Thrips Settling, Oviposition and IYSV Distribution on Onion Foliage

Comparison of Bioassays Used to Determine Onion Thrips (Thysanoptera: Thripidae) Susceptibility to Spinetoram

Onion thrips (Thrips tabaci Lindeman) is one of onion's most damaging insect pests and has a history of developing resistance across insecticide classes. The susceptibility of T. tabaci populations to insecticides can be determined using laboratory bioassays. Three types of bioassays have been documented in the literature specifically for use with T. tabaci: vial assay (contact only), feeding assay (ingestion only), and leaf-dip assay (contact + ingestion). The objectives of this study were to 1) compare insecticide susceptibility levels of a T. tabaci population using these three assays and 2) determine which bioassay's results were most similar to those generated from exposing thrips to whole plants treated with insecticide. All experiments were conducted using a colony of T. tabaci known to be susceptible to insecticides and all were evaluated for their susceptibility to spinetoram (Radiant SC). Results indicated that 1) each bioassay generated a unique concentration-mortality relationship and LC50 value (0.01, 0.03 and 1.6 ppm for leaf-dip, vial, and feeding assays, respectively), and 2) all bioassays overestimated the susceptibility of the population relative to the whole-plant assay (LC50 = 5.3 ppm). Attributes of these bioassays are discussed relative to their future use in insecticide resistance monitoring programs for T. tabaci.

Could Broccoli and Cauliflower Influence the Dispersal Dynamics of Western Flower Thrips (Thysanoptera: Thripidae) to Lettuce in the Salinas Valley of California?

The western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), is an important, worldwide vector of two tospoviruses (Family Bunyaviridae; Genus Tospovirus), Impatiens necrotic spot virus (INSV) and Tomato spotted wilt virus (TSWV). In the Salinas Valley of California, INSV causes severe necrosis, stunting, and crop losses to lettuce (Lactuca sativa L.). Because broccoli (Brassica oleracea var. italica L.) and cauliflower (Brassica oleracea var. botrytis L.) are major rotation crops with lettuce in the Salinas Valley, it is critical to understand if these Brassicas influence the dispersal dynamics of F. occidentalis. In 2013 and 2014, surveys were conducted evaluating yellow sticky cards and plant samples in lettuce, broccoli, and cauliflower fields in three regions of the Salinas Valley. F. occidentalis were collected on sticky cards from April to November and on foliage in all three regions. Two-choice oviposition assays as well as no-choice and two-choice feeding assays were conducted where F. occidentalis adults were exposed to leaves of lettuce, broccoli, and cauliflower. A significantly greater number of eggs was found on lettuce than on broccoli or cauliflower leaves in the choice assay, whereas for the broccoli and cauliflower choice assay, the number of eggs retrieved was low and not significantly different between hosts. The number of F. occidentalis feeding spots and leaf area injury ratings were similar on broccoli and lettuce leaves but were significantly lower on cauliflower than on lettuce and broccoli leaves. The implications of these results on thrips and plant virus management are discussed.

Iris Yellow Spot Virus Prolongs the Adult Lifespan of Its Primary Vector, Onion Thrips (Thrips tabaci) (Thysanoptera: Thripidae)

Iris yellow spot virus (IYSV) from the genus Tospovirus, family Peribunyaviridae, reduces yield in several crops, especially Allium spp. IYSV is primarily transmitted by onion thrips (Thrips tabaci), but little is known about how IYSV impacts the biology of its principal vector. In a controlled experiment, the effect of IYSV on the lifespan and fecundity of onion thrips was examined. Larvae were reared on IYSV-infected onions until pupation. Individual pupae were confined until adults eclosed, and the lifespan and total progeny produced per adult were monitored daily. Thrips were tested for the virus in reverse-transcriptase polymerase chain reaction using specific primers to confirm the presence of IYSV. Results indicated that 114 and 35 out of 149 eclosing adults tested positive (viruliferous) and negative (nonviruliferous) for IYSV, respectively. The viruliferous adults lived 1.1-6.1 d longer (average of 3.6 d) than nonviruliferous adults. Fecundity of viruliferous and nonviruliferous onion thrips was similar with 2.0 ± 0.1 and 2.3 ± 0.3 offspring produced per female per day, respectively. Fecundity for both viruliferous and nonviruliferous thrips also was significantly positively correlated with lifespan. These findings suggest that the longer lifespan of viruliferous onion thrips adults may allow this primary vector of IYSV to infect more plants, thereby exacerbating IYSV epidemics.