Changes in green peach aphid responses to potato leafroll virus-induced volatiles emitted during disease progression.

Previous research has shown that green peach aphids, Myzus persicae (Sulzer), preferentially settle on leaflets of potato plants (Solanum tuberosum L.) infected with potato leafroll virus (PLRV) compared with sham-inoculated controls, at least in part because of aphid responses to volatile cues from the plants. The prior work used plants 4 wk after inoculation. In this study, aphid emigration from the vicinity of leaflets of PLRV-infected plants at 2, 4, 6, 8, and 10 wk after inoculation was compared with emigration from leaflets of sham-inoculated control plants. For the bioassay, 30 aphids were placed directly above a test leaflet on screening to exclude gustatory and tactile cues and in darkness to exclude visual cues. The numbers emigrating were recorded every 10 min for 1 h. Volatile organic compounds (VOCs) were collected from the headspace of the test plants, quantified, and compared among treatments. In bioassays with leaflets of upper nodes of the plants, aphid immigration rates were significantly lower from leaflets of PLRV-infected plants than from sham-inoculated plants at 4 and 6 wk after inoculation, but not at 2, 8, and 10 wk after inoculation. In bioassays with leaflets from lower nodes, emigration did not differ between PLRV-infected plants and sham-inoculated plants at any stage in the infection. Volatile compounds detectable in the headspace of intact plants at 2, 4, and 8 wk after inoculation (or sham inoculation) changed with plant age and with disease progression, potentially explaining behavioral responses of the aphids.

Influence of the Potato leafroll virus and virus-infected plants on the arrestment of the aphid, Myzus persicae.

A series of experiments was conducted using membrane sachets containing MP148 diet or phosphate-buffered sucrose with and without purified Potato leafroll virus to determine if direct encounter with the virus would arrest the aphid, Myzus persicae (Sulzer) (Homoptera: Aphididae). In only two out of 36 tests were there significantly more aphids settled on sachets containing the virus. In all other tests, there were either significantly fewer aphids on sachets containing virus or there were no differences between virus treatments and control sachets without virus. In an experiment using excised Physalis floridana leaves, twice as many M. persicae settled on virus-infected leaves as on noninfected control leaves. Taken together, the results indicate that arrestment of M. persicae on potato leaf roll virus-infected plants may be due to enhanced nutritional qualities resulting from disease, but not from direct encounter with or detection of the virus.

Parasitism of Lygus spp. nymphs by the parasitoid wasp, Peristenus howardi, in the alfalfa seed-growing region of the Pacific Northwest.

The parasitoid, Peristenus howardi Shaw (Hymenoptera: Braconidae) has been found to parasitize a large proportion of Lygus species in some Washington and Idaho alfalfa seed fields. During 2002-2003 a survey was conducted to estimate the proportion of Lygus spp. (Hemiptera: Miridae) parasitized and the amountof that parasitism attributable to P. howardi in crop and non-crop plants attacked by Lygus in the alfalfa seed growing region of southwestern Idaho and eastern Oregon. Percentage parasitism was estimated from dissection of field-collected Lygus nymphs. Polymerase chain reaction (PCR) was used to amplify DNA extracted from parasitoid larvae followed by restriction endonuclease digestion of PCR products to distinguish P. howardi from other potentially co-occurring Peristenus species. Peak parasitism of Lygus nymphs occurred between the first and third weeks of July for both years for all host plants sampled. Of the parasitoid larvae recovered from Lygus nymphs in our study, 75% to 80% tested positive as Peristenus spp. and 76% of these matched the endonuclease digestion banding pattern for P. howardi. The identity of the remaining 20% to 25% of the parasitoids is not known.

Distinguishing the parasitic wasp, Peristenus howardi, from some of its congeners using polymerase chain reaction and restriction endonuclease digestion.

A molecular procedure incorporating polymerase chain reaction (PCR) of the COI gene and restriction endonuclease digestion of PCR products was used to distinguish Peristenus howardi (Hymenoptera: Braconidae) from four other Peristenus species. Non-solvent extraction of parasite DNA using a commercially available kit proved to be very effective in producing amplifiable template. Use of SfcI endonuclease produced restriction fragments with banding patterns in agarose gel electrophoresis that readily separated P. howardi, P. digoneutis, P. conradi, P. pallipes, and P. pseudopallipes. However, while the restriction fragment banding patterns of both P. pallipes and P. pseudopallipes were easily distinguishable from the other Peristenus species, they could not be reliably separated from one another. This molecular procedure can be used in applied and ecological research to better understand the role of P. howardi in the Peristenus-Lygus parasite-host system within the Pacific Northwest. Consensus sequences of our amplimers for all five Peristenus spp. are deposited in GenBank.