Comparison of Relative Bias, Precision, and Efficiency of Sampling Methods for Natural Enemies of Soybean Aphid (Hemiptera: Aphididae).

Generalist natural enemies play an important role in controlling soybean aphid, Aphis glycines (Hemiptera: Aphididae), in North America. Several sampling methods are used to monitor natural enemy populations in soybean, but there has been little work investigating their relative bias, precision, and efficiency. We compare five sampling methods: quadrats, whole-plant counts, sweep-netting, walking transects, and yellow sticky cards to determine the most practical methods for sampling the three most prominent species, which included Harmonia axyridis (Pallas), Coccinella septempunctata L. (Coleoptera: Coccinellidae), and Orius insidiosus (Say) (Hemiptera: Anthocoridae). We show an important time by sampling method interaction indicated by diverging community similarities within and between sampling methods as the growing season progressed. Similarly, correlations between sampling methods for the three most abundant species over multiple time periods indicated differences in relative bias between sampling methods and suggests that bias is not consistent throughout the growing season, particularly for sticky cards and whole-plant samples. Furthermore, we show that sticky cards produce strongly biased capture rates relative to the other four sampling methods. Precision and efficiency differed between sampling methods and sticky cards produced the most precise (but highly biased) results for adult natural enemies, while walking transects and whole-plant counts were the most efficient methods for detecting coccinellids and O. insidiosus, respectively. Based on bias, precision, and efficiency considerations, the most practical sampling methods for monitoring in soybean include walking transects for coccinellid detection and whole-plant counts for detection of small predators like O. insidiosus. Sweep-netting and quadrat samples are also useful for some applications, when efficiency is not paramount.

Within-plant distribution of soybean aphid (Hemiptera: Aphididae) and development of node-based sample units for estimating whole-plant densities in soybean.

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is capable of reducing soybean, Glycine max (L.) Merr., yield up to 40% during severe outbreaks. Frequent sampling, which can be costly and time-consuming, is essential to making informed management decisions. However, one way to decrease sampling effort is to use a reduced sample unit when possible. The objectives of this study were to describe the vertical distribution of soybean aphid within soybean over time and to define node-based sample units of varying sizes by testing the ability of selected units to accurately estimate whole-plant aphid density. Within-plant distribution of soybean aphid changed significantly with time. However, the average nodal position where soybean aphids were found on soybean remained within the top half of the plant at all three locations studied across all sample dates. Consequently, selecting the node with the highest aphid density multiplied by the total number of infested nodes (N(MAX)') was the best predictor of aphids on remaining soybean components in both the original (r2 = 0.855) and validation (r2 = 0.824) data sets. For sample units that included more than a single node to estimate densities, a weighted formula, which incorporated changes observed in the within-plant aphid distribution, improved model performance (higher r2 values) and reduced variability around parameter estimates compared with a node-averaged formula. Our results suggest that smaller sample units provide reliable estimations of whole-plant aphid density throughout the growing season for differently maturing soybean, which is essential to their use in pest management decisions and development of future sampling plans.

Maize and oat antixenosis and antibiosis against Delphacodes kuscheli (Homoptera: Delphacidae), vector of "Mal de Rio Cuarto" of maize in Argentina.

"Mal de Rio Cuarto" (MRC) is the most important virus disease of maize, Zea mays L., in Argentina. Several maize lines show different levels of resistance to MRC in the field; however, no studies have been conducted to investigate resistance mechanisms against its insect vector, Delphacodes kuscheli Fennah (Homoptera: Delphacidae). Oat, Avena spp., is the main overwintering host of D. kuscheli and main source of populations that infest maize. Although oat varieties resistant to the greenbug, Schizaphis graminum (Rondani) (Homoptera: Aphididae) are commercially available, their effect on D. kuscheli is unknown. We conducted laboratory experiments to test for the presence of antixenosis and antibiosis resistance mechanisms on six maize lines with different levels of field resistance to MRC, and seven commercial oat cultivars that include two S. graminum-resistant varieties. We did not find antibiotic effects of maize lines on D. kuscheli longevity and survivorship patterns, but we obtained antixenotic effects from the LP2 line (field moderate) due to reduced settling preference and feeding. Oat 'Bonaerense Payé and 'Suregrain INTA' showed both antixenosis and antibiosis, with significantly less settling preference, oviposition in the no-choice test, and reduced total fecundity in comparison with the other varieties studied. The S. graminum-resistant 'Boyera F. A.' and 'Tambera F. A.' did not showed a consistent pattern of resistance versus D. kuscheli across all experiments. Our results indicate the presence of potential sources of insect resistance in the maize lines and oat cultivars tested that may be used in MRC integrated pest management programs.