Juan M. Alvarez

Evaluation of the Ribosomal ITS2 DNA Sequences in Separating Closely Related Populations of the Parasitoid Ageniaspis (Hymenoptera: Encyrtidae)

Abstract Different genes have been used to evaluate the genetic divergence of closely related species or populations of insects. The taxonomic level at which specific genes or nucleotide regions are useful varies across taxa. This study assesses the relative phylogenetic usefulness of a segment of a noncoding ribosomal region (ITS2) in separating populations of the parasitoid Ageniaspis citricola (Hymenoptera: Encyrtidae) from different geographic areas (Australia and Taiwan). Intra- and interindividual variations in ITS2 sequence and length have affected previous phylogenetic studies with other arthropods. To determine whether these variations would affect our phylogenetic studies, clones (2–3/individual) containing ITS2 polymerase chain reaction (PCR) products were sequenced from two to five individuals for each of the two populations. Three clones from one A. fuscicollis (a species present only in Europe) individual were used for comparison. Intra- and interindividual variations in ITS2 sequence and length were present in the three Ageniaspis populations. Intraindividual sequence variation was sometimes greater than between individuals in each Ageniaspis population. All clones isolated from a single individual were different, and no single variant was common to all wasps in any Ageniaspis population. The variation in sequence and length of the ITS2 region in Ageniaspis populations suggests that concerted evolution has not homogenized all rDNA copies within individuals. Despite the level of intraindividual variation found, the sequences of this ITS2 region were phylogenetically informative and defined the three populations (A. citricola Australian and Taiwanese, and A. fuscicollis). These results confirm previous molecular work using RAPD-PCR and Actin genes and suggest that the two populations of Ageniaspis from Australia and Taiwan are cryptic species.

Effect of Mixed Viral Infections (Potato Virus Y–Potato Leafroll Virus) on Biology and Preference of Vectors Myzus persicae and Macrosiphum euphorbiae (Hemiptera: Aphididae)

Abstract Mixed viral infections of heterologous viruses such as Potato virus Y (familyPotyviridae, genusPotyvirus, PVY) and Potato leafroll virus (familyLuteoviridae, genusPolerovirus, PLRV) are a regular occurrence in Idaho’s potato,Solanum tuberosum (L.), cropping systems. An increased number of plant samples from Idaho’s potato fields over the past 2 yr has serologically tested positive for both PVY and PLRV via double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) and exhibited more severe symptoms than singly-infected plants (PVY or PLRV). Several studies have extensively examined the mixed infection phenomenon but to the best of our knowledge, none have examined the effect of such infections on vector biology and preference. Laboratory studies were conducted to examine the effect of mixed viral (PVY–PLRV) infection on the fecundity and preference of two of the most efficient PVY and PLRV vectors, the green peach aphid,Myzus persicae (Sulzer), and the potato aphid,Macrosiphum euphorbiae (Thomas) (Hemiptera: Aphididae).M.persicae andM.euphorbiae adults were clip-caged (one adult per cage) to leaflets of PVY, PLRV, PVY–PLRV-infected, and noninfected potato plants. The number of nymphs produced in all four treatments was recorded after 96 h.M.persicae andM.euphorbiae fecundity was significantly higher on mixed infected plants than on singly infected plants or noninfected plants. Preference of alatae and apterae ofM.persicae andM.euphorbiae was determined with the use of settling bioassays. Both alatae and apterae ofM.persicae andM.euphorbiae preferentially settled on PVY–PLRV-infected plants than on singly infected plants (PVY or PLRV) or noninfected plants.

Evaluation of Hairy Nightshade as an Inoculum Source for Aphid-Mediated Transmission of Potato Leafroll Virus

Abstract Potato leafroll virus (PLRV) causes one of the most serious aphid-transmitted diseases affecting yield and quality of potatoes, Solanum tuberosum (L.), grown in the United States. The green peach aphid, Myzus persicae (Sulzer), is considered to be by far the most efficient vector of this virus. Even the most strict aphid control strategy may not prevent the spread of PLRV unless measures also are taken to keep virus source plants within and outside the crop at a minimum. Hairy nightshade, Solanum sarrachoides (Sendtner), is one of the preferred weed hosts for green peach aphid. The potential of this weed as an aphid reservoir and virus source and its spread or perpetuation were investigated. With the use of double antibody sandwich enzyme-linked immunosorbent assay, it was confirmed that green peach aphid can transmit PLRV to hairy nightshade and that aphids can become viruliferous after feeding on infected hairy nightshade plants. Transmission from hairy nightshade to potato is 4 times the rate of potato to potato or potato to hairy nightshade. The green peach aphid preferred hairy nightshade over potato plants and reproduced at a higher rate on hairy nightshade than on potato. Therefore, a low level of PLRV–hairy nightshade infection could enhance the disease spread in the field.

Effect of an Alternate Weed Host, Hairy Nightshade, Solanum sarrachoides, on the Biology of the Two Most Important Potato Leafroll Virus (Luteoviridae: Polerovirus) Vectors, Myzus persicae and Macrosiphum euphorbiae (Aphididae: Homoptera)

Abstract Hairy nightshade, Solanum sarrachoides (Sendtner), is a ubiquitous weed in potato agro-ecosystems and nonagricultural lands of southeastern Idaho and the Pacific Northwest. This weed increases the complexity of the Potato leafroll virus (PLRV) (Luteoviridae: Polervirus)-potato pathosystem by serving as aphid and virus reservoir. Previous field studies showed higher densities of green peach aphid, Myzus persicae (Sulzer), and potato aphid, Macrosiphum euphorbiae (Thomas), the two most important vectors of PLRV, on S. sarrachoides compared with potato plants in the same fields. Some of the S. sarrachoides plants sampled in these surveys tested positive for PLRV. Viral infections can alter the physiology of plant hosts and aphid performance on such plants. To understand better the potential effects of S. sarrachoides on the PLRV-potato-aphid pathosystem, the life histories of M. persicae and M. euphorbiae were compared on virus-free and PLRV-infected S. sarrachoides and potato. Individual nymphs of each aphid species were held in clip cages on plants from each treatment to monitor their development, survival, and reproductive output. Nymphal survival for both aphids across plant species was higher on S. sarrachoides than on potato, and, within plant species, it was higher on PLRV-infected plants than on noninfected plants. With a few exceptions, similar patterns occurred for fecundity, reproductive periods, adult longevity, and intrinsic rate of increase. The enhanced performance of aphids on S. sarrachoides and on PLRV-infected plants could alter the vector population dynamics and thus the PLRV-disease epidemiology in fields infested with this weed.

Current status of insecticidal control of wireworms in potatoes

Thomas P. Kuhar (Department of Entomology, Virginia Tech), John Speese III (Eastern Shore Agricultural Research and Extension Center, Virginia Tech), Joanne Whalen (Department of Entomology and Applied Ecology, University of Delaware), Juan M. Alvarez (Department of Plant, Soil and Entomological Sciences, University of Idaho), Andrei Alyokhin (Department of Biology, University of Maine), Gerald Ghidiu (Department of Entomology, Rutgers University) and Martin R. Spellman (Extension IPM Associate, University of Delaware) summarize the performance of various insecticides in efficacy trials for control of wireworms in potatoes and discuss the future outlook.

Within plant distribution of Potato Virus Y in hairy nightshade (Solanum sarrachoides): An inoculum source affecting PVY aphid transmission

Potato virus Y (PVY) is vectored by several potato-colonizing and non-colonizing aphid species in a non-persistent manner and has a wide host range. It occurs naturally in several plant families. Myzus persicae and Macrosiphum euphorbiae are the most efficient potato-colonizing aphid vectors of PVY. Rhopalosiphum padi, a cereal aphid that migrates in large numbers through potato fields during the middle of the growing season, does not colonize potato plants but can transmit PVY. Hairy nightshade, Solanum sarrachoides, a prevalent annual solanaceous weed in the Pacific Northwest (PNW) of the United States, is an alternative host for PVY and a preferred host for M. persicae and M. euphorbiae. Hence, hairy nightshade plants might play an important role as an inoculum source in the epidemiology of PVY. We looked at titre accumulation and distribution of PVY(O), PVY(N:O) and PVY(NTN) in S. sarrachoides and potato after aphid inoculation with M. persicae and studied the transmission of PVY(O) and PVY(NTN), by M. persicae, M. euphorbiae and R. padi from hairy nightshade to potato plants. Virus titre at different positions on the plant was similar in S. sarrachoides and potato plants with strains PVY(O) and PVY(N:O). Titres of PVY(NTN) were similar in S. sarrachoides and potato but differences in titre were observed at different positions within the plant depending on the plant phenology. Percentage transmission of PVY(NTN) by M. persicae and M. euphorbiae was twice as high (46 and 34%, respectively) from hairy nightshade to potato than from potato to potato (20 and 14%). Percentage transmission of PVY(O) by M. persicae and M. euphorbiae was not affected by the inoculum source. No effect of the inoculum source was observed in the transmission of either PVY strain by R. padi. These results show that hairy nightshade may be an equal or better virus reservoir than potato and thus, important in the epidemiology of PVY.

Specialized Host Utilization of Macrosiphum euphorbiae on a Nonnative Weed Host, Solanum sarrachoides, and Competition With Myzus persicae

ABSTRACT The potato aphid, Macrosiphum euphorbiae (Thomas), (Homoptera: Aphididae), is a polyphagous aphid known to feed on over 200 plant species across 20 families. Although this aphid is known as one of the potato (Solanum tuberosum L.) colonizing aphids, field observations for populations of M. euphorbiae in southern Idaho indicated that they preferentially settled and colonized hairy nightshade, Solanum sarrachoides (Sendtner), plants rather than potato plants. Laboratory investigations revealed that M. euphorbiae survived only on S. sarrachoides and not on potato. A series of subsequent laboratory experiments conducted using laboratory-reared and field-collected aphids confirmed that M. euphorbiae survived only on S. sarrachoides. Experiments also showed that M. euphorbiae produced more winged morphs (alatae) than Myzus persicae on S. sarrachoides when reared alone under similar conditions. Furthermore, we documented intraguild competition between M. euphorbiae and M. persicae on S. sarrachoides. Results suggested that M. euphorbiae can potentially suppress M. persicae on S. sarrachoides. Competition studies reiterated that M. euphorbiae produced more alatae than M. persicae. Results also indicated that M. euphorbiae may have specialized on S. sarrachoides. The loss of polyphagous abilities of this M. euphorbiae “biotype” is intriguing and the factors responsible for specialized host utilization pattern remain to be elucidated.

Response of Potatoes to Soil-Applied Insecticides, Fungicides, and Herbicides

Non-target impacts of pesticides are well documented in crops such as soybeans and corn. However, little effort has been made to evaluate these impacts in potatoes. Field trials were conducted at the Parma and Aberdeen Research and Extension Centers during 2005 and 2006 to evaluate the potential for systemic insecticides, fungicides, and herbicides to affect disease development, crop injury, and tuber yield. The first set of trials evaluated six insecticide treatments (aldicarb, oxamyl, phorate, imidacloprid, thiamethoxam, and non-treated) in a factorial combination with in-furrow application of the fungicide azoxystrobin. Azoxystrobin significantly decreased Rhizoctonia stem canker, while aldicarb and phorate tended to increase it compared to the non-treated control. There was a significant insecticide by fungicide interaction due to a bigger reduction in disease index by the fungicide when certain insecticides were used. Despite the association of these insecticides with increased disease incidence, there was no significant impact on yield. In the second set of trials, sulfentrazone herbicide was applied just prior to plant emergence to potatoes treated with the same six insecticides to evaluate the potential for the in-furrow treatments to increase plant injury. Sulfentrazone caused stunting and visible plant injury, but the level of plant injury was influenced by insecticide treatment in only one out of four trials.