Dectes Stem Borer (Kansas Sunflower)

With appropriate cultural management, sunflower producers can tolerate dectes infestation without sustaining yield losses. Dectes larvae cannot physically girdle a radius greater than about 1.0 cm (1/2 inch). Whereas this can completely sever a slender stalk, it will only slightly weaken a stout one. In confection sunflowers, growers target lower plant populations to increase seed size and girdled plants rarely lodge because larvae are unable to completely girdle stalks of such large girth. Thus manipulation of stalk diameter via careful control of plant spacing is a primary tactic for mitigating sunflower losses to dectes. An additional advantage of large plants is slower stalk desiccation post-maturity which may extend the period of larval feeding and delay girdling. In oil sunflowers, yield per acre remains relatively constant across a wide range of plant populations, so growers should employ relatively low plant populations in dryland plantings where dectes infestation is expected. For example, studies in Nebraska have shown that seed yield did not vary significantly between 11,000 and 20,000 plants per acre; the larger plants obtained in lower populations not only have much higher per-plant yield, they benefit from improved resistance to lodging. Studies in Kansas suggest that seed oil content is not negatively affected by flower size unless the heads exceed 9 or 10 inches in diameter, and even then the oil penalty is minor. Reduced planting density is further enabled by seed treatments that improve seedling emergence and help to ensure good stand establishment. Growers of oil sunflowers without irrigation should target established plant populations between 12,000 to 15,000 plants per acre to prevent losses due to dectes-induced lodging. While the higher range may be acceptable for eastern Kansas where rainfall tends to be greater, the lower range is more appropriate further west. 

Studies in sunflower suggest that stalk desiccation is an important cue triggering the girdling behavior of dectes larvae. In one data set, variation in plant spacing within rows (and hence variation in plant size) explained about 60% of the variation in dectes larval girdling one two dates – a majority of small, closely spaced plants were girdled and lodging while girdling was delayed in larger, more widely spaced plants. Fortunately, stalk and seed desiccation appear to proceed independently in sunflowers post-maturity. Accelerated stalk desiccation brought about by hot, dry winds can lead to significant larval girdling before seeds are dry enough to harvest, whereas delayed stalk desiccation can result in seed being harvestable before any girdling occurs. This is because soil moisture can rehydrate stalks without impeding seed desiccation. Dead stalks wick up moisture from the soil through capillary action, keeping tissues moist enough to elicit continued feeding by larvae, but this moisture does not reach seeds in the flower recepticle. This is a key insight because seed moisture must be below 10% for delivery, and yet dectes larvae begin girdling plants when stalk moisture falls beow 70%. When soil moisture is high as harvest approaches, stalks remain damp and larvae continue feeding until lower temperatures eventually limit their activity. Under these conditions, few larvae complete girdles, many girdle only partially, and some do not girdle at all. Thus, growers can assess their risk of loss by monitoring soil moisture in the field post-maturity. If dry soil conditions threaten to elicit early girdling, it may be advisable to harvest earlier at higher seed moisture and accept the cost of drying seed before delivery.