Field and laboratory evaluations of soybean lines against soybean aphid (Hemiptera: Aphididae).

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is a major pest of soybean, Glycine max (L.). Merr., that significantly reduces yield in northern production areas of North America. Insecticides are widely used to control soybean aphid outbreaks, but efforts are underway to develop host plant resistance as an effective alternative management strategy. Here, previously identified resistant lines were evaluated in laboratory tests against field-collected populations of soybean aphid and in field-plot tests over 2 yr in South Dakota. Six lines previously identified with resistance to soybean aphid--Jackson, Dowling, K1639, Cobb, Palmetto and Sennari--were resistant in this study, but relatively high aphid counts on Tie-feng 8 in field plots contrasted with its previously reported resistance. Bhart-PI 165989 showed resistance in one of two laboratory tests, but it had relatively large aphid infestations in both years of field tests. Intermediate levels of soybean aphid occurred in field plots on lines previously shown to have strong (Sugao Zairai, PI 230977, and D75-10169) or moderate resistance to soybean aphid (G93-9223, Bragg, Braxton, and Tracy-M). Sugao Zairai also failed to have a significant proportion of resistant plants in two laboratory tests against aphids field-collected in 2008, but it was resistant in laboratory tests with aphids collected in 2002, 2005, and 2006. Overall, results showed that lines with Rag (i.e., Jackson) or Rag1 gene (i.e., Dowling) had low aphid numbers, whereas lines with Rag2 (i.e., Sugao Zairai, Sennari) had mixed results. Collectively, responses of soybean aphid populations in laboratory and field tests in 2008 resembled a virulence pattern reported previously for biotype 3 soybean aphids, but virulence in soybean aphid populations was variable and dynamic over years of the study. These results, coupled with previous reports of biotypes virulent to Rag1, suggest that deployment of lines with a single aphid-resistance gene is limited for soybean aphid management, and that deployment strategies relying on multiple resistance genes may be needed to effectively use plant resistance against soybean aphid.

Generalist-feeding subterranean mites as potential biological control agents of immature corn rootworms.

Predatory mites are important components of subterranean food webs and may help regulate densities of agricultural pests, including western corn rootworms (Chrysomelidae: Diabrotica virgifera virgifera). Implementing conservation and/or classical biocontrol tactics could enhance densities of specialist or generalist predatory mites and lead to pest suppression, but first relevant mite species must be identified and their predatory capabilities evaluated. We conducted lab assays to quantify consumption of immature rootworms and oviposition rates of various mite species. Our study indicates that rootworms are a sub-optimal food source for the mite taxa tested. However, all mite species fed upon rootworms to some degree, although consumption by nematophagous Eviphis ostrinus was extremely low. Predators consumed more rootworm larvae than eggs, and mite size was correlated with prey consumption, with larger predators eating more prey. Four mite taxa (Gaeolaelaps sp., S. miles, Gl. americana, and G. aculeifer) had detrimental effects on survival of rootworm larvae, and the latter two species also had negative impacts on densities of pest eggs. Although it is unlikely that any of these mite species by itself has a major impact on rootworm control, the community of generalist soil-dwelling mites may play an important role in regulating immature rootworm populations in the field.

Analysis of the predator community of a subterranean herbivorous insect based on polymerase chain reaction.

The identity and impact of trophic linkages within subterranean arthropod communities are challenging to establish, a fact that hinders the development of conservation biological control programs of subterranean herbivores. Diabrotica virgifera (the western corn rootworm) is a severe agricultural pest that lives subterraneously during its pre-imaginal stages and succumbs to high levels of pre-imaginal mortality from unknown agents. The guts of 1500 field-collected arthropod predators were analyzed for D. virgifera-specific DNA sequences using quantitative polymerase chain reaction (qPCR). These gut analyses were used to generate relative and taxon-specific prey consumption indices for the major predator taxa and to determine relative consumption levels during D. virgifera egg and larval stages by predator feeding guilds. Laboratory feeding assays were used to determine the meal size consumed during 5 min and digestion rates of D. virgifera DNA of four predators abundant in D. virgifera-infested cornfields. More than 17 taxa consumed D. virgifera in the field. Harvestmen and small rove beetles were the most abundant predators captured, and the most frequent predators within the community to consume D. virgifera. The largest proportions of individual species' populations testing positive for D. virgifera DNA were found in ground beetles (Scarites quadriceps and Poecilus chalcites) and spiders, wolf spiders, and predaceous mites. Because of the longer duration of the egg stage, significantly more predators consumed D. virgifera eggs than larvae, but a similar proportion of the predator community fed on eggs and larvae. Predators with sucking mouthparts had a higher consumption index than chewing predators. Laboratory assays confirmed that sucking predators consume more D. virgifera DNA during 5 min than the chewing predators, and all four predators digested this DNA at a similar rate. This research substantiates that a diverse community of soil-dwelling and subterranean predators contribute to the high level of mortality incurred by D. virgifera in cornfields (approximately 99% pre-adult mortality). Moreover, qPCR is a useful tool for describing trophic relationships within subterranean food webs, a crucial step in determining the relative contributions of a diverse predator community to the population dynamics of an herbivorous arthropod.

Conventional screening overlooks resistance sources: rootworm damage of diverse inbred lines and their B73 hybrids is unrelated.

The western corn rootworm, Diabrotica virgifera virgifera (LeConte), is a major pest of maize, Zea mays L., in the United States and Europe, and it is likely to increase in importance as a trend toward increased nonrotated maize favors larger rootworm populations. Options for rootworm management in nonrotated maize in Europe and in nontransgenic "refuge" areas in countries that permit transgenic maize are limited to insecticides. Development of additional options for growers would be helpful. Screening maize germplasm (e.g., landraces, populations, inbreds) for native resistance to western corn rootworm is labor-intensive and is usually conducted on unfinished germplasm and not on hybrid materials. However, we have recently observed that topcrossed (hybrid) materials tend to have reduced western corn rootworm damage. To formally test whether rootworm damage to inbreds and associated hybrids were correlated, we evaluated 25 diverse inbred lines and their B73 hybrids for western corn rootworm damage in seven environments. Overall, hybrids had significantly less damage than inbreds, but unfortunately, the correlation between inbreds and hybrids was not significant. These findings have important implications regarding screening germplasm for western corn rootworm resistance, namely, that inbred materials and perhaps populations should be topcrossed to form hybrid materials before screening for western corn rootworm damage to ensure that valuable sources of resistance to western corn rootworm are not missed during the screening process.