Matthew P. Daugherty

Invertebrate Carcasses as a Resource for Competing Aedes albopictus and Aedes aegypti (Diptera: Culicidae)

Abstract Terrestrial invertebrate carcasses are an important resource for insects developing in pitcher plants. However, little is known of the role of these carcasses in other containers, which also receive leaf fall and stemflow inputs. This experiment investigated effects of accumulated invertebrate carcasses as a resource for two competing mosquitoes, Aedes albopictus (Skuse) and Aedes aegypti (L.), whether either species differentially benefited from accumulated carcasses, and if such a benefit affected interspecific competition. First, we measured accumulation of invertebrate carcasses in standard containers at a field site. We then used a replacement series with five different species ratios at the same total density, and varied the input of invertebrate carcasses [dead Drosophila melanogaster (Meigen)] in three levels: none, the average input from our field site, or the maximum input recorded at our field site. Survivorship, development time, and mass were measured for each mosquito species as correlates of population growth, and were used to calculate a population performance index, λ′. There were strong positive effects of invertebrate carcass additions on all growth correlates and λ′. Differences in performance between species were pronounced in small or no carcass additions and absent in large inputs of invertebrate carcasses, but there was little evidence that inputs of invertebrate carcasses altered the competitive advantage in this system. These results suggest that terrestrial invertebrate carcasses may be an important resource for many types of container communities, and large accumulations of dead invertebrates may reduce resource competition between these mosquitoes, thus favoring coexistence. We propose that the total amount of resource, including accumulated invertebrate carcasses, may explain observed patterns of replacement involving these mosquitoes.

Indirect Effects of Temperature on Stink Bug Fitness, via Maintenance of Gut-Associated Symbionts

ABSTRACT Impacts of climate change on organisms are already apparent, with effects ranging from the individual to ecosystem scales. For organisms engaged in mutualisms, climate may affect population performance directly or indirectly through mediated effects on their mutualists. We tested this hypothesis for two stink bugs, Acrosternum hilare and Murgantia histrionica, and their gut-associated symbionts. We reared these species at two constant temperatures, 25 and 30°C, and monitored population demographic parameters and the presence of gut-associated symbionts with diagnostic PCR primer sets. Both stink bugs lost their respective gut symbionts within two generations at 30°C. In addition, the insect survivorship and reproductive rates of both A. hilare and M. histrionica at 30°C were lower than at 25°C. Other demographic parameters also indicated a decrease in overall insect fitness at the high temperature. Collectively our data showed that the decrease in host fitness was coupled with, and potentially mediated by, symbiont loss at 30°C. This work illustrates the need to better understand the biology of animal-symbiont associations and the consequences of local climate for the dynamics of these interactions.

Estimating Xylella fastidiosa transmission parameters: decoupling sharpshooter number and feeding period.

Compared to human‐ and wildlife‐transmitted pathogens, less emphasis has been placed on developing models of plant pathogen transmission by insects. Here, we describe the transmission ecology of the bacterium Xylella fastidiosa Wells et al., the causal agent of Pierce’s disease in grapevines, by its leafhopper vectors. First, we performed a meta‐analysis of transmission studies of X. fastidiosa by its two most important vectors in the Western USA, the invasive glassy‐winged sharpshooter, Homalodisca vitripennis Germar, and the native blue‐green sharpshooter, Graphocephala atropunctata Signoret (both Hemiptera: Cicadellidae). The importance of vector number, pathogen acquisition period, and inoculation access period (IAP) for transmission differed between the two species. We fit these transmission datasets to two biologically derived transmission models, i.e., a binomial and a Poisson probability model. The Poisson model provided substantially better fit and produced estimates of H. vitripennis transmission efficiency that were dramatically lower than for G. atropunctata. We also conducted a separate pair of experiments that decoupled vector number from IAP. These experiments supported the results of the meta‐analysis. Interestingly, high vector loads not only increased transmission rate, but also shortened X. fastidiosa incubation period in grapevines. This work provides quantitative estimates of transmission of an economically important pathogen that is analogous to risk models for arthropod‐vectored human and wildlife diseases. In addition, this work suggests that heterogeneous vector loads may accelerate the disease cycle, increasing the potential for secondary spread in vineyards.

Context‐dependent transmission of a generalist plant pathogen: host species and pathogen strain mediate insect vector competence

The specificity of pathogen–vector–host interactions is an important element of disease epidemiology. For generalist pathogens, different pathogen strains, vector species, or host species may all contribute to variability in disease incidence. One such pathogen is Xylella fastidiosa Wells et al., a xylem‐limited bacterium that infects dozens of crop, ornamental, and native plants in the USA. This pathogen also has a diverse vector complex and multiple biologically distinct strains. We studied the implications of diversity in this pathogen–vector–host system, by quantifying variability in transmission efficiency of different X. fastidiosa strains (isolates from almond and grape genetic groups) for different host plants (grape, almond, and alfalfa) by two of the most important vectors in California: glassy‐winged sharpshooter [Homalodisca vitripennis (Germar)] and green sharpshooter (Draeculacephala minerva Ball) (both Hemiptera: Cicadellidae). Transmission of isolates of the almond strain by H. vitripennis did not differ significantly, whereas transmission varied significantly among isolates from the grape strain (15–90%). Host plant species did not affect H. vitripennis transmission. Conversely, D. minerva efficiency was mediated by both host plant species and pathogen strain. No acquisition of an almond isolate occurred regardless of plant type (0/122), whereas acquisition of a grape isolate from alfalfa was 10‐fold higher than from grape or almond plants. These results suggest that pathogen, vector, and host diversity impose contingencies on the transmission ecology of this complex disease system. Studies aimed at the development of management strategies for X. fastidiosa diseases should consider the complexity of these interactions as they relate to disease spread.

Vector within‐host feeding preference mediates transmission of a heterogeneously distributed pathogen

1. Ecological theory predicts that vector preference for certain host species or discrimination between infected versus uninfected hosts impacts disease incidence. However, little information exists on the extent to which vector within‐host feeding preference mediates transmission. This may be particularly important for plant pathogens, such as sharpshooter transmission of the bacterium Xylella fastidiosa, which are distributed irregularly throughout hosts.

Testing for context-dependence in a processing chain interaction among detritus-feeding aquatic insects

Abstract 1. Scirtid beetles may benefit mosquitoes Ochlerotatus triseriatus (Say) by consuming whole leaves and leaving behind fine particles required by mosquito larvae. Such interactions based on the sequential use of a resource that occurs in multiple forms are known as processing chains.

Demography of Gut Symbiotic and Aposymbiotic Nezara viridula L. (Hemiptera: Pentatomidae)

ABSTRACT Nezara viridula L. is a highly polyphagous and cosmopolitan pentatomid stink bug. Despite its economic importance, aspects of its biology are poorly understood. N. viridula has one primary bacterium associated with its gastric caeca, which females provide to offspring by smearing it on the surface of eggs during oviposition. We studied the impact of three temperatures and egg mass surface sterilization on N. viridulas nymphal development rate and reproductive performance. Our results show that maintenance of the symbiont is affected both by temperature and egg mass surface sterilization. We detected the symbiont in 100, 84, and 8.3% of the untreated control insects at 20, 25, and 30°C, respectively, by using polymerase chain reaction. In insects originated from surface sterilized egg masses, the symbiont was never detected at 20 or 30°C and was detected in only 1 of 21 insects at 25°C. Nymphal mean development time decreased with increasing temperature, but there were no differences between the sterilized and control treatments. Sterilized insects at 20°C lived longer than insects in any other treatment but never laid eggs. Life table analysis of N. viridula adults showed that net reproductive rate, intrinsic rate of increase, finite rate of increase, and gross reproductive rate were not significantly different among treatments except at 20°C for the surface sterilized treatment. Mean generation time, however, was significantly longer at 20°C (70.96 ± 4.43 d), regardless of the surface sterilization treatment. Our results highlight the effect that temperature has on the maintenance of this symbiosis and its relationship with N. viridula hosts development and reproduction.

Vector preference for hosts differing in infection status: sharpshooter movement and Xylella fastidiosa transmission

1. Epidemiological theory predicts that vector preference for hosts differing in infection status (i.e. healthy or infected) affects disease dynamics.

Strain-specific alfalfa water stress induced by Xylella fastidiosa

Differences in the virulence of a pathogen among host species can occur because hosts differ in their resistance or tolerance to infection or because of underlying genetic variation in the pathogen. The xylem-limited bacterium Xylella fastidiosa is pathogenic to dozens of plant species throughout the Americas, and is structured into genetically and biologically distinct strains. In some plants X. fastidiosa causes striking leaf scorch symptoms and in others, such as alfalfa, stunting is the primary symptom. The mechanism by which these symptoms occur has been debated. We tested the hypothesis that symptoms result from X. fastidiosa-induced water stress, and that the magnitude of water stress is strain-dependent. We mechanically inoculated alfalfa plants with one of 14 isolates (5 identified genetically as “almond” and 9 as “grape” isolates), and compared stable carbon isotope ratios among isolates. Infected plants showed significant isotopic shifts (up to 2% on average) relative to healthy plants that were consistent with water stress. Moreover, there were significant differences in water stress among isolates, with a tendency for grape isolates to cause more severe water stress than almond isolates. There was also a positive relationship between plant infection level and isotopic shift (slope ± SE = 0.273 ± 0.068), which supports the hypothesis that X. fastidiosa symptoms result from bacterial multiplication and vessel occlusion. Unexpectedly, however, water stress was not correlated with measures of alfalfa stunting. These results suggest X. fastidiosa induces strain-specific levels of water stress, but factors other than water stress alone are responsible for stunting.

Leaf Scraping Beetle Feces are a Food Resource for Tree Hole Mosquito Larvae

Abstract In tree holes, leaf scraping scirtid beetles increase the rate at which leaf litter is converted to fine particles, which may benefit fine particle feeding mosquitoes if these fine particles are valuable to mosquitoes. We tested whether the products of scirtid feeding are a valuable food resource for mosquito larvae [Ochlerotatus triseriatus (Say)] by introducing different amounts of scirtid feces to mosquito larvae and measuring mosquito performance. Mosquito larvae survived longer and developed to later instars in treatments with many scirtids (and, therefore, a lot of feces) compared to treatments with few or no scirtids. This result suggests that scirtid feces (and attached microorganisms) constitute a valuable food resource for O. triseriatus. Thus, other members of tree hole communities may have complex effects on the population growth of O. triseriatus.