1) Project Title: Soybean aphid research I.

Investigators: Robert J. O’Neil, Ho Jung Yoo

Contact: Robert J. O’Neil (rjoneil@purdue.edu)

Project Description: Our overall soybean aphid (SBA), Aphis glycines Matsumura (Homoptera: Aphididae), research program focused on study of aphid-primary host plant relationships; endemic natural enemy impact on aphid dynamics, and life history characteristics of an important soybean aphid predator, Orius insidiosus (Say). This particular project focuses on the aphid primary host plants.

Aphid-primary host relationships: A collaborative study with the Illinois Natural History Survey and Iowa State University is being used to determine the (primary) host suitability of 10 rhamanceous taxa, the overwintering capability of the aphid, and its reproductive potential on its primary host(s). In a “choice” cage study conducted in Illinois, three species of Rhamnus, R. cathartica, R. lanceolata, and R. alnifolia were confirmed as hosts; defined as plants that that aphid would deposit overwintering eggs. Two non-choice experiments were conducted in the laboratory to determine suitability of 10 taxa of Rhamnaceae (R. lanceolata was unavailable for this study) as primary host plants of A. glycines. First, a non-choice host suitability test was conducted on gynoparae. Individual gynoparae were offered one of 10 host plant cuttings, and subsequent survivorship and reproduction were observed. Gynoparae survived longest on R. cathartica and R. alnifolia plants, with mean durations of 19.9 and 16.0 days, respectively. Survival on R. frangula ‘columnaris’ (11.1 d) and R. caroliniana (9.6 d) were not statistically different from survival on R. alnifolia. Survival on the remaining taxa, however, was significantly lower and ranged from a mean of 4.8 to 6.3 d. The greatest mean numbers of nymphs produced per gynopara were obtained on R. cathartica (5.0) and R. alnifolia (3.9). Significantly lower fecundities were observed on R. frangula ‘columnaris’, R. caroliniana, and R. frangula. Although a few nymphs were produced on R. frangula asplenifolia and Rhamnella franguloides, none survived more than 5 d.

A second host suitability experiment was conducted to measure survivorship and fecundity of oviparae on the potential host taxa. One day-old oviparous nymphs were placed in clip cages on potted Rhamnaceae, then transferred to cuttings as they matured, to allow for mating and oviposition. Among the taxa tested, oviparous nymphs were able to survive to maturity only on R. cathartica and R. alnifolia. 83% (25/30) and 73% (22/30) of nymphs matured on R. cathartica and R. alnifolia, respectively. 88% (22/25) of mature oviparae successfully oviposited on R. cathartica, but only 23% (5/22) of mature oviparae oviposited on R. alnifolia. Longevity of oviparae that successfully matured was greater on R. cathartica (32.6 d vs. 17.8 d). The mean number of eggs laid by each reproductive female was also significantly greater on R. cathartica (4.2 vs. 2.0). However, there was no difference in egg load between the host plant treatments (4.9 vs. 4.3), where egg load was calculated as the sum of eggs laid per female and ova dissections upon death.

2) Project Title: Soybean aphid research II.

Investigators: Robert J. O’Neil, Ho Jung Yoo, Nicolas Desneux, Casey Butler

Contact: Robert J. O’Neil (rjoneil@purdue.edu)

Project Description: This part of our soybean aphid (SBA), research program focused on endemic natural enemy impact on aphid dynamics, and life history characteristics of an important soybean aphid predator, Orius insidiosus (Say).

Endemic natural enemy impact on aphid dynamics: Throughout the summer, we monitored insect populations in a field in Tippecanoe County, focusing particularly on populations of soybean aphid (SBA) and the predator, Orius insidiosus. Five plots of 30 m x 40 m within this field were surveyed semi-weekly from 1 Jun to 7 Sep 2004. Insects were allowed to colonize naturally. After colonization of the field by O. insidiosus, its population mean varied between 0.2 and 1 per plant, peaking both on 22 June and 23 July (Fig. 1a). SBA first appeared in the field on 15 June, but did not begin to increase (from negligible, nearly zero levels) until 13 July. At this date, O. insidiosus density was very low, between its two peaks. The second peak was slightly higher than the first, and during this period, SBA density increased, albeit slowly. Why SBA increased when O. insidiosus density per plant (“relative” density) was high in late July is easier to understand when we consider densities expressed per unit leaf area (“absolute” density) (Fig. 1b). The second peak in O. insidiosus absolute density was much lower than the first, such that it only achieved about one-third the level of the first peak. SBA density began to grow when the predator’s density dropped below ~0.0002/cm2.
As we saw last year, the dynamics of O. insidiosus were not dependent on SBA densities, but likely to be driven to a large extent by thrips dynamics (Fig. 1). The two peaks in thrips densities are followed by peaks in O. insidiosus densities that consist primarily of nymphs. Thus, the numerical response by the predator to thrips is reproductive rather than aggregative at the field level.

Weekly samples: To further document aphid dynamics, weekly surveys were conducted by Purdue cooperators in 23 fields throughout the state and by our group in 15 fields in north-central Indiana. The 38 fields are located in 28 counties in Indiana. Of the 35 fields whose data have already been submitted (we expect 3 more data submissions this year), only 3 lacked aphid populations (Adams, Cass, and Clinton). In cooperator-surveyed fields where SBA populations were present, peak SBA severity ranged from 0.1 to 2.5. The mean number of aphids per plant exceeded 100 in only 4 fields, all located in the SE corner (Dearborn and Jefferson) or north-central (Grant and Tippecanoe) part of the state. One of those fields (Tippecanoe), however, was the only field in our survey that was double-cropped after wheat. An adjacent field planted at the standard time for soybean failed to reach aphid densities of greater than 100 per plant. SBA arrived into each field at widely ranging dates, as early as 2 Jun in Miami and as late as 1 Sep in Bartholomew. Only in the northern half of the state, however, were aphids found as early as June.

In the 15 fields in north-central Indiana that our group surveyed weekly, O. insidiosus arrived earlier than SBA in all but two fields. The earlier the predator arrived relative to SBA arrival, the lower was the peak SBA density per plant in that field during the season (P=0.0017). In addition to the data in Figure 1, these data suggest that predator impact on the prey occurs at both the within-field and regional (several counties) scales.
Cage studies: The impacts of abiotic (temperature) and biotic (endemic natural enemies) factors on growth of soybean aphid populations were studied this summer in the field. Based on a comparison of aphid population growth in the field and in exclusion cages, we showed that the soybean aphid populations were significantly limited by natural enemies. In 60% of cases the aphid populations in the field did not grow while in the exclusion cages they increased in 85% of cases. Moreover, we found a significant negative correlation between aphid population growth and O. insidiosus density, and no correlation with degree-day, temperature and plant complexity. Additionally, the spatial distribution of aphids among plants (clumped and random distributions were tested) seemed to have little impact on the action of O. insidiosus on aphid population growth. Further studies on predation activity, spatio-temporal dispersion of the predator as a function of prey and alternative prey available are planned. A simulation model of predator-prey dynamics in soybeans is under construction.

Life History Characteristics of Orius insidiosus (Say): The goal of this research is to examine the life history characteristics of O. insidiosus to understand its role as a soybean aphid predator. Life history studies are being conducted with nymphal and adult O. insidiosus being fed different levels of soybean aphid, soybean thrips (Neohydatothrips variabilis [Beech]), and combination diets of soybean thrips and aphids. Life history components on the various diets for adults will measure survivorship and fecundity, while life history components on the various diets for the nymphs will measure developmental time and survivorship. This work is the research project of the MSstudent, Casey Butler.

3) Project Title: Garlic mustard research.

Investigators: Robert J. O’Neil, Kimberly Rebek

Contact: Robert J. O’Neil (rjoneil@purdue.edu)

Project Description: Garlic mustard, Alliaria petiolata, is an exotic biennial herb that invades forest communities throughout eastern North America. Researchers in Switzerland are investigating several natural enemy species for use as potential control agents. In the US, several groups (Cornell, University of Illinois, and others) have studied various aspects of garlic mustard ecology in invaded habitats. Our research has investigated the ecological aspects of the plant pertinent to the biological control agents under investigation, including studies in garlic mustard life history, simulated injury and winter mortality. We found relatively little impact of garlic mustard density on plant survival, but a considerable effect on its reproduction. Simulating damage to the plant also affected reproduction more than survival. Overall, plants damaged by simulated insect attack or in lowered density situation contributed little to the seed bank, suggesting that biological control has potential to reduce garlic mustard population and associated ecological damage.

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Heimpel, G. E., D.W. Ragsdale, R. Vennette, K., R. Hopper, R. J. O’Neil, C. Rutledge, and Z. Wu. 2004. Prospects for importation biological control of the soybean aphid: anticipating potential costs and benefits. Annals Entomol. Soc. Amer 97: 249-258.

Ragsdale, D. W, D. J. Voegtlin, and R. J. O’Neil 2004. Soybean aphid biology in North America. Annals. Ent. Soc. Amer. 97:204-208.

Rebek, K. A. 2004. Garlic mustard ecology in Indiana: opportunities for biological control. MS Thesis, Purdue University, W. Lafayette, IN. 100pp.

Rutledge, C. E., R. J. O’Neil, T. B. Fox, and D. A. Landis. 2004. Soybean aphid predators and their use in pest management. Annals. Ent. Soc. Amer. 97:240-248.

Voegtlin, D. J., R. J. O’Neil, and W. Graves 2004. Tests of suitability of overwintering hosts of Aphis glycines: identification of a new host association with Rhamnus alnifolia L’Hér. Annals Entomol. Soc. Amer. 97:

Wu, Z. K. R. Hopper, R. J. O’Neil, D. J. Voegtlin, D. R. Prokrym, and G. E. Heimpel. 2004. Reproductive Compatibility and Genetic Variation between Two Strains of Aphelinus albipodus (Hymenoptera: Aphelinidae), a Parasitoid of the Soybean Aphid, Aphis glycines (Homoptera: Aphididae). Biological Control (In press).

Robert J. O’Neil, R. J., L. A. Cañas, and J. J. Obrycki. 2005. Foreign Exploration for Natural Enemies of the Colorado Potato Beetle in Central and South America. Biological Control (accepted for publication, in revision).

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