Sources of Variation in the Adult Flight of Walnut Husk Fly (Diptera: Tephritidae): A Phenology Model for California Walnut Orchards

Leveraging satellite observations to reveal ecological drivers of pest densities across landscapes

Landscape ecologists have long suggested that pest abundances increase in simplified, monoculture landscapes. However, tests of this theory often fail to predict pest population sizes in real-world agricultural fields. These failures may arise not only from variation in pest ecology, but also from the widespread use of categorical land-use maps that do not adequately characterize habitat-availability for pests. We used 1163 field-year observations of Lygus hesperus (Western Tarnished Plant Bug) densities in California cotton fields to determine whether integrating remotely-sensed metrics of vegetation productivity and phenology into pest models could improve pest abundance analysis and prediction. Because L. hesperus often overwinters in non-crop vegetation, we predicted that pest abundances would peak on farms surrounded by more non-crop vegetation, especially when the non-crop vegetation is initially productive but then dries down early in the year, causing the pest to disperse into cotton fields. We found that the effect of non-crop habitat on pest densities varied across latitudes, with a positive relationship in the north and a negative one in the south. Aligning with our hypotheses, models predicted that L. hesperus densities were 35 times higher on farms surrounded by high versus low productivity non-crop vegetation (EVI area 350 vs. 50) and 2.8 times higher when dormancy occurred earlier versus later in the year (May 15 vs. June 30). Despite these strong and significant effects, we found that integrating these remote-sensing variables into land-use models only marginally improved pest density predictions in cotton compared to models with categorical land cover metrics alone. Together, our work suggests that the remote sensing variables analyzed here can advance our understanding of pest ecology, but not yet substantively increase the accuracy of pest abundance predictions.

Cold winters drive consistent and spatially synchronous 8-year population cycles of cabbage stem flea beetle

Population cycles have been observed in mammals as well as insects, but consistent population cycling has rarely been documented in agroecosystems and never for a beetle. We analysed the long-term population patterns of the cabbage stem flea beetle Psylliodes chrysocephala in winter oilseed rape over 50 years. Psylliodes chrysocephala larval density from 3045 winter oilseed rape fields in southern Sweden showed strong 8-year population cycles in regional mean density. Fluctuations in larval density were synchronous over time across five subregional populations. Subregional mean environmental variables explained 90.6% of the synchrony in P. chrysocephala populations at the 7-11 year time-scale. The number of days below -10°C showed strong anti-phase coherence with larval densities in the 7-11 year time-scale, such that more cold days resulted in low larval densities. High levels of the North Atlantic Oscillation weather system are coherent and anti-phase with cold weather in Scania, Sweden. At the field-scale, later crop planting date and more cold winter days were associated with decreased overwintering larval density. Warmer autumn temperatures, resulting in greater larval accumulated degree days early in the season, increased overwintering larval density. Despite variation in environmental conditions and crop management, 8-year cycles persisted for cabbage stem flea beetle throughout the 50 years of data collection. Moran effects, influenced by the North Atlantic Oscillation weather patterns, are the primary drivers of this cycle and synchronicity. Insect pest data collected in commercial agriculture fields is an abundant source of long-term data. We show that an agricultural pest can have the same periodic population cycles observed in perennial and unmanaged ecosystems. This unexpected finding has implications for sustainable pest management in agriculture and shows the value of long-term pest monitoring projects as an additional source of time-series data to untangle the drivers of population cycles.

Sensitivities to Chill Durations and No-Chill Temperatures Regulating Eclosion Responses Differ Between Rhagoletis zephyria (Diptera: Tephritidae) and its Braconid Parasitoids (Hymenoptera: Braconidae)

Seasonal temperatures select for eclosion timing of temperate insects and their parasitoids. In western North America, the fruit fly Rhagoletis zephyria Snow (Diptera: Tephritidae) is parasitized by the hymenopterous wasps Utetes lectoides (Gahan), an egg parasite, and Opius downesi Gahan, a larval parasite (both Braconidae). Eclosion of wasps should be timed with the presence of susceptible fly stages, but reports indicate U. lectoides ecloses in the absence of flies under no-chill conditions. Based on this, we tested the hypotheses that chill durations and no-chill temperatures both differentially regulate eclosion times of R. zephyria and its parasitic wasps. When fly puparia were chilled at ~3°C for 130-180 d, U. lectoides and O. downesi always eclosed on average later than flies. However, after 180-d chill, flies eclosed on average earlier than after 130- and 150-d chill, whereas eclosion times of U. lectoides and O. downesi were less or not affected by chill duration. When fly puparia were exposed to 20-22°C (no chill), U. lectoides eclosed before flies, with 88.9% of U. lectoides versus only 0.61% of flies eclosing. Taken together, findings show that eclosion times of flies are more sensitive to changes in chill duration than those of wasps. Flies are less sensitive than wasps to no-chill in that most flies do not respond by eclosing after no-chill while most wasps do. Our results suggest that shorter winters and longer summers due to climate change could cause mismatches in eclosion times of flies and wasps, with potentially significant evolutionary consequences.

Evaluating the Quality of Ecoinformatics Data Derived From Commercial Agriculture: A Repeatability Analysis of Pest Density Estimates

Each year, consultants and field scouts working in commercial agriculture undertake a massive, decentralized data collection effort as they monitor insect populations to make real-time pest management decisions. These data, if integrated into a database, offer rich opportunities for applying big data or ecoinformatics methods in agricultural entomology research. However, questions have been raised about whether or not the underlying quality of these data is sufficiently high to be a foundation for robust research. Here I suggest that repeatability analysis can be used to quantify the quality of data collected from commercial field scouting, without requiring any additional data gathering by researchers. In this context, repeatability quantifies the proportion of total variance across all insect density estimates that is explained by differences across populations and is thus a measure of the underlying reliability of observations. Repeatability was moderately high for cotton fields scouted commercially for total Lygus hesperus Knight densities (R = 0.631) and further improved by accounting for observer effects (R = 0.697). Repeatabilities appeared to be somewhat lower than those computed for a comparable, but much smaller, researcher-generated data set. In general, the much larger sizes of ecoinformatics data sets are likely to more than compensate for modest reductions in measurement precision. Tools for evaluating data quality are important for building confidence in the growing applications of ecoinformatics methods.

Seasonal Abundance and Infectivity of Philaenus spumarius (Hemiptera: Aphrophoridae), a Vector of Xylella fastidiosa in California Vineyards

The meadow spittlebug, Philaenus spumarius (Linnaeus) (Hemiptera: Aphrophoridae), is a vector of the plant pathogen Xylella fastidiosa; however, its role in recent outbreaks of Pierce's disease of grapevine (PD) in California is unclear. While the phenology and ecology of P. spumarius can help determine its contributions to PD epidemics, both remain poorly described in the North Coast vineyards of California. We assessed the phenology of P. spumarius in the region. Spittlemasses were first observed in February or March, while the emergence of adult spittlebugs did not occur until April or May depending on the year. Analysis of sweep and trap data from 2016 to 2018 revealed significant effects of survey month, vineyard site, and year on adult abundance in sweep and trap surveys. Spittlebug adults were present in the vineyards from April until December, with the greatest number of adults by sweep net in May or June, whereas adults on traps peaked between July and November. Analysis of natural infectivity in groups of field-collected spittlebug adults showed significant difference in transmission rates among months. Spittlebugs successfully transmitted Xylella fastidiosa (Wells) (Xanthomonadales: Xanthomonadaceae) to potted grapevines between July and December. The greatest risk of X. fastidiosa transmission by P. spumarius was in December (60%) followed by October (30%). However, the infectivity patterns of the meadow spittlebug did not align with the historical paradigm of California North Coast PD. We discuss alternative hypotheses in which P. spumarius could play a role in the epidemiology of this disease.

Emergence of Walnut Husk Maggot Adults in Central Illinois and Potential for Control with Metarhizium brunneum

The walnut husk maggot, Rhagoletis sauvis (Loew) (Diptera: Tephritidae), causes damage to walnuts when maggots feed inside the husk. September applications of the entomopathogenic fungi Metarhizium brunneum F52 as microsclerotia laced granules to the soil in Illinois were evaluated for pest control based on adult emergence during the following summer. Over 3 yr in central Illinois, adult emergence began near 1 July, peaked before 23 July, and emergence extended as late as 23 August. One summer application of fungus (30 June) when pupae were present, did not reduce fly emergence. Of two September applications that targeted maggots as they move to the soil to pupate, one significantly reduced the number of flies emerging from treated plots when compared with untreated plots for one 7-d sample collected 29 July 2020. Emergence trap data show a defined peak adult emergence in July for central Illinois while September applications of granules containing Metarhizium brunneum (Petch) (Hypocreales: Clavicipitaceae) show shows potential to reduced subsequent fly emergence.

Effects of photoperiod and relative humidity on diapause termination and post-winter development of Rhagoletis cerasi pupae

The European cherry fruit fly, Rhagoletis cerasi (Diptera: Tephritidae), is a univoltine species that undergoes obligatory summer-winter diapause at pupal stage in the soil (2-5 cm) beneath host trees. To study the effects of photoperiod and relative humidity on diapause termination and post-winter developmental duration of R. cerasi, pupae collected from Dossenheim (Germany) were exposed to different photoperiod or relative humidity regimes during a chilling period ranging from 2 to 8.5 months. Specifically, pupae were exposed to four photoperiod regimes: (a) light conditions (24L:00D), (b) dark conditions (00L:24D), (c) short photoperiod (08L:16D) and (d) long photoperiod (16L:08D), as well as to three relative humidity regimes: (a) low (40% RH), (b) medium (60% RH) and (c) high (70-80% RH). Data revealed that relative humidity is not a significant predictor of diapause termination, but it affects the post-winter developmental period. Higher relative humidity promotes post-winter pupae development. On the other hand, photoperiod significantly affected both diapause termination and post-winter development of R. cerasi pupae. Light conditions (24L:00D) accelerate adult emergence, particularly for females. Regardless of the photoperiod (24L:00D, 00L:24D, 08L:16D), rates of adult emergence were high (>75%) for chilling intervals longer than 6.5 months. Nonetheless, exposure to a long day photoperiod (16L:08D), during chilling, dramatically reduced the proportion of adult emergence following 6 months exposure to chilling. Our findings broaden the understanding of factors regulating diapause responses in European cherry fruit fly, local adaptation and synchronization of adult emergence with the ripening period of major hosts.

The Effect of Winter Length on Duration of Dormancy and Survival of Rhagoletis completa (Diptera: Tephritidae) and Associated Parasitoids From Northeastern Mexico

The walnut husk fly Rhagoletis completa (Cresson), native to the Midwestern United States and Mexico, is invasive in California and Europe. It is one of the most important pests of walnuts in areas gathering 30% of the world production. Knowledge of life-history regulation is important for the design of management strategies. Research on dormancy has been performed on invasive populations, and not on populations at the southern extreme of its native range. Here, we examined the effect of winter length on fly and parasitoid emergence, survival, and duration of dormancy. Percent emergence was higher for chill periods at 5°C ranging from 8 to 20 wk. No or insufficient chill resulted in low emergence and a significant proportion of individuals in prolonged dormancy (>1 yr). Duration of dormancy was longer for pupae at constant temperatures and a 4-wk chill period than longer winter durations. Dormancy was longer for Mexican than that reported for U.S. populations, suggesting the existence of a latitudinal cline where populations at southern latitudes have evolved slower metabolic rates. Three parasitoid species were found associated with R. completa (Aganaspis alujai (Wharton and Ovruski) (Hymenoptera: Figitidae), Diachasmimorpha juglandis Muesebeck, and Diachasmimorpha mellea Gahan) (Hymenoptera: Braconidae). Results suggest that rearing of R. completa is possible by subjecting pupae to chill periods between 8 and 20 wk. Overwintering mortality of flies and A. alujai could be further reduced above 5°C. Our findings can contribute for the accurate development of predictive models on invasion potential, development, fly and parasitoid rearing, and biological control.