Biology, ecology, and pest management of the red sunflower seed weevil (Coleoptera: Curculionidae)

Abstract

The red sunflower seed weevil, Smicronyx fulvus LeConte (Coleoptera: Curculionidae), is a native pest of cultivated sunflower in North America. Larvae consume a portion of the seed, reducing the amount of extractable oil and the marketability of confection seeds. Aerial application of insecticides during sunflower bloom is the primary method of weevil population management. However, sole reliance on chemical control appears to have led to insecticide resistance and crop failure in some areas. Furthermore, insecticide applications may negatively affect yields in this pollinator-dependent crop. Consequently, a holistic approach to red sunflower seed weevil management is needed. We provide an overview of red sunflower seed weevil biology, ecology, and current management practices, including scouting, economic thresholds, and insecticides. Complementary methods, such as cultural control, biological control, and host plant resistance also are discussed as tools to improve management of this pest.

Cultivated sunflower, Helianthus annuus L. (Asterales: Asteraceae), is a multipurpose crop with primary uses including oilseed (edible oil) and confection (in-shell or dehulled snack) (Giannini et al. 2022). For oilseed crop production, sunflower is the third or fourth most important oil crop by volume (USDA-FAS 2024), with a global value of approximately 14 to 20 billion USD (USDA-FAS 2024). Confection sunflower represents a small part of the market (11% to 12% by area, USDA-NASS 2024a), but sells at a higher price compared to oilseed sunflower. In 2023, North American farmers planted over 570,000 hectares of sunflower, with the top producing states being North Dakota and South Dakota (Statistics Canada 2024, USDA-NASS 2024a).

Because Helianthus spp. are native to North America, cultivated sunflowers host a wide array of insect pests, including the banded sunflower moth, Cochylis hospes Walsingham (Lepidoptera: Tortricidae), the sunflower moth, Homoeosoma electellum (Hulst) (Lep.: Pyralidae), sunflower midge, Contarinia schulzi Gagné (Diptera: Cecidomyiidae), the gray sunflower seed weevil, Smicronyx sordidus LeConte (Coleoptera: Curculionidae), and the red sunflower seed weevil (RSSW) Smicronyx fulvus LeConte (Col.: Curculionidae). For more complete lists, see Charlet et al. (1997) and Bradshaw et al. (2016). The frequency and severity of pest species have changed significantly over time, but the RSSW remains among the most destructive of sunflower insect pests (Prasifka 2024).

LeConte first described RSSW from Missouri specimens in 1876 (LeConte and Horn 1876, Anderson 1962). The range of RSSW extends from the Pacific Mountain system in the west through the Great Plains to the Appalachian Mountains in the east and from Alberta in the north to Texas in the south (Anderson 1962). The first mention of the RSSW as a pest was by Forbes in 1915, who claimed that damage caused by RSSW led to the abandonment of oilseed production in Illinois and Missouri (Cockerell 1915, Satterthwait 1946).

Red sunflower seed weevil larvae damage the crop by feeding on the developing seed inside the achene (technically a cypsela), reducing yield and oil content (Oseto and Braness 1980). However, even low levels of RSSW infestation can be problematic for confection seeds, since contracts limit damage to no more than 1% (John Sandbakken, pers. comm.). The area of sunflower production in North America was at its greatest in 1979 (over 2.2 million hectares, USDA-NASS 2019) when Oseto and Braness (1979a) reported that RSSW populations reached economic levels every year in southeastern North Dakota. Other reports on the severity of RSSW have indicated that within individual fields, 50% of heads had RSSW larvae and that 80% of seeds within these heads were infested (Cobia and Zimmer 1978 as by Oseto and Braness 1979a).

The fatty acid composition of RSSW adults in sunflower-growing areas indicates that most weevils complete larval development on cultivated sunflower (Prasifka et al. 2021), though RSSW have been found on several wild Helianthus and other Asteraceae. However, it is unlikely that the RSSW can successfully develop in all wild species. As illustrated in Table 1, only H. annuus, H. maximiliani, and H. petiolaris have been demonstrated as suitable hosts for larval development.

The need for a review of RSSW biology and management is prompted by 2 different (but almost certainly related) changes in recent years. First, biennial crop surveys sponsored by the National Sunflower Association indicate substantial increases in mean RSSW damage (% of damaged seeds) in South Dakota from 2019 (5%) to 2021 (13%) and 2023 (26%) (Prasifka 2024). Location-specific data on RSSW-damaged seeds in Minnesota, North Dakota, and South Dakota show that the increase in damage between 2019 and 2023 appears to be absent or delayed outside of South Dakota (Fig. 1). Additionally, the clustering of like values is indicative of high positive spatial autocorrelation (Fávero et al. 2023, Moraga 2024, Supplementary Materials). In this case Moran’s I tests indicated that high RSSW damaged fields were located near other high sunflower damaged fields, a pattern consistent with a RSSW outbreak in South Dakota with potential for spread to neighboring states (Supplementary Materials). Second, there has been a loss of efficacy for labeled pyrethroids that are commonly used against RSSW adults to prevent oviposition in achenes. Reports of aerial applications that failed to reduce adult populations (and prevent larval damage) were noted in 2017, and by 2020, early testing of adults showed evidence of reduced susceptibility to pyrethroid active ingredients in South Dakota (but not North Dakota; Varenhorst et al. 2021). Later data from South Dakota confirmed resistance to the commonly used active ingredients lambda-cyhalothrin and esfenvalerate (Table 2, Varenhorst et al. 2023a). Thus, a more holistic approach to RSSW management is needed. To facilitate improved understanding and management of this sunflower pest, we summarize RSSW biology and ecology, present and potential future options for pest management, and discuss related issues such as insecticide interference with pollination.

Description of Life Stages

Adult

Weevils are 2.5 to 3.0 mm long with reddish-orange scales (Figs. 2 and 3) and a black, moderately curved rostrum (Fig. 3). Older weevils are often darker because their scales are lost over time (Charlet et al. 1997). Morphological differences between males and females include the shape of the prepygidial tergite (Hyder and Oseto 1987) and components of their stridulatory apparatus (Hyder and Oseto 1989), though the concealed locations of these characters likely limit their practical value in determining the sex of living weevils. Female RSSW tend to be larger than males; however, there is overlap within RSSW populations. The rostrum length and length before antennal insertion are visible characters whose ranges appear distinct for male and female weevils with both lengths being longer in females (i.e., 1.06 to 1.45 mm vs. 0.83 to 1.00 mm for rostral length and 0.59 to 0.66 mm vs. 0.33 to 0.40 for length before antennal insertion) (Anderson 1962).

Egg, Larva, and Pupa

Eggs are white, approximately 0.70 mm long by 0.28 mm wide, and embedded in the hull toward the distal end of the achene (Oseto and Braness 1979a, Charlet et al. 1997). Red sunflower seed weevils have 5 larval instars. Early instar larvae are 1 to 2 mm long, pale, legless, and C-shaped (Fig. 4) (Charlet et al. 1997, Bradshaw et al. 2016) and are typically located in the distal end of the achene (Oseto and Braness 1979a, Brewer 1991). Fifth instar RSSW are pale with an orange head capsule and are 2.3 to 3.0 mm long (Oseto and Braness 1979b). Pupae are elongated with a white body that darkens to a reddish brown before adult emergence (Fig. 5) (Oseto and Braness 1979b).

Life History

The RSSW has one generation per year throughout its range (Oseto and Braness 1979a, Pinkham and Oseto 1988, Bradshaw et al. 2016). After overwintering in the soil, larvae pupate, and adults emerge. For RSSW in North Dakota, Pantzke (2022) developed a degree-day model for adult emergence using a 5 °C lower threshold with 50% adult emergence occurring when ≈ 1,160 degree-days had been accumulated starting 1 January. Field observations have shown that RSSW emergence begins in July and extends into August with specifics dependent upon climatic conditions and ground cover (Pantzke 2022, Prasifka unpublished). Furthermore, populations from more southerly locations (eg Arizona, Texas) may have different degree-day responses, as collection dates of museum specimens (adults) caught from light traps are similar to those observed in North and South Dakota (JRP, pers. obs.). After emergence RSSW will seek out cultivated or wild Helianthus (Oseto and Braness 1979a). Roseland et al. (1990) observed that males tend to arrive on plants earlier in the season than females; they attribute this to earlier emergence of males.

In the Dakotas, adult RSSW are found on sunflowers from early-July into September (Oseto and Braness 1979a, Gednalske and Walgenbach 1984, Oseto and Korman 1986). Little is known about initial S. fulvus host finding except for a study by Roseland et al. (1992), which indicated that a mixture of α-pinene, β-pinene, limonene, camphene, and bornyl acetate was attractive to RSSW. Experiments by Roseland et al. (1990) suggest that male RSSW produce an aggregation pheromone after they land on a host. This behavior is similar to boll weevils, Anthonomus grandis grandis Boheman (Col.: Curculionidae), where females are attracted to the aggregation pheromone produced by pioneering males (Cross 1983, Sorenson and Stevens 2019). Additionally, because of differences in male and female stridulatory apparatuses, Hyder and Oseto (1989) postulate that males stridulate to attract females.

Adult RSSW are diurnal and initially feed upon stems, petioles, buds, and involucral bracts before feeding on disk florets and pollen; adult longevity is approximately 53 d (Oseto and Braness 1979a, Korman and Oseto 1989, Brewer 1991, Charlet et al. 1997, Rana and Charlet 1997). According to Oseto and Braness (1979a), a RSSW female produces an average of 19.9 larvae; however, this is likely an underestimate as Oseto and Braness (1979a) gave RSSW females only 7 d to oviposit after exposure to pollen (Peng and Brewer 1995a). Pollen feeding (for 4 to 5 d) is necessary for female RSSW to complete egg maturation (Korman and Oseto 1989).

Female RSSW prefer to oviposit eggs on plants in anthesis that have achenes with intermediate maturity as RSSW are no longer attracted to sunflower plants after pollen shed (Oseto and Braness 1979a, Brewer 1991). Eggs are deposited singly toward the distal end of the achene between the pericarp and the embryo (Oseto and Braness 1979a, Pinkham and Oseto 1988, Brewer, 1991). The time from oviposition to larval eclosion is approximately 1 wk (Oseto and Braness 1979a). Over the course of 7 to 10 d, larvae will consume about one-third of the developing seed and pass through 5 instars (Oseto and Braness 1979a, b, Pinkham and Oseto 1988).

In late-August or September, fifth instar larvae chew a circular hole in the side of the pericarp through which they exit and drop to the soil (Oseto and Braness, 1979a, Oseto and Charlet 1981, Charlet et al. 1997). However, a small percentage of larvae may remain in the achenes especially if the heads are harvested early (Peng et al. 1997). After larvae exit the achene and drop to the soil, they appear to move very little horizontally across the soil surface (Oseto and Braness 1979a, Oseto and Charlet 1981). After moving downward into the soil, most larvae (> 90%) are located within 7 cm of the soil surface, though some may be found as deep as 15 cm (Gednalske and Walgenbach 1984). However, data by Pantzke et al. (2023) suggest that larvae may move slightly deeper (i.e., 1 to 2 cm) between November and January. Pupation occurs during June and July and lasts approximately 14 d (Oseto and Braness 1979a, Charlet et al. 1997).

Natural Sources of Mortality

Pinkham and Oseto (1988) developed a life table for RSSW in North Dakota and estimated that RSSW experiences approximately 95% mortality. The life table showed low mortality for eggs (3%) and developing larvae (first to fourth instar; 5%), but high overwintering mortality (fifth instar and pupa; 87%). Most of the overwintering mortality was attributed to unknown causes. However, predators, parasitoids [Nealiolus curculionis (Fitch) (Hymenoptera: Braconidae], and pathogens (Metarhizium spp.) were noted as minor (< 10%) sources of mortality (Pinkham and Oseto 1988). Similar work by Pinkham and Oseto (1987) also showed low levels of predation and parasitism and concluded that “natural enemies of the RSSW are not effective control agents.” Parasitism by N. curculionis was also reported by Bigger (1931, 1932) and Oseto and Braness (1979a).

Several parasitoid species have been associated with RSSW (Table 3). However, Triaspis aequoris Martin (Hym.: Braconidae) (Fig. 6) was the only parasitoid species found by Charlet (2002) in a survey of cultivated sunflower in Minnesota, Nebraska, North Dakota, and South Dakota, with RSSW numbers ranging from 5.2 to 73.0 larvae per head and parasitism often exceeding 20%. Discrepancies in parasitoid species associated with RSSW may be related to differences in when and where weevils were sampled, as well as the presence of the congener S. sordidus in some samples (Charlet and Seiler 1994). Commercially available pathogens such as Beauveria bassiana and Steinernema feltiae may have an effect on the soil dwelling stages of RSSW. However, no published accounts of augmentative biological control exist for RSSW management (including with the use of pathogens).

Aside from biotic causes of mortality (i.e., predators, parasitoids, and pathogens), abiotic conditions are likely important in limiting RSSW populations. A laboratory-based evaluation by Barker et al. (1991) indicated that either too much or too little moisture was a substantial cause of larval mortality. Freezing temperatures during overwintering may also be a source of significant mortality. Rojas et al. (1991) noted that RSSW larvae do not appear to be freeze-tolerant; rather, larvae are able to avoid freezing down to temperatures as low as −22 °C (the supercooling point). Though soil temperatures equal to or lower than −22 °C do not appear to occur even in areas with high overwintering mortality, such as North Dakota, there are 2 ways in which somewhat warmer temperatures might still explain overwintering mortality (Pantzke 2022). First, freezing can occur above the supercooling point via inoculative freezing, wherein external moisture acts as an ice nucleation agent. Rojas et al. (1992) noted that this inoculative freezing could elevate the supercooling point by up to 10 °C, reaching more realistic winter soil temperatures for the Northern Great Plains. The second explanation for mortality at temperatures warmer than the supercooling point is accumulated chill injury, where a mismatch in metabolic pathways, changes in lipid membranes, and ion imbalances leads to mortality (Koštál et al. 2007, Pantzke et al. 2023). Pantzke et al. (2023) observed elevated RSSW larval mortality following a week-long exposure to −8 °C and attributed this observation to accumulated chill injury.

Injury and Monitoring

Injury

Characteristics of RSSW feeding include the presence of a small exit hole with irregular edges on the side of the achene (a quarter to halfway between the distal and proximal ends), and the consumption of only a third of the embryo (Fig. 7B) (Oseto and Braness 1980, Brewer 1991, Peng and Brewer 1995b). However, most of the embryo can be consumed in situations of high RSSW pressure owing to the presence of multiple larvae (Fig. 7C). Frass is always present on the kernel surface (Peng and Brewer 1995b). Unlike the feeding damage by the gray sunflower seed weevil (GSSW), feeding by RSSW does not cause the achene to be enlarged and protrude above other achenes (Brewer 1991, Peng and Brewer 1995b). In X-ray radiographs, banded sunflower moth damage may be distinguished from RSSW damage as the banded sunflower moth consumes a larger portion of the achene (the distal end will always be consumed) and leave an exit hole on the distal end of the achene that is larger than that from RSSW (Fig. 7D). Additionally, frass may be present if the entire kernel is consumed (Peng and Brewer 1995b).

For researchers, manual analysis of feeding damage can be inefficient and time-consuming; furthermore, damage from weevils that are still inside the achenes may go unnoticed (Peng and Brewer 1995b, Pearson et al. 2014). While using X-ray radiographs is more efficient than visually examining achenes (Peng and Brewer 1995b), even more efficient methods are possible; for instance, Pearson et al. (2014) stated that the time spent on damage assessment could be reduced by about 60% if machine rating is used to score radiographs.

In oilseeds, feeding by a single larva reduces the total extractable oil of a seed by at least 30% (Oseto and Braness 1980, Peng and Brewer 1995a). The offspring of a single adult female RSSW can damage 53.76 achenes (Peng and Brewer 1995a). Thus, a population of one adult weevil per head would result in an approximately 13 kg ha^-1 loss, assuming a population of 50,000 plants ha^-1. Furthermore, issues selling seed may result as Canadian export standards limit all forms of excreta, including insect, to 0.02% (Canadian Grain Commission 2023). Heating and moisture problems also can occur to harvested achenes in storage if larvae remain present (Peng et al. 1997).

Monitoring

Current scouting plans for RSSW vary, but most state that scouting should begin when 80% of plants reach the R5 stage (start of anthesis) and that it should be repeated every 4 to 7 d until R5.7 (i.e., when 70% of disk florets have reached anthesis) (Michaud 2014, Varenhorst 2021). (See Schneiter and Miller 1981 for a description of sunflower development stages.) The small differences in bloom time within a planting likely contribute to the aggregated or “clumped” dispersion seen in weevil populations (Peng and Brewer 1994). Furthermore, the number of weevils (or weevil damage) may be twice as high at field edges (Charlet and Oseto 1982). As a result, sampling should be conducted by sampling 5 plants at 5 locations in the field, all at least 23 m (75 ft) from the field’s edge (Peng et al. 1997, Michaud 2014, Varenhorst 2021). More accurate counts may be facilitated by spraying heads with a 25% or higher DEET-based insect repellent as this causes the weevils to leave their sheltered locations (Weiss and Brewer 1988, Peng et al. 1997, Knodel and Beauzay 2024). While not formally tested against other methods of RSSW sampling, some sample by shaking sunflower heads over a bucket to dislodge RSSW and other insects (JRP, pers. obs.).

Economic thresholds for oilseed sunflowers range from 4 to 12 adult RSSW per head (Michaud 2014, Varenhorst 2021, Manitoba Agriculture 2024), depending on the cost of treatment, plant population, and crop value. The threshold for RSSW in oilseed sunflower may be calculated using a formula developed by Peng et al. 1997 (Formula 1). In confection sunflowers, industry contracts dictate that all seed damage remains below 1%; thus, thresholds are often only one weevil per head (Peng et al. 1997, Michaud 2014). However, contracts often stipulate that one or more insecticide applications must be applied to confection sunflowers during bloom regardless of pest populations; thus, it could be argued that no threshold exists in this market class. Of course, making insecticide applications based solely on the stage of crop development is contrary to the fundamental principles of IPM and likely results in unnecessary insecticide applications which may harm beneficial insects (including pollinators).

Formula 1.

Calculation of the economic threshold for the red sunflower seed weevil, Smicronyx fulvus (Peng et al. 1997)

Instead of fixed effort (= fixed sample size) sampling, the aggregated distribution of weevils may make sequential sampling more suitable (Peng and Brewer 1994). Peng and Brewer (1996) developed a sequential sampling plan for oilseed sunflower with thresholds of 6 and 8 weevils per head wherein the decision intercepts are ± 63.02, and the slope is 5.89 for total weevils. In this plan, a scout would sample 12 heads, and if the total number of weevils is fewer than 8, no application is needed; if more than 134, an application is needed; if the number is somewhere between 8 and 134, two more heads must be sampled at a time until a decision is reached (Peng et al. 1997). The sequential sampling plan is advantageous since it provides more accuracy when populations are near the threshold and more efficiency when not (Peng and Brewer 1996).

Pest Management

Insecticides

Chemical management is the primary, and sometimes the only strategy of RSSW management (Brewer and Schmidt 1995, de Greef et al. 2020). Application timing is key to preventing oviposition by female weevils as applications are ineffective against larvae inside the achene. Adults should be targeted when plants are in the early R5 (i.e., flowering) stages (Peng et al. 1997, Bradshaw et al. 2016). Early trials of insecticides for RSSW management were unsuccessful, likely due in part to poor application timing. For instance, Satterthwait (1946) noted that management with various insecticides was ineffective due to the extended bloom period. Similarly, Muma et al. (1950) saw that benzene hexachloride effectively reduced Smicronyx spp. if applications were properly timed.

In the past, the organophosphate active ingredients ethyl parathion (1B), methyl parathion (1B), and 6-3 parathion (1B), were commonly used for RSSW management (Dahl et al. 1991, Lamey et al. 1992, Zollinger et al. 1993). However, the uses of ethyl parathion and methyl parathion were forbidden as of 13 December 2006 and 31 December 2013, respectively (USEPA 2006, 2016). In the mid-1990s, pyrethroids (3A) became more commonly used for RSSW management (Lamey et al. 1999). Chlorpyrifos (1B), an organophosphate, was also commonly used. However, its use was effectively banned on 28 February 2022 by a ruling from the Ninth Circuit Court of Appeals (USEPA 2023). Thus, the only options available to growers for RSSW management were pyrethroids (CDMS 2024). In the United States, an emergency use exemption was granted for malathion (1B); however, this insecticide is less effective against RSSW than pyrethroids (Varenhorst et al. 2023b, Knodel et al. 2024).

In 2023, a decision by the Eighth Circuit Court of appeals restored food tolerances for chlorpyrifos (USEPA 2024a). Thus, sunflower growers had the option to use chlorpyrifos in 2024; however, it is not among the 11 currently approved food uses (USEPA 2024b). In Canada, application of insecticides containing chlorpyrifos was prohibited as of 10 December 2023 (Health Canada 2023). However, even if chlorpyrifos was still permitted it would not have been an option for those exporting to the European Union or the United Kingdom as maximum residue limits for sunflower seed are only 0.01 mg kg^-1 (EU 2018, HSE 2024). Thus, pyrethroids may be the only available option except for a mix of chlorantraniliprole (28) and lambda-cyhalothrin (3A) (Syngenta 2021).

Most growers reported that pyrethroids provided good to excellent insect management in 1997 (Lamey et al. 1999). Similarly, Knodel et al. (2009, 2024) observed successful RSSW management with pyrethroids in North Dakota. Since 2016, RSSW populations have been at 10 to 100× the threshold in South Dakota (SD-PUC 2022). Farmer reports of pyrethroid failures for RSSW management have dramatically increased in South Dakota starting in 2017 (Varenhorst et al. 2020, SD-PUC 2022, Lilleboe 2024). In 2018, field trials by South Dakota State researchers found reduced efficacy of pyrethroids, particularly lambda-cyhalothrin (Varenhorst et al. 2020). Reduced susceptibility was subsequently confirmed by glass vial assays (Varenhorst et al. 2020, 2021, 2022).

Observed insecticide failures can be attributed to 3 issues. First, the large populations of RSSW observed in South Dakota during recent years (i.e., exceeding the economic threshold by over 100 times) mean weevils in an area can infest and later re-infest the same treated sunflower fields causing significant yield loss even if an application caused high mortality (i.e., mortality greater than or equal to 80% still results in a surviving RSSW population that greatly exceeds the thresholds) (AJV, pers. obs.). Second, pyrethroid resistance driven by repeated use has been documented in South Dakota (Table 2). Pyrethroids were the most commonly used insecticides, and the primary insecticide class labeled for RSSW control in South Dakota from 2022 to 2023. Third, the sunflower production area is concentrated in central South Dakota (USDA-NASS 2024b), providing intense weevil pressures in a small area (i.e., Haakon, Hughes, Hyde, Jones, Lyman, Potter, and Sully counties) and increased genetic selection pressures for pyrethroid resistance among the RSSW populations.

Additional concerns with insecticides as a primary management strategy include toxicity and negative health impacts to pollinators, including honey bees and native bees (Brittain and Potts 2011). Flowering sunflowers are attractive to bees and are often used for forage and honey production (Erlandson and Hauff 1981, Knodel 2020). Thus, the proper use of economic thresholds is critical to protect honey bees. When insecticides are necessary, communication among beekeepers, growers, and aerial applicators is essential to minimize negative impacts on honey bees (Sauter et al. 2016, SDDANR 2020).

From a crop-centric perspective, repeated insecticide applications during bloom may inhibit sunflower pollination (de Oliveira et al. 2019). Though sunflower breeding produces hybrids that generally self-pollinate (Fick and Miller 1997, Degrandi-Hoffman and Chambers 2006, Sun et al. 2012, Perrot et al. 2019), adverse conditions during bloom can limit yields from self-pollination, leading to some dependence on pollinators for high, stable yields (Dag et al. 2002, Degrandi-Hoffman and Chambers 2006, Mallinger et al. 2019, Perrot et al. 2019). Impact on pollination can be minimized by applying insecticides outside bee foraging hours (Johansen 1977, Riedl et al. 2006, Bradshaw et al. 2016). As most insecticide applications in sunflower are contracted aerial applications, this will require knowledge and careful planning by both growers and aerial applicators.

Additionally, aerial applications have high input costs ranging from $32–43 ha^-1 (Klein and McClure 2023, Haugen 2024), with most of the cost going to the custom application rather than the insecticide. Furthermore, sunflower growers’ reliance on aerial applications during bloom means they have less ability to control the composition (i.e., active ingredient and concentration) of applications. Since demand for aerial applications are consistently high (across multiple crops) in the Nebraska panhandle, growers and applicators have decided to re-examine the efficacy of border-only insecticide applications (Ochoa 2023, JDB, pers. obs.).

Cultural Practices

Early planting and early-blooming sunflowers allow for development beyond the vulnerable stages (R5.9) prior to RSSW peak populations, thus reducing injury (Oseto and Korman 1986, Oseto et al. 1987, Prasifka et al. 2016). For instance, Oseto and Korman (1986) saw 38% achene damage in sunflowers planted on 4 May and 85% on sunflowers planted on 25 June. In a 4-yr study, Oseto et al. (1987) also saw reduced achene damage to sunflowers planted in early-May compared to those planted in mid-June. More recent data from North and South Dakota trials suggest that moving planting dates from mid-June to early-May reduces weevil damaged seeds in the absence of insecticides (Prasifka et al. 2023, 2024) without reduced yield or quality (i.e., % oil). However, the results of Alessi et al. (1977) and Oseto et al. (1987) show that the effects of planting date on sunflower yield and quality vary depending on conditions in a particular year.

Early planting to avoid damage by RSSW may conflict with recommended management practices for other pests, including sunflower moth, banded sunflower moth, sunflower stem weevil, or dectes stem borer (Rogers and Jones 1979, Oseto et al. 1982, 1989, Rogers 1985, Aslam and Wilde 1991, Charlet and Aiken 2005, Charlet et al. 2007). However, these concerns may be of little practical importance where RSSW is the most significant pest and since other serious pests (eg sunflower moth, banded sunflower moth) are more easily managed with insecticides (Prasifka 2015).

The habit of RSSW to infest the edges of the fields before moving to the center (Charlet and Oseto 1982) suggests trap cropping or limited area insecticide treatments could be effective. When Brewer and Schmidt (1995) planted a trap crop of early-blooming sunflowers around the edges of sunflower fields and applied insecticide to those rows, they noted comparable yields but lower costs in the trap crop fields compared to conventionally managed fields. Likewise, Ochoa (2023) found that effective control was obtained by applying insecticide only to the field borders, though utility of this strategy may be limited to when RSSW populations are relatively low.

Shallow overwintering by RSSW suggests tillage may be an effective cultural management practice. In South Dakota, moldboard and chisel plowing reduced adult RSSW emergence by 30% to 40%, possibly due to aeration, drying, soil temperature changes, and physical damage to the larvae (Gednalske and Walgenbach 1984). However, tillage may have negative effects on the soil and may prove problematic to recommend in semi-arid regions (Aase and Siddoway 1980, Michaud 2014, DeJong-Hughes and Daigh 2022). Consequently, the adoption of tillage as part of a RSSW management program is likely limited to certain areas of sunflower production where soil moisture or precipitation are not concerns and widespread adoption of no-till practices has not occurred.

Host Plant Resistance

Since the introduction of Bt corn in 1996, transgenic crops expressing Bacillus thuringiensis proteins have been deployed to control lepidopteran and coleopteran pests (Suszkiw 2010, Watters 2018, DiFonzo 2023). However, the commercial release of Bt sunflower is unlikely due to the risk of gene flow into wild Helianthus populations, opposition within the sunflower industry (i.e., a desire to position sunflower as a non-GM oil), and the costs of approval (Cantamutto and Poverene 2007, 2010, NSA 2023). Thus, conventional breeding for host plant resistance is currently the only viable option (Prasifka 2020).

Varying degrees of S. fulvus resistance exist in the sunflower germplasm. Brewer and Charlet (1995) stated that Plant Introduction (PI) accessions 251465, 175730, 170407, and 170408 exhibited a greater degree of larval antibiosis than USDA hybrid 894. However, the value of this is questionable since the total damage (exit holes plus unemerged larvae) is similar to the historical USDA hybrid 894 (Prasifka 2020). Gao and Brewer (1998) attributed the resistance in PI 170424 and PI 170411 to non-preference. The reason for this non-preference is unclear but could be due to variation in host volatiles (Lokumana 2017). A RSSW-resistant inbred line, HA 488, was developed and released, providing an 80% reduction in damage compared to the susceptible parent used in inbred line development (de Greef et al. 2020). In a selection of other public inbred lines, no resistance to RSSW damage approaching that of HA 488 was found (Prasifka 2020). Additionally, the mechanism of the resistance remains unknown and the absence of genetic markers currently limits its use in commercial sunflower breeding programs.

Conclusions

The red sunflower seed weevil has become the primary pest of cultivated sunflower in many areas of the Northern Plains, especially in central South Dakota, where injury can be sufficient to make harvested fields unmarketable. Current management practices, especially insecticides, are ineffective in areas where pyrethroid resistance and high populations of RSSW exist. Furthermore, continued reliance on insecticides for management of RSSW is unsustainable and could increase insecticide resistance development. Therefore, movement toward a more holistic approach to RSSW management is needed. Such an approach could combine planting as early as crop insurance allows, tillage (where practicable), conservation of natural enemies, trap cropping, and host plant resistance.

To encourage the use of economic thresholds, extension information could be disseminated detailing their importance to RSSW management and pollinator protection. Additionally, insecticides with modes of action other than pyrethroids could be registered. Ideally, these would have selectivity such that harm to pollinators and other beneficial insects is minimized. Furthermore, the development of an insecticide resistance management plan for growers would be useful when new insecticides or modes of action become available for RSSW management.

To enable growers to better manage RSSW, future research could focus on assessing the efficacy of alternative modes of action (including entomopathogens). Additionally, the elucidation of mechanisms of host plant resistance and the mapping of resistance traits could facilitate the widespread use of resistant germplasm. The edge effect could be reassessed under situations of high RSSW populations as this will greatly affect the efficacy of scouting (i.e., how far one needs to go into a field), trap cropping, and border only insecticide applications. Knowledge about adult RSSW flight distance could be informative as to how spatial isolation may reduce RSSW damage.

Acknowledgments

This work was supported by the National Sunflower Association (Projects 20-E02, 23-E01), and U.S. Department of Agriculture, Agricultural Research Service through project 3060-21000-0473-000-D, and the National Institute of Food and Agriculture, Crop Protection and Pest Management Applied Research and Development Program through grant 2021-05135. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. We acknowledge the use of the artificial intelligence program R Tutor for compiling the R code used in the production of maps and spatial analysis. However, the data collation and interpretation were solely that of the authors.

Author contributions

Jeffrey D. Cluever (Software [lead], Visualization [equal], Writing—original draft [equal], Writing—review & editing [equal]), Jeffrey Bradshaw (Conceptualization [equal], Writing—review & editing [equal]), Adam Varenhorst (Conceptualization [equal], Writing—review & editing [equal]), Janet Knodel (Conceptualization [equal], Writing—review & editing [equal]), Patrick Beauzay (Conceptualization [equal], Writing—review & editing [equal]), and Jarrad Prasifka (Conceptualization [equal], Investigation [equal], Supervision [equal], Writing—original draft [equal])

Funding

None declared.

Conflicts of interest. None declared.

References