Impact of the Stem Borer,Dectes texanus, on Yield of the Cultivated Sunflower,Helianthus annuus.

Abstract

Foliar and soil-drench insecticide treatments were used in attempts to manipulate infestation of cultivated sunflower plants, Helianthus annuus LeConte (Asterales: Asteraceae) by Dectes texanus LeConte, (Coleoptera: Cerambycidae) a serious pest of sunflowers in the High Plains of the USA. Seed yields were assessed on a per-plant basis for both oilseed and confection type sunflower hybrids in two years. Both insecticide treatments (foliar ë-cyhalothrin and soil-drench carbofuran) improved yield of oilseed sunflowers in 2004, but not in 2005. Yield of confection hybrids was improved by a systemic fungicide (thiophanate methyl) in 2005, but insecticides did not improve yield in either year. Both insecticide treatments gave good control of various stalk-boring insects such as Cylindrocopturus adspersus (Coleoptera: Curculionidae), Mordellistena sp. (Coleoptera: Mordellidae), and Pelochrista womanana (Lepidoptera: Tortricidae), but neither gave better than 50% control of D. texanus. Plants were sorted according to the presence or absence of D. texanus larvae and no reduction was found in total seed weight, seed size, or oil content as a result of infestation. However, mature larvae of D. texanus girdle stalks at the base in preparation for overwintering, a behavior that reduced stalk breakage force by 34–40%, leading to yield losses through lodging. At harvest in 2005, there were differences between cultivars and among treatments in the proportions of D. texanus larvae that had girdled their plants at harvest. It was concluded that further research aimed at reducing crop losses to D. texanus should focus on means of delaying stalk desiccation and/or deterioration, factors that appear to trigger girdling behavior.

Plant Spacing and Weed Control Affect Sunflower Stalk Insects and the Girdling Behavior of <I>Dectes texanus</I> (Coleoptera: Cerambycidae)

“…Excessive soil moisture can also reduce the oil content of sunßower seed (Unger 1982, Nielsen 2005, and the wet conditions in 2007 may have contributed to the 7Ð9% reduction in oil content relative to 2006. A similar pattern of lower seed oil content after wet summer conditions was observed in a previous study conducted at the same location (Michaud et al 2007a).…”

“…Infestation by D. texanus was not affected by planting date because this species has extended periods of adult emergence and activity in the crop (Michaud and Grant 2005). The larger numbers of stem weevils and root moths per plant in low-density plots in 2006 reßects the fact that larger plants typically host larger numbers of these insects (Michaud et al 2007a). Excessive soil moisture can also reduce the oil content of sunßower seed (Unger 1982, Nielsen 2005, and the wet conditions in 2007 may have contributed to the 7Ð9% reduction in oil content relative to 2006.…”

“…Preharvest lodging of plants girdled by D. texanus is frequently a major cause of yield loss. Despite the fact that larval boring has no impact on plant productivity (Michaud et al 2007a), each lodged plant becomes unharvestable and represents a complete loss. Our results clearly show a strong correlation between plant population and progression of girdling by D. texanus larvae; small, closely spaced plants are girdled much earlier than larger, widely spaced ones, ostensibly because of more rapid desiccation of their stalks.…”

“…Thus, farmers dealing with large D. texanus populations in rain-fed plantings should target plant populations within the low range, especially in drought-prone regions, while keeping in mind that weed control becomes increasingly important in sparser stands. However, ßowers Ͼ20 cm across are undesirable for oil seed production because oil content tends to be negatively correlated with ßower diameter in larger size classes, likely because oil content declines in very large seeds (Michaud et al 2007a).…”

“…Similarly, certain stem boring species such as the sunßower stem weevil, Cylindrocopturus adspersus (LeConte) (Coleoptera: Curculionidae), are found in much greater abundance in cultivated than in wild H. annuus plants and tend to have lower rates of parasitism in the former (A.K.G., unpublished data). It is also very rare to Þnd D. texanus infesting wild H. annuus in Kansas (Michaud and Grant 2005), whereas it is common for 70 Ð 80% of sunßowers in cultivated Þelds to be infested (Michaud et al 2007a).…”