Habitat Effects on Population Density and Movement of Insect Vectors of Xylellafastidiosa in California, USA

Abstract

Xylella fastidiosa is a xylem-limited bacterium that causes disease in grapevines, almonds, citrus, pear, alfalfa, and many other economically important plants. In California, USA, the bacteria are transmitted by several species of leafhoppers including the cicadellids Draeculacephala minerva Ball and Homalodisca vitripennis (Germar), the glassy-winged sharpshooter (GWSS). The pathogen and vectors have a wide host range including natural vegetation, cultivated crops, and ornamental plants in urban areas. Management of the diseases caused by X. fastidiosa requires knowledge of all possible infection pathways and biotic and abiotic factors that affect primary and secondary spread of the pathogen into and within agricultural landscapes. Two field studies were conducted to (i) determine patterns of insect vector population dynamics and temporal distribution of X. fastidiosa-infected plants relative to host plant assemblages in natural and cultivated habitats, and (ii) quantify movement and net dispersal rates of insect vectors in a manipulated experimental area. The first study investigated the role of D. minerva on movement of X. fastidiosa from different habitats into commercial almond nurseries, whereas the second study investigated the effects of deficit irrigated citrus trees on the spatiotemporal distribution and net dispersal rates of GWSS within the orchard. Surveys near commercial nurseries revealed that only habitats with permanent grass cover sustained D. minerva populations throughout the season. A total of 87 plant samples tested positive for X. fastidiosa (6.3%), with a higher number of X. fastidiosa-infected plants found in weedy alfalfa fields than in other habitat types. Among plant species infected by X. fastidiosa, 33% were winter annuals, 45% were biennials or perennials, and 22% were summer annuals. Collectively, these findings identified a potential pathway for X. fastidiosa infection of almonds in nursery situations. Sex-specific net dispersal rates showed that GWSS males and females moved consistently and contributed equally to the level of population change within the citrus orchard. Trees under severe water stress were the least preferred by GWSS and yet, ca. 80% of the population were inflow individuals. Movement towards less preferable plants indicates that in agricultural landscapes dominated by perennial monocultures, there is a random component to GWSS movement, which may result from the inability of GWSS to use plant visual and/or olfactory cues to make well-informed long-range decisions.