There were no differences in mortality, plant preference, or plant damage when diamondback moth (DBM) (Plutella xylostella L.) larvae were tested in no-choice and free-choice tests using leaf disks of resistant (`Green Glaze') or susceptible (`Vates', standard commercial cultivar) collards (Brassica oleracea Acephala group). No residuals of the pyrethrin insecticide Asana-XL (esfenvalerate) were detected 6 days after its application when DBM larvae were exposed to excised foliage for 72 hours. In a field test, more imported cabbage worm (ICW) (Pieris rapae L.) eggs were found on `Vates' treated with the insecticide than on nontreated Vates' or nontreated or treated `Green Glaze'. The fewest ICW, cabbage looper (CL) (Trichoplusia ni Hubner), and DBM larvae were found on the insecticide-treated cultivars. Fewer caterpillars were found on `Green Glaze' than Yates'. An additive effect of plant resistance and insecticide application lowered counts of DBM, ICW, and CL larvae. Percent parasitism of DBM by Diadegma insulare Cresson (Hymenoptera: Ichneumonidae) was lower on cultivars treated with the insecticide. Field plant damage ratings were higher for nontreated `Vates' and lowest for treated cultivars, but nontreated `Green Glaze' had a significantly lower feeding damage rating than nontreated `Vates'. Chemical name used: (S)-cyano (3-phenoxy phenyl) methyl-(S)-4 chloro-alpha (1-methylethyl) benzeneacetate [esfenvalerate (Asana-XL)].
Two hundred nineteen U.S. plant introductions (PI) belonging to the watermelon core collection were evaluated for broad mite, Polyphagotarsonemus latus (Banks), infestation and injury that occurred naturally in a field planting. Of the 219 PIs, nine (4%) had no visible broad mite injury in the field compared with a commercial cultivar, ‘Mickey Lee’, which was severely injured. Injury mainly occurred on the growing terminals and the tender apical leaves. The growing terminals and the apical leaves were bronzed, grew poorly and, in some cases, they were distorted and curled upward. Broad mites were extracted by washing the growing terminals of 22 selected PIs with boiling water and counting the mites under a stereomicroscope. ‘Mickey Lee’ had more broad mites on growing terminals compared with some of the PIs with no visible injury. Fourteen selected PIs were further evaluated in the greenhouse to confirm their resistance by artificially infesting them with broad mites that had been cultured on susceptible watermelon plants. PIs in accessions belonging to Citrullus lanatus var. lanatus (PI 357708), Citrullus lanatus var. citroides (PI 500354), Citrullus colocynthis (PI 386015, PI 386016, PI 525082), and Parecitrullus fistulosus (PI 449332) had significantly lower broad mite injury ratings and counts compared with ‘Mickey Lee’ and other susceptible PIs. Broad mites have not been reported on watermelons in the United States before; however, it can emerge as a serious pest. The previously mentioned accessions can serve as potential sources of broad mite resistance for use in breeding programs aimed at enhancing pest resistance in watermelon.
Selection for and incorporation of host plant resistance into cultivars is a fundamental strategy to control insects and diseases and may help reduce reliance on synthetic pesticides. The sweetpotato whitefly, Bemisia tabaci (Gennadius), is an important pest of watermelon [Citrullus lanatus (Thunberg) Matsum. and Nakai var. lanatus] and is among the most damaging pests in many agricultural systems worldwide. Citrullus colocynthis L., a perennial melon species indigenous to arid regions of northern Africa, the Mediterranean region, and southwestern Asia, is a valuable source of resistance to insect pests and diseases of watermelon. Laboratory and greenhouse experiments were conducted to evaluate selected C. colocynthis genotypes for sources of resistance to B. tabaci. Thirty genotypes of C. colocynthis, collected in different geographic regions, were evaluated against the heirloom cultivar Calhoun Gray using first a horizontal Y-tube olfactometer in the laboratory. A selected subset of the genotypes was evaluated in a second experiment in the laboratory using a vertical monitoring assay. In this assay, whiteflies could freely move upward to feed and oviposit on leaves placed in the upper portion of a Y-tube. In a third experiment, a choice assay was conducted on selected genotypes in cages in the greenhouse. Of the 30 C. colocynthis genotypes evaluated, PI 346082 (collected in Afghanistan) exhibited the highest level of resistance against B. tabaci based on all three experiments. PI 537277 (collected in Pakistan) exhibited a significantly high level of whitefly resistance based on low survival of adult whiteflies and a low ratio of nymphs to eggs. PI 346082 and PI 537277 should be a useful source for breeding projects aiming to improve whitefly resistance in watermelon cultivars.
The two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), often causes serious damage to watermelon (Citrullus spp.), and there is a need to evaluate and identify watermelon germplasm resistant to T. urticae. Watermelon cultivars (Citrullus lanatus var. lanatus), and U.S. plant introduction (PI) accessions of C. lanatus var. citroides and C. colocynthis, were evaluated for preference by T. urticae (number of adults and eggs on leaves). In open-choice experiments in the greenhouse and in laboratory rearing cages, there was a significant preference by T. urticae for watermelon cultivars, Citrullus lanatus var. lanatus PIs, and C. lanatus var. citroides PIs over C. colocynthis PIs. All watermelon cultivars and PIs were infested, but the C. colocynthis PIs were significantly less infested with T. urticae. The C. colocynthis PIs may be useful sources for enhancing T. urticae resistance in cultivated watermelon.