Plant spacing and weed control affect sunflower stalk insects and the girdling behavior of Dectes texanus (Coleoptera: Cerambycidae).

We conducted a 2-yr study to determine the effects of crop density and weeds on levels of damage caused by stalk-boring insects in rain-fed sunflowers in west-central Kansas. Weed-free sunflower had higher seed weight and oil content in 2007, but not in 2006, but weeds did not affect infestation by stalk-boring insects in either year. High-density sunflower had lower estimated seed yield per unit area than low-density sunflower in both years, but percentage oil was slightly greater in the high-density treatment in 2006. Sunflowers were more heavily infested by larvae of Ataxia hubbardi Fisher, Cylindrocopturus adspersus (Leconte), and Pelochrista womanana (Kearfott) in 2006 than in 2007, ostensibly as a result of being planted earlier. Larvae of Dectes texanus LeConte appeared unaffected by planting date and were present in > 70% of plants in both years. Conditions during the period of crop maturity were much drier in 2006 than in 2007 and were associated with higher seed oil content and earlier and faster progression of stalk girdling by D. texanus larvae in both low- and high-density plots. There was also a strong effect of plant density on girdling behavior that seemed to be mediated by effects on soil moisture. Stalk girdling began earlier in high-density plots and a larger proportion of plants were girdled compared with low-density plots on all sampling dates in both years. Certain cultural tactics, in particular reduced plant spacing, have potential to delay the onset of girdling behavior by D. texanus larvae and thus mitigate losses that otherwise result from the lodging of girdled plants.

Impact of the stem borer, Dectes texanus, on yield of the cultivated sunflower, Helianthus annuus.

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.

Positive correlation of fitness with group size in two biotypes of Russian wheat aphid (Homoptera: Aphididae).

Changes in fitness parameters as a function of colony size (one versus 10 aphids) were measured in two biotypes (RWA1 and RWA2) of the Russian wheat aphid, Diuraphis noxia (Mordvilko) (Homoptera: Aphididae), feeding on three cultivars of wheat, Triticum aestivum L., at two temperatures. 'Trego' is a cultivar with no specific resistance to D. noxia, whereas, 'Stanton' and 'Halt' express Dny and Dn4 resistance sources, respectively. Feeding in a group accelerated the development of RWA1 on Trego and Stanton at 20 degrees C, but not at 24 degrees C, whereas grouped RWA2 developed faster than solitary RWA2 on all three cultivars at 24 degrees C, but not at 20 degrees C. Survival (first instar-adult) of RWA2 also was improved by grouping on Stanton and Halt at 24 degrees C, but solitary RWA2 survived better at 20 degrees C on all three cultivars. The reproductive rate of RWA1 was improved by grouping on Trego and Stanton at both temperatures, but only on Halt at 24 degrees C. Lifetime fecundity of RWA1 also was increased by grouping in all cases except for Trego at 20 degrees C. Grouped development increased the reproductive rate of RWA2 on all three cultivars at 24 degrees C, but had no effect at 20 degrees C. Grouped RWA2 developed and reproduced faster than grouped RWA1 on all three cultivars at 24 degrees C. Thus, the fitness of D. noxia was positively correlated with group size during colony establishment, but the effects were sensitive to temperature, being more pronounced at 20 degrees C for RWA1 and at 24 degrees C for RWA2.

Comparative biology of a novel strain of Russian wheat aphid (Homoptera: Aphididae) on three wheat cultivars.

In spring 2003, several outbreaks of the Russian wheat aphid, Diuraphis noxia (Mordvilko), were reported in fields of supposedly resistant wheat cultivars ('Stanton', 'Halt', and 'Prairie Red') in eastern Colorado. We conducted two laboratory experiments to compare the biological performance of this new biotype 2 (B2) to that of two D. noxia collections of biotype 1 (B1) from western Kansas by using three wheat cultivars as host plants: 'Trego', a susceptible cultivar, and Stanton and Halt, two cultivars with different genetic sources of resistance. Survival of solitary nymphs from first instar to adult for the two clones of B1 on Trego was 96 and 90%, respectively, compared with 67 and 43% on Stanton, and 65 and 57% on Halt. In contrast, B2 had 60% survival on Trego, 43% survival on Halt, and 85% survival on Stanton. One clone of B1 required longer to mature on Halt compared with Trego or Stanton, but no other differences in developmental time among cultivars were significant. The standardized fecundity of solitary foundresses of the B1 clones was 19.6 and 20.1 nymphs on Trego, compared with 4.6 and 0.9 on Stanton, and 2.8 and 1.1 on Halt, respectively, over the same period. In contrast, fecundity of B2 was 21.1, 20.8, and 19.7 on Trego, Stanton, and Halt, respectively. When larger colonies developed on individual plants over longer periods, Trego supported the largest number of B1 aphids by experiment's end, whereas Stanton and Halt yielded the largest numbers of B2. The order of overall plant damage was Trego > Stanton > Halt when infested with B1, with no significant differences for B2. Trego had more pronounced leaf rolling than other cultivars, independent of biotype. Collectively, the results suggest that D. noxia B2 from Colorado has evolved cross-virulence to both Dn4- and Dny-based resistance sources.

Identifying sources and mechanisms of resistance in crucifers for control of cabbage maggot (Diptera: Anthomyiidae).

The cabbage maggot, Delia radicum (L.) is an important insect pest of eruciferous crops in upstate New York. This species causes considerable damage to seedlings and young plants by feeding on roots and stems, resulting in plant stand loss and yield loss. Five crucifer accessions (Brassica oleracea variety italica L.,'Green Comet'; B. oleracea L.,'Rapid Cycling' [Crucifer Genetics Cooperative 3-1 ]; B. oleracea variety botrytis L., a standard cauliflower cultivar'Amazing'; B. carinata L.; and Sinapis alba L., 'Cornell Alt 543') were evaluated to identify sources and mechanisms of resistance for D. radicum. Of the accessions tested, S. alba Cornell Alt 543 demonstrated reduced oviposition by D. radicum, reduced weights and survivorship of larvae, pupae or adults, and reduced damage to plants. Thus, S. alba Cornell Alt 543 could be a potential source for resistance to be bred into cruciferous crops for control of D. radicum.

Resistance in sunflower and interaction with Bacillus thuringiensis for control of banded sunflower moth (Lepidoptera: Tortricidae).

Four sunflower accessions were compared with a susceptible check, hybrid '894', in the greenhouse to determine their resistance to the banded sunflower moth, Cochylis hospes Walsingham, and their interaction with Bacillus thuringiensis Berliner variety kurstaki. Antibiosis, expressed as lower larval weight, was detected in all of the accessions. In addition to being antibiotic, sunflower accession Ames 3291 was antixenotic to banded sunflower moth oviposition and exhibited an additional impact on larval weight when B. thuringiensis was applied. By itself, B. thuringiensis provided better control of banded sunflower moth than the resistance tested. However, banded sunflower moth-resistant sunflower would be a good option when B. thuringiensis or another insecticide is not applied, and it may prevent the economic threshold from being reached.