Fatty Acid Data and Crop Surveys Indicate Sources of Red Sunflower Seed Weevil (Coleoptera: Curculionidae), Populations and Suggest Strategies for Management

The red sunflower seed weevil, Smicronyx fulvus L., is a univoltine seed-feeding pest of cultivated sunflower, Helianthus annuus L. Artificial infestations of S. fulvus onto sunflowers with traditional (<25% oleic acid), mid-oleic (55–75%), or high oleic (>80%) fatty acid profiles were used to test if fatty acids could be used as natural markers to estimate the proportion of weevils developing on oilseed sunflowers rather than wild Helianthus spp. and confection (non-oil) types. Oleic acid (%) in S. fulvus confirmed the fatty acid compositions of mature larvae and weevil adults reflected their diets, making primary (oleic or linoleic) fatty acids feasible as natural markers for this crop-insect combination. Oleic acid in wild S. fulvus populations in North Dakota suggests at least 84 and 90% of adults originated from mid-oleic or high oleic sunflower hybrids in 2017 and 2018, respectively. Surveys in 2017 (n = 156 fields) and 2019 (n = 120 fields) extended information provided by S. fulvus fatty acid data; no significant spatial patterns of S. fulvus damage were detected in samples, damage to oilseed sunflowers was greater than confection (non-oil) types, and the majority of damage occurred in ≈10% of surveyed fields. Combined, data suggest a few unmanaged or mismanaged oilseed sunflower fields are responsible for producing most S. fulvus in an area. Improved management seems possible with a combination of grower education and expanded use of non-insecticidal tactics, including cultural practices and S. fulvus-resistant hybrids.

Effects from Early Planting of Late-Maturing Sunflowers on Damage from Primary Insect Pests in the United States

Delayed planting is recommended to reduce damage from sunflower insect pests in the United States, including the sunflower moth, Homoeosoma electellum (Hulst) and banded sunflower moth, Cochylis hospes Walsingham. However, in some locations, planting earlier or growing later-maturing hybrids could improve yield or oil content of sunflowers which would partially offset any added costs from insect pests or their management. Because the abundance and distribution of some sunflower insects have changed since recommendations for delayed planting were developed, experimental plots were grown in 2012 and 2013 at sites in North Dakota, Nebraska, Iowa, and Illinois. Sunflowers were planted two to four weeks earlier than normal, including hybrids that flower two to three weeks later than elite commercial hybrids. The sum of seed damaged by sunflower moth, banded sunflower moth, and red sunflower seed weevil, Smicronyx fulvus LeConte, (i. e., total percentage) was influenced by location, but not the relative maturity of tested entries. However, when damage attributed solely to the red sunflower seed weevil was analyzed, more damaged seed were found for late-maturing entries in North Dakota and Nebraska. In addition to the trial data, current pest populations are lower than when delayed planting was first recommended and insecticide use during sunflower bloom is both common and effective. Together, these observations suggest factoring insect pests into planting time decisions may be unnecessary, except for areas with a history of problems with severe pests that cannot be managed using insecticides (e. g., sunflower midge, Contarinia schulzi Gagné).

Trap cropping to manage the red sunflower seed weevil in oilseed sunflower

The red sunflower seed weevil,Smicronyx fulvusLeConte, is a major pest of sunflower in North and South Dakota and Minnesota. Typically, insecticides are used to reduce oviposition and prevent economically damaging larval populations from developing. In a test of a reduced-pesticide alternative, we planted early maturing sunflower in the margins around commercial fields of oilseed sunflower. The margins flowered earlier than the field interiors and acted as a trap for the red sunflower seed weevil. They were treated with an insecticide while the trap rows were blooming and before the field interior began to flower. Conventional fields entirely treated with insecticide served as checks. We compared the trap and check fields with regard to yield, the fraction of larval-damaged seeds, and economic costs (insect damage and the cost of insecticide materials and application). Yield and larval damage of seeds were comparable between check and trap fields. However, the economic costs of weevil control in the check fields was greater than in the trap fields. Trap cropping is a recommended strategy for the red sunflower seed weevil, especially where economically damaging populations of the weevil are expected.

ECONOMIC INJURY LEVELS FOR THE RED SUNFLOWER SEED WEEVIL (COLEOPTERA: CURCULIONIDAE) INFESTING OILSEED SUNFLOWER

The control of the red sunflower seed weevil, Smicronyx fulvus LeConte, relies on the use of insecticides. An economic injury level is essential to the judicious deployment of insecticidal control. Field studies were conducted in 1992 and 1993 to quantify the weight loss and oil content loss of damaged kernels as a result of larval infestation and the relationship between the adult weevil population and the subsequent number of damaged achenes. We found that for each weevil sampled in plant stages R5.0–5.3, 26.88 damaged achenes resulted. Damaged kernels lost an average of 9.86 ± 2.36 mg (mean ± SD) per kernel, approximately equal to the amount consumed by a single larva. The oil content loss in damaged kernels was 3.34%. Economic injury levels were calculated as a function of the cost of controls, the market value, and the plant population density. Use of adult weevil counts when most plants in the fields are in plant stages R5.0–5.3 are recommended for calculating economic status.