Glenn Y. Yokota

Ground Vegetation Survey for Xylella fastidiosa in California Almond Orchards

Xylella fastidiosa is a xylem-limited bacterium that causes almond leaf scorch (ALS), Pierces disease of grapevines, and other plant diseases. We surveyed ground vegetation in ALS-infected almond orchards in Californias Central Valley for the presence of this bacterium. Plant tissue samples were collected throughout a 2-year period and processed for the presence of X. fastidiosa using restriction enzyme digestion of RST31 and RST33 polymerase chain reaction (PCR) products and bacterial culture on selective media. Overall disease incidence was low in the ground vegetation species; only 63 of 1,369 samples tested positive. Of the 38 species of common ground vegetation tested, 11 tested positive for X. fastidiosa, including such common species as shepherds purse (Capsella bursa-pastoris), filaree (Erodium spp.), cheeseweed (Malva parvifolia), burclover (Medicago polymorpha), annual bluegrass (Poa annua) London rocket (Sisymbrium irio), and chickweed (Stellaria media). There was a seasonal component to bacterial presence, with positive samples found only between November and March. Both ground vegetation and almond trees were most commonly infected with the almond strain of X. fastidiosa (six of seven surveyed sites). ALS-infected almond samples had an X. fastidiosa concentration within previously reported ranges (1.84 × 106 to 2.15 × 107 CFU/g); however, we were unable to accurately measure X. fastidiosa titer in sampled ground vegetation for comparison. These results are discussed with respect to ground vegetation management for ALS control.

Biology of Macrocentrus iridescens (Hymenoptera: Braconidae): A Parasitoid of the Obliquebanded Leafroller (Lepidoptera: Tortricidae)

Abstract Macrocentrus iridescens French is a polyembryonic parasitoid with a wide host and geographic range in North America. In a survey of California pistachio orchards, M. iridescens was the most common parasitoid species reared from the obliquebanded leafroller, Choristoneura rosaceana Harris. To determine its potential as a biological control agent of the obliquebanded leafroller, we conducted laboratory studies and described M. iridescens immature development. We evaluated adult female longevity, brood size, sex ratio, and host stage preference for oviposition. The parasitoid egg develops polyembryonically. At 26.8°C, larval development required 25–30 d, and pupal development 10–12 d. There was a positive relationship between mandible size and development time, although individual stages could not be determined. Adult females survived on average 15.9, 1.6, and 0.9 d at 25°C when provided with honey and fructose solution, water only, or neither, respectively. Oviposition of fertilized eggs was observed 24 h after adult eclosion, and continued for 27 d. Under laboratory conditions, each brood produced an average of 27.2 adult M. iridescens. More broods were of mixed gender (56%) than exclusively male (17%) or female (27%), with an average of 30.9, 25.8, and 20.2 adult M. iridescens per brood, respectively. M. iridescens oviposited in all host larval stages, with significantly more second (26.6%) and third (42.2%) instar larvae parasitized than first, fifth, or sixth instars. The potential of mass rearing and augmentative release of M. iridescens is discussed.

Biology of Habrobracon gelechiae (Hymenoptera: Braconidae), as a Parasitoid of the Obliquebanded Leafroller (Lepidoptera: Tortricidae)

ABSTRACT Habrobracon gelechiae Ashmead (Hymenoptera: Braconidae) was studied as a parasitoid of the obliquebanded leafroller, Choristoneura rosaceana (Harris) (Lepidoptera: Tortricidae) in California pistachio (Pistacia vera L.) orchards. Ovipositional behavior, adult longevity and fecundity, and the effects of temperature on developmental time and survival were determined. Habrobracon gelechiae develops as a gregarious, ectoparasitic idiobiont on late-instar C. rosaceana larvae. At 25°C, adult female wasps survived longer when provided honey and water (35.4 ± 4.9 d) or honey, water, and host larvae (34.4 ± 2.4 d) than when provided water (8.9 ± 1.1 d) or no food (5.9 ± 0.8 d). Over the adult lifespan, females parasitized 20.6 ± 2.1 hosts and deposited 228.8 ± 24.6 eggs. The intrinsic rate of increase was 0.24, the mean generation time was 18.15 d, and the double time 2.88 d. At constant temperatures, H. gelechiae successfully developed (egg to adult) from 15 to 35°C. The developmental rate was fit to a nonlinear model, providing estimates of the parasitoids lower (10.5°C), upper (36.0°C), and optimal (33.3°C) development temperatures. Based on a linear model, 155 degree days were estimated for egg to adult eclosion. Temperature-dependent nonlinear model of survival showed similar shape with the model of development rate. The wasp developed under two diurnal temperature regimes, with 31.0 ± 13.3% survival at low (4–15°C) and 63.0 ± 11.4% survival at high (15–35°C) temperature regimes. The results are discussed with respect to H. gelechiae potential as a parasitoid of C. rosaceana in Californias San Joaquin Valley.

Native grass ground covers provide multiple ecosystem services in Californian vineyards

Mechanisms responsible for the success or failure of agricultural diversification are often unknown. Most studies of arthropod pest management focus on enhancing natural enemy effectiveness. However, non‐crop plants can also change crop host quality by reducing or adding soil nutrients or water, and therefore improve or hamper pest suppression. Native perennial ground covers may provide food or habitat to natural enemies and, in terms of competition for soil nutrients or water, be more compatible with crop management than exotic annuals. We conducted a 3‐year vineyard study to examine the impacts of native perennial grasses on pests, natural enemies, crop plant condition and soil properties. We included three ground cover treatments: bare soil with a grower standard drip irrigation; native grasses with drip irrigation; and native grasses with drip irrigation as well as an additional flood irrigation to keep the grasses green and growing during the season. Numbers of leafhopper pests Erythroneura spp. decreased in both native grass treatments, where parasitism rates were higher. Vine petiole nitrate levels were lower in grass treatments, indicating competition for soil nitrogen, which is most often considered to be detrimental. Berry weight was higher in the irrigated treatment but did not differ between the bare soil and non‐irrigated grass treatment. Grape °Brix was similar in the bare soil and native grass treatments, suggesting native grasses did not compromise grape quality. In fact, leaf water stress was lower and soil moisture higher not only in the irrigated grass treatment but, at times, in the non‐irrigated grass treatment, compared with the bare soil treatment. Synthesis and applications. Our work shows that native grasses contribute to a reduction in vineyard leafhopper pests by reducing host quality through competition for soil nitrogen and providing food resources and/or habitat for natural enemies. Native grasses also improve soil water content and may be part of a water conservation program for perennial crops in dry climate regions.