in those areas are sporadic and dependent on migration aided by southerly winds. The host range of the sunflower moth includes wild and cultivated sunflower and other species of Compositae. Eggs are deposited on the surface of open sunflower heads. First instars feed primarily on pollen. Second instars feed on pollen, and may burrow through the corolla to feed on pollen inside disk flowers. Feeding by third instars may sever the style and prevent the ovary from being fertilized, resulting in empty seeds. Third instars also begin feeding on ovaries. Larval feeding until maturity results in an average of about 96 damaged disk flowers and about 23 damaged ovaries per larva. As they feed, larvae spin a webbing over the face of the sunflower head. The accumulated debris in the larval webbing and damage caused by larval feeding predispose the head to Rhizopus infection. Mature larvae move to the ground where they spin overwintering cocoons. A number of tachinid and hymenopteran parasitoids attack the sunflower moth and aid in its control, but other methods are often needed to protect commercial sunflower from economic loss. Sex-pheromone traps can be used to monitor populations. In Kansas, early plantings usually have higher infestations than later plantings. However, in other locations, planting dates have to be adjusted for conditions such as moth flight, available moisture, and length of the growing season. Phytomelanin based plant resistance is used to manage the closely related European sunflower moth, H. nebulella (Hubner), and may impart resistance to H. electellum. The microbial insecticide, Bacillus thuringiensis (Berliner), can be used to suppress sunflower moth infestations and may be as effective as synthetic insecticides. Despite the research on cultural and biological controls and plant resistance, the control method of necessity is usually an insecticide.
The banded sunflower moth, Cochylis hospes Walsingham (Lepidoptera: Tortricidae), is widely distributed. It is found throughout the Great Plains and the Midwest, in the east coast states of North Carolina and New Jersey, and in Oregon on the west coast. Based on its known distribution, the moth probably occurs wherever wild Helianthus spp. grow. Moderate banded moth damage consistently occurs on sunflower grown in North and South Dakota and Minnesota. The moth is a potential pest in Kansas, Colorado, and Nebraska. The adult moth possesses a distinct, dark-brown triangular scale patch covering the mid‑portion of the forewings. The body color of larvae differs among instars. The early instars are light‑pink and late instars are red or red and green in color. Moths often stage in vegetation along field margins during the day. At twilight, females move into the fields to oviposit. In North Dakota, oviposition begins during early July. The majority of eggs are oviposited on the outer whorl of the involucral bracts. The remainder occur on the underside of the sunflower head. Newly emerged larvae are found on the involucral bracts and later move to the face of the capitulum to feed on florets and pollen. Third instars chew into the seeds. Each larva will penetrate and consume the contents of several seeds. At maturity, larvae drop from the head and enter the soil to overwinter in silken cocoons. Pupation occurs in late June. More than one generation is possible in the southern regions. Sampling plans have been developed using both adult and egg counts. Sampling is most accurate when conducted in early morning or late evening when moths are active within the sunflower field. During this period the economic threshold is estimated to be one adult per two plants. Insecticide efficacy can be maximized if application timing is based on the plant developmental stage. Chemicals should be applied to sunflower from the late bud stage (R4) to early bloom (R5.1) to significantly reduce banded sunflower moth damage. Damage caused by the banded sunflower moth can be minimized by manipulating planting dates to avoid oviposition. Sunflower planted late (early June) in southeastern North Dakota had less damaged seeds than sunflower planted early (early to mid-May). Resistance to the banded sunflower moth is present in some species of native sunflower. Honey bees have been shown to effectively act as vectors of Bacillus thuringensis to sunflower heads depositing enough spores to cause banded sunflower moth larval mortality. A new strain of Bt isolate from sunflower head extracts collected in Kansas has been shown to be highly active against larvae of the banded sunflower moth. Eggs and young larvae are preyed upon by general predators and larvae are attacked by a number of parasitic Hymenoptera. Ground beetles were found to destroy about 40% of overwintering larvae and pupae.
The red sunflower seed weevil, Smicronyx fulvus LeConte (Coleoptera: Curculionidae) occurs from the Appalachian Mountains westward through the Great Plains and to the Pacific Northwest. It is a consistent, economic pest of sunflower in the Dakotas and Minnesota and is the most common of the two sunflower seed weevil species in the northern latitudes. Red sunflower seed weevil adults are 2.5 to 3 mm long and are covered with reddish‑orange, oval scales. Larvae are found inside and usually in the upper third of developing sunflower seeds. Adults appear during late June on volunteer sunflower and feed on the involucral bracts where they form pinpoint holes. As the bud develops and opens, adults move to the inflorescence and feed on pollen produced by the disk flowers. Females oviposit inside the pericarp of developing seeds. Weevil populations are highest on plants at 50% anthesis. The oviposition pattern follows seed filling which progresses from the periphery to the center of the head. Usually, an infested achene contains a single larva, but approximately 12% of the achenes contain two or more larvae. In late August, fifth instars chew an exit hole in the seed, drop to the ground directly beneath the sunflower head, and overwinter in the soil. Pupation occurs in the soil during early June through early July. The red sunflower seed weevil occurs on various species of Helianthus and a few weed species. Larval feeding reduces seed weight and oil content. Insecticides, sometimes in combination with trap-cropping, remain the major management tool to reduce red sunflower seed weevil damage. Treatment decisions consider the impact of larval feeding on both seed yield and oil content. Adult numbers are estimated by using a sequential sampling plan. Insecticide efficacy is maximized if chemicals are applied when the majority of plants are at 10 to 30% anthesis. Early planting in the northern Plains region of the United States results in lower seed damage because early planted sunflower completes anthesis and is no longer susceptible to oviposition at the time of peak weevil populations. Fall or spring moldboard plowing can reduce adult emergence. Natural enemies of larvae in the seed include a number of species of parasitic Hymenoptera. Stilleto fly (Diptera: Therevidae) larvae attack larvae and pupae in the soil. Sources of resistance to the red sunflower seed weevil have been identified.
The gray sunflower seed weevil, Smicronyx sordidus LeConte (Coleoptera: Curculionidae) is found throughout the Great Plains from Mexico to the Canadian Prairie Provinces of Manitoba and Saskatchewan. However, it is more common in the southern regions than the red sunflower seed weevil. Adults resemble those of the red sunflower seed weevil except for being gray in color and larger in size. The larvae also resemble red sunflower seed weevil larvae but are larger (3 to 3.5 mm). Eggs are placed in the tips of immature disk flowers. Seeds infested by larvae of the gray sunflower seed weevil enlarge, protruding above the surrounding seeds, and lack a kernel. The damage caused by a single larva of the gray sunflower seed weevil exceeds that of the red sunflower seed weevil larva because 100% rather than about 33% of the kernel is missing. However, the populations of the gray sunflower seed weevil usually are low and normally do not cause economic damage. Tillage treatments, especially disking and moldboard plowing, reduce emergence of overwintering adults. Larvae in the soil are susceptible to Metarrhizium, a fungal pathogen.
The sunflower midge, Contarinia schulzi Gagné (Diptera: Cecidomyiidae) is distributed throughout the Great Plains of North America from Manitoba to Texas. However, its economic impact has been restricted to sunflower in North and South Dakota, Minnesota, and Manitoba, especially in the Red River Valley. Eggs are laid singly or in groups of about 50 on the involucral bracts of the bud. Because larvae feed and develop at the base of the bracts and seeds, they are not normally seen unless the tissues are pulled back. Often the first indication of a midge infestation is a brown scarring at the base of the bracts or the failure of ray petals to develop normally. In North Dakota, pupation occurs in the spring with adult emergence beginning in late June. Adults live only 2 to 3 days and are difficult to detect. Buds are preferred for oviposition, but receptacles of all stages and even leaf axils receive eggs when infestations are high or when buds are not available. Mature larvae drop from the head and burrow into the soil. The life cycle ranges from 31 to 35 days with several generations per year. In North Dakota, a second generation of adults emerge in August. Sunflower midge overwinter as larvae. The host range of the sunflower midge is restricted to the genus Helianthus. If sufficient numbers of larvae are present during the plant’s early reproductive stage, growth of the sunflower head (capitula) is altered. Heavily damaged heads are gnarled and cupped inwardly, often with a hole or depression in the center, and produce few if any seeds. Severe infestations early in the bud stage may kill the floral tissues. Economic damage is usually sporadic and localized and restricted to field margins. However, in severe infestations, damage extends through the field and in some cases has resulted in 100% yield loss throughout the field. Because of the difficulty in timing insecticide applications to the emergence of adults and in affecting the cryptic larvae, insecticides are not effective in controlling sunflower midge. Altering of planting dates has not been shown to have a consistent effect on midge infestations. Some commercial hybrids are tolerant or resistant to the sunflower midge. Larvae are attacked by a number of general predators and a platygastrid parasitic wasp. Growth deformity of sunflower heads that results from sunflower midge infestation is probably due to elevated auxin levels. Sunflower midge management relies on cultural practices done prior to planting because of the absence of effective chemical controls. In locations when sunflower midge infestations have occurred, new fields should be established away from fields damaged the previous season. To minimize the risk of all plantings being at their most susceptible stage at midge emergence, several planting dates should be used and tolerant hybrids should be planted.
The sunflower headclipping weevil, Haplorhynchites aeneus (Boheman) (Coleoptera: Curculionidae) occurs throughout the United States and north to Manitoba and Saskatchewan, Canada. It is most abundant in the Midwest, occurring on native species of Helianthus. Adults appear in July on sunflower at the early bud stage. Females deposit eggs in the base of the disk flowers and then move to the stem, below the head, and create a series of punctures. The punctures encircle the stem and eventually cause the head to drop to the soil after hanging from the stem for a few days. Larval development takes place in the decaying tissue of the head. At maturity, fourth-instar larvae leave the heads and enter the soil at the base of the plant to overwinter. Pupation occurs the first week of July the following year. Damage is usually insignificant, although losses of up to 10% have been noted in North Dakota. Differences in damage due to planting date were evident in certain locations in Kansas.
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