Density Of Nymphs Research Articles

Ixodes ricinus and Borrelia burgdorferi sensu lato in the Royal Parks of London, UK

Assessing the risk of tick-borne disease in areas with high visitor numbers is important from a public health perspective. Evidence suggests that tick presence, density, infection prevalence and the density of infected ticks can vary between habitats within urban green space, suggesting that the risk of Lyme borreliosis transmission can also vary. This study assessed nymph density, Borrelia prevalence and the density of infected nymphs across a range of habitat types in nine parks in London which receive millions of visitors each year. Ixodes ricinus were found in only two of the nine locations sampled, and here they were found in all types of habitat surveyed. Established I. ricinus populations were identified in the two largest parks, both of which had resident free-roaming deer populations. Highest densities of nymphs (15.68 per 100 m2) and infected nymphs (1.22 per 100 m2) were associated with woodland and under canopy habitats in Richmond Park, but ticks infected with Borrelia were found across all habitat types surveyed. Nymphs infected with Borrelia (7.9%) were only reported from Richmond Park, where Borrelia burgdorferi sensu stricto and Borrelia afzelii were identified as the dominant genospecies. Areas with short grass appeared to be less suitable for ticks and maintaining short grass in high footfall areas could be a good strategy for reducing the risk of Lyme borreliosis transmission to humans in such settings. In areas where this would create conflict with existing practices which aim to improve and/or meet historic landscape, biodiversity and public access goals, promoting public health awareness of tick-borne disease risks could also be utilised.

Lake Michigan insights from island studies: the roles of chipmunks and coyotes in maintaining Ixodes scapularis and Borrelia burgdorferi in the absence of white-tailed deer

Deer management (e.g., reduction) has been proposed as a tool to reduce the acarological risk of Lyme disease. There have been few opportunities to investigate Ixodes scapularis (blacklegged tick) and Borrelia burgdorferi sensu stricto dynamics in the absence of white-tailed deer (Odocoileus virginianus) in midwestern North America. A pair of islands in Lake Michigan presented a unique opportunity to study the role of alternative hosts for the adult stage of the blacklegged tick for maintaining a tick population as a deer herd exists on North Manitou Island but not on South Manitou Island, where coyotes (Canis latrans) and hares (Lepus americanus) are the dominant medium mammals. Additionally, we were able to investigate the maintenance of I. scapularis and B. burgdorferi in small mammal communities on both islands, which were dominated by eastern chipmunks (Tamias striatus). From 2011 to 2015, we surveyed both islands for blacklegged ticks by drag cloth sampling, bird mist netting, and small and medium-sized mammal trapping. We assayed questing ticks, on-host ticks, and mammal biopsies for the Lyme disease pathogen, B. burgdorferi. We detected all three life stages of the blacklegged tick on both islands. Of the medium mammals sampled, no snowshoe hares (Lepus americanus, 0/23) were parasitized by adult blacklegged ticks, but 2/2 coyotes (Canis latrans) sampled on South Manitou Island in 2014 were parasitized by adult blacklegged ticks, suggesting that coyotes played a role in maintaining the tick population in the absence of deer. We also detected I. scapularis ticks on passerine birds from both islands, providing support that birds contribute to maintaining as well as introducing blacklegged ticks and B. burgdorferi to the islands. We observed higher questing adult and nymphal tick densities, and higher B. burgdorferi infection prevalence in small mammals and in adult ticks on the island with deer as compared to the deer-free island. On the islands, we also found that 25% more chipmunks were tick-infested than mice, fed more larvae and nymphs relative to their proportional abundance compared to mice, and thus may play a larger role compared to mice in the maintenance of B. burgdorferi. Our investigation demonstrated that alternative hosts could maintain a local population of blacklegged ticks and an enzootic cycle of the Lyme disease bacterium in the absence of white-tailed deer. Thus, alternative adult blacklegged tick hosts should be considered when investigating deer-targeted management tools for reducing tick-borne disease risk, especially when the alternative host community may be abundant and diverse.

Interactions between sympatric invasive European fire ants (Myrmica rubra) and blacklegged ticks (Ixodes scapularis).

The blacklegged tick (Ixodes scapularis) and the invasive European fire ant (Myrmica rubra) are both expanding throughout their sympatric range in coastal New England. Ixodes scapularis is the primary vector of the bacterium Borrelia burgdorferi, which is the causative agent of Lyme disease, and Mount Desert Island, Maine, home to Acadia National Park, currently is affected by a high Lyme disease burden. Ticks have many natural predators, including ants, although no previous studies have investigated interactions between these two species. To test the hypothesis that the presence of M. rubra alters I. scapularis abundance, we collected ticks by drag-sampling at eight ant-infested sites and eight uninfested control sites in Acadia National Park. We found that nymph density was significantly higher at ant-infested sites, while larval density was significantly higher at control sites. In addition, we conducted a laboratory bioassay to measure M. rubra aggression against I. scapularis larvae, nymphs, and adults and Dermacentor variabilis adults, and found that ant aggression was significantly higher against D. variabilis adults than I. scapularis adults. Our findings support the hypothesis that M. rubra has divergent effects across I. scapularis life stages, and we discuss possible ecological mechanisms, including optimal microclimate and predation, that could promote density of nymphs while inhibiting density of larvae.

Taxa-specific responses to flooding shape patterns of abundance in river rock pools

Connectivity and patch size are important landscape characteristics that drive patterns of abundance and diversity across scales. However, responses to connectivity and patch size are dependent on species traits. Riverine landscapes are highly dynamic both spatially and temporally, with hydrologic connectivity being a major driver of abundance and diversity. Here we modeled the densities of 2 taxa that differ in life history and dispersal ability, the Virginia River Snail (Elimia virginica) and skimmer dragonfly larvae (Pantala spp.), as a function of flooding, patch area, and season in >300 riverine rock pools. We found key differences in how each taxon responded to these predictors. Specifically, increasing pool flood height had a strong negative effect on snail densities, whereas dragonfly nymph densities increased as pools became isolated from the river channel for longer durations of time. Increasing pool surface area had a positive effect on snail densities, whereas dragonfly nymph densities showed no such relationship. Dragonfly nymph densities were greater in summer and autumn than in spring, but snails showed no difference in their temporal distribution across seasons. Our study highlights how differential responses to landscape characteristics are dependent on organism traits. These findings give insight into patterns of abundance and diversity across spatiotemporal scales.

Local and landscape scale drivers of Euschistus servus and Lygus lineolaris in North Carolina small grain agroecosystems

Crop production sequences influence arthropod populations in temporally unstable row crop systems. Winter wheat (Triticum aestivum L.) represents one of the earliest abundant crops in south‐eastern United States. This study aims to understand primary source habitats driving brown stink bug, Euschistus servus (Say), and tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), population abundance in wheat. To better understand these relationships, adult and nymphal densities were in wheat fields weekly from flowering through harvest in 2019 and 2020. Geospatial data were used to measure landscape composition surrounding sampled fields. We investigated the influence of landscape predictors on E. servus and L. lineolaris abundance using generalized linear mixed modelling. Field size, proportion of agriculture, proportion of wheat area, and proportion of soybean Glycine max L.) area from the previous year in the surrounding landscape were associated with E. servus abundance in wheat. Similarly, L. lineolaris abundance was associated with proportion of wheat area and soybean area from the previous year. These results reveal the influence of soybean area planted the previous year on insect pest densities the following spring in wheat. Further, results suggest agricultural landscapes dominated by wheat are associated with decreased pest abundance across the sampled region.

Tree Diversity and Damage by Cypress Aphid, Cinara cupressi, on Juniperus procera in Gulele Botanical Garden and Entoto

Juniperus procera is an important tree species in Ethiopia. Increasing demand for fuelwood and construction material has also created markets for J. procera products. The impact of cypress aphid, Cinara cupressi, on J. procera is becoming catastrophic in the country. However, the level of cypress aphid damage on J. procera has never been studied in Entoto Mountain forest and Gullele Botanical Garden (GBG) in Ethiopia. The objective of this study was to assess the cypress aphid densities and extent of damage caused by the cypress aphid on solely and mixed planted J. procera in Entoto Mountain forest and GBG in Addis Ababa, Ethiopia, by evaluating aphid adult and nymph densities and damage to J. procera trees. Eight plots of 50 m ∗ 50 m planted with pure J. procera and mixed J. procerawere selected at each site. Five 5 m ∗ 5 m subplots were formed. To assess aphid density hthree tree twigs per whorl were sampled from each tree in the subplots randomly. The collected samples were examined in the laboratory for the presence of nymph and adult cypress aphid. Tree damage was assessed by visually observing the percentage of infestation in the tree crown and was scored on a 1–5 scale. The results showed significant differences in aphid densities between the sites. The highest aphid count was observed at Entoto Mountain. Besides, the solely planted J. procera trees had higher cypress aphid numbers than the mixed plantation. The mean percentage of aphid-infested trees in Entoto and GBG was 53.6 and 46.4, respectively. Tree mortality was extremely low in both sites and planting systems. The level of aphid damage was also significantly different between the planting systems, where mixed plantations had a lower aphid infestation level. Tree damage was directly correlated with the density of aphids collected. In conclusion, cypress aphid was apparent in both sites, and most of the trees were infested by this pest. However, the extent of damage varied with the planting system. Therefore, it is recommended to consider a mixed plantation of trees as one of cypress aphid management tools in J. procera.

Population dynamics and damage status of the dusky cotton bug, Oxycarenus hyalinipennis Costa (Hemiptera: Lygaeidae) in cotton in Çukurova region of Turkey

The dusky cotton bug, Oxycarenus hyalinipennis (Costa) (Hemiptera: Lygaeidae) is a common insect species in cotton fields of the Eastern Mediterranean region of Turkey during the critical developmental period of the plants (i.e. squaring-boll formation period). Although the adults of the pest are mostly detected in young squares and bolls of cotton plants, its damage status on them is not yet known. With this study, the population dynamics and damage status of the O. hyalinipennis were investigated in a cotton field in Adana Province, Turkey in 2017 and 2018. Oxycarenus hyalinipennis adult population density increased after mid-July on the square, flower, and green bolls, and then it decreased after the first week of August. The females started to lay eggs in a period close to the opening of the mature green bolls. The population density of nymphs was higher when the mature green bolls were opened (in September). Adults were mostly found in squares (30.37% and 34.44% in 2017 and 2018, respectively) or green bolls (35.19% and 27.16% in 2017 and 2018, respectively), while nymphs were mostly found in opened bolls (65.03% and 57.45% in 2017 and 2018, respectively). With the cage trials, it has been found that adult females did not cause any damage to squares and green bolls. It has been commented that the presence of adults in these generative organs may be its behavioral characteristic rather than its feeding on these plant parts.

A Comparison of Questing Substrates and Environmental Factors That Influence Nymphal Ixodes pacificus (Acari: Ixodidae) Abundance and Seasonality in the Sierra Nevada Foothills of California.

In California, the western blacklegged tick, Ixodes pacificus Cooley and Kohls, is the principal vector of the Borrelia burgdorferi sensu lato (sl) complex (Spirochaetales: Spirochaetaceae, Johnson et al.), which includes the causative agent of Lyme disease (B. burgdorferi sensu stricto). Ixodes pacificus nymphs were sampled from 2015 to 2017 at one Sierra Nevada foothill site to evaluate our efficiency in collecting this life stage, characterize nymphal seasonality, and identify environmental factors affecting their abundance and infection with B. burgdorferi sl. To assess sampling success, we compared the density and prevalence of I. pacificus nymphs flagged from four questing substrates (logs, rocks, tree trunks, leaf litter). Habitat characteristics (e.g., canopy cover, tree species) were recorded for each sample, and temperature and relative humidity were measured hourly at one location. Generalized linear mixed models were used to assess environmental factors associated with I. pacificus abundance and B. burgdorferi sl infection. In total, 2,033 substrates were sampled, resulting in the collection of 742 I. pacificus nymphs. Seasonal abundance of nymphs was bimodal with peak activity occurring from late March through April and a secondary peak in June. Substrate type, collection year, month, and canopy cover were all significant predictors of nymphal density and prevalence. Logs, rocks, and tree trunks had significantly greater nymphal densities and prevalences than leaf litter. Cumulative annual vapor pressure deficit was the only significant climatic predictor of overall nymphal I. pacificus density and prevalence. No associations were observed between the presence of B. burgdorferi sl in nymphs and environmental variables.

Biopesticide and Parasitoid Effects on Megacopta cribraria (Hemiptera: Plataspidae) Life Stage Density and Egg Parasitism in Soybean

Biopesticide and Parasitoid Effects on <i>Megacopta cribraria</i> (Hemiptera: Plataspidae) Life Stage Density and Egg Parasitism in Soybean

Influence of Vineyard Vegetational Borders on Western Grape Leafhopper (Erythroneura elegantula Osborn), its Egg Parasitoids (Anagrus spp.) and Generalist Insect Predators

Studies have shown that vegetational diversity in or around cropping systems can enhance naturalenemy abundance, although the impact on herbivores is less certain. We studied the influence of vineyardvegetational borders on density of the western grape leafhopper, Erythroneura elegantula, its majorparasitoids, Anagrus spp., and two generalist predators, Orius spp. and Leptothrips mali. Two studyvineyards had planted, perennial flowering hedgerows, one bordered a natural riparian zone, and onehad a sparse border of native trees. From April to September 2007, we counted leafhopper nymphs andadults of Anagrus spp., Orius spp. and L. mali within 10 m of the border, and at additional 20 m intervalsup to 90 m. At two sites with a diverse border, leafhopper density was stable nearest the border; at otherdistances leafhopper density decreased between the first and second generations. This effect was notseen at the sparse border site. Anagrus spp. density was enhanced at one site with a diverse border, butonly late season. There was no border effect on the generalist insect predators, by site or early vs. lateseason. Regression analysis showed a positive relationship between leafhopper and Anagrus spp. density,suggesting that the parasitoids were responding to higher leafhopper density. We conclude that, dependingon the nature of the border vegetation, there can be an effect on leafhopper nymphal density, but in thisstudy there is no evidence that it was due to natural enemies.

Masting by beech trees predicts the risk of Lyme disease

BackgroundThe incidence of Lyme borreliosis and other tick-borne diseases is increasing in Europe and North America. There is currently much interest in identifying the ecological factors that determine the density of infected ticks as this variable determines the risk of Lyme borreliosis to vertebrate hosts, including humans. Lyme borreliosis is caused by the bacterium Borrelia burgdorferi sensu lato (s.l.) and in western Europe, the hard tick Ixodes ricinus is the most important vector.MethodsOver a 15-year period (2004–2018), we monitored the monthly abundance of I. ricinus ticks (nymphs and adults) and their B. burgdorferi s.l. infection status at four different elevations on a mountain in western Switzerland. We collected climate variables in the field and from nearby weather stations. We obtained data on beech tree seed production (masting) from the literature, as the abundance of Ixodes nymphs can increase dramatically 2 years after a masting event. We used generalized linear mixed effects models and AIC-based model selection to identify the ecological factors that influence inter-annual variation in the nymphal infection prevalence (NIP) and the density of infected nymphs (DIN).ResultsWe found that the NIP decreased by 78% over the study period. Inter-annual variation in the NIP was explained by the mean precipitation in the present year, and the duration that the DNA extraction was stored in the freezer prior to pathogen detection. The DIN decreased over the study period at all four elevation sites, and the decrease was significant at the top elevation. Inter-annual variation in the DIN was best explained by elevation site, year, beech tree masting index 2 years prior and the mean relative humidity in the present year. This is the first study in Europe to demonstrate that seed production by deciduous trees influences the density of nymphs infected with B. burgdorferi s.l. and hence the risk of Lyme borreliosis.ConclusionsPublic health officials in Europe should be aware that masting by deciduous trees is an important predictor of the risk of Lyme borreliosis.Graphical

Seasonal patterns and spatial variation of Borrelia burgdorferi (sensu lato) infections in Ixodes ricinus in the Netherlands

BackgroundThe incidence of Lyme borreliosis varies over time and space through as yet incompletely understood mechanisms. In Europe, Lyme borreliosis is caused by infection with a Borrelia burgdorferi (s.l.) genospecies, which is primarily transmitted by a bite of Ixodes ricinus nymphs. The aim of this study was to investigate the spatial and temporal variation in nymphal infection prevalence of B. burgdorferi (s.l.) (NIP), density of questing nymphs (DON) and the resulting density of infected nymphs (DIN).MethodsWe investigated the infection rates in I. ricinus nymphs that were collected monthly between 2009 and 2016 in 12 locations in the Netherlands. Using generalized linear mixed models, we explored how the NIP, DON and DIN varied during the seasons, between years and between locations. We also determined the genospecies of the Borrelia infections and investigated whether the genospecies composition differed between locations.ResultsThe overall NIP was 14.7%. A seasonal pattern in infection prevalence was observed, with higher estimated prevalences in the summer than in the spring and autumn. This, combined with higher nymphal densities in summer, resulted in a pronounced summer peak in the estimated DIN. Over the 7.5-year study period, a significant decrease in infection prevalence was found, as well as a significant increase in nymphal density. These two effects appear to cancel each other out; the density of infected nymphs, which is the product of NIP × DON, showed no significant trend over years. Mean infection prevalence (NIP, averaged over all years and all months) varied considerably between locations, ranging from 5 to 26%. Borrelia genospecies composition differed between locations: in some locations almost all infections consisted of B. afzelii, whereas other locations had more diverse genospecies compositions.ConclusionIn the Netherlands, the summer peak in DIN is a result of peaks in both NIP and DON. No significant trend in DIN was observed over the years of the study, and variations in DIN between locations were mostly a result of the variation in DON. There were considerable differences in acarological risk between areas in terms of infection prevalence and densities of ticks as well as in Borrelia genospecies composition.Graphical

&lt;p class="Body"&gt;&lt;strong&gt;Fun&lt;/strong&gt;&lt;strong&gt;ctional and numerical responses of &lt;em&gt;Neoseiulus barkeri&lt;/em&gt; (Acari: Phytoseiidae) on two-spotted spider mite: the effect of patch condition and additional food source&lt;/strong&gt;&lt;/p&gt;

Parameters that describe foraging behavior play a key role in selection of natural enemies used in biological control programs. These parameters are greatly affected by patch condition and food source type. This study was carried out to evaluate foraging behavior in Neoseiulus barkeri (Hughes) (Acari: Phytoseiidae), a predatory mite of Tetranychus urticae Koch under different patch and diet conditions. The predation rate of N. barkeri at different egg and nymph densities of T. urticae (2, 4, 8, 16, 32, 64, 128) was investigated in both limited and unlimited patches either in the presence or absence of pollen as an additional food source under laboratory conditions. A Type II functional response was determined for N. barkeri on eggs or nymphs of T. urticae in both unlimited and limited patches. The type of functional response was not affected by presence or absence of pollen. The numerical response, in terms of eggs laid, increased curvilinear with increasing prey density. The numerical response was similar in shape to a Type II functional response. The efficiency of conversion of ingested food (ECI) of female N. barkeri decreased in all treatments when prey density increased. The type of patch (limited or unlimited) and prey stage (egg or nymph in the presence or absence of pollen) can affect searching efficiency (a) and handling time (Th) (functional response) as well as daily fecundity and ECI (numerical response) of the phytoseiid predator mite, N. barkeri with implications for its use in biological control.

Effective control of the motile stages of Amblyomma americanum and reduced Ehrlichia spp. prevalence in adults via permethrin treatment of white-tailed deer in coastal Connecticut, USA

The lone star tick, Amblyomma americanum, is a common human-biting species whose range has been largely restricted to the southeastern United States, until recent detections of established populations on Long Island, New York and throughout coastal southern New England. We evaluated the effectiveness of topical treatment of 10 % permethrin delivered via 4-poster devices to white-tailed deer, Odocoileus virginianus, in the management of a newly discovered A. americanum population in Norwalk, Connecticut. Using a high-density deployment of one 4-poster device/12.7 ha, we were successful in significantly reducing densities of host-seeking adults (93 % reduction), nymphs (92 %), and larvae (96 %) from 2018 to 2020. We also documented a significant reduction (87 %) in parasitizing adults and nymphs on white-tailed deer from 2018 to 2019. The prevalence of Ehrlichia chaffeensis and Ehrlichia ewingii combined in host-seeking adults declined significantly from 47 % at the time the A. americanum population was discovered in 2017 to 7% in 2020. However, the prevalence in nymphs remained static (∼9%) throughout the study period. These data demonstrate that, when properly deployed in a density-dependent manner in terms of deer abundance, 4-poster devices can effectively manage parasitizing and host-seeking A. americanum populations and reduce the prevalence of two ehrlichial species of public health importance.

Spatial distribution and colonization pattern of Bemisia tabaci in tropical tomato crops.

In precision integrated pest management, management tactics are implemented only where and when needed, by identifying the sites where the pest population has reached economic thresholds. Tomato, Solanum lycopersicum (Linn.), is a vegetable cultivated worldwide, but its production is reduced by insect pests such as the whitefly, Bemisia tabaci (Genn.). To improve management, there is a need to understand B. tabaci spatial dynamics in tomato fields, which will elucidate colonization patterns and may improve management of this pest. Thus, this study was conducted to assess the spatial autocorrelation, distribution, and colonization patterns of B. tabaci in 19 commercial tomato fields through the growing season. A total of 69 isotropic variograms were fit for B. tabaci. The insect distribution was aggregated with a strong level of spatial dependence. Ranges of spatial dependence varied from 0.53 to 19.05 m and 0.5 to 20 m for adults and nymphs, respectively. Overall, densities of adults and nymphs were higher and reached the economic threshold mainly at the field edges. Our results suggest a colonization pattern for B. tabaci starting at the edges and spreading inwards in to the tomato fields. This study can improve B. tabaci management in tomato fields, especially scouting and decision-making to treat fields. Scouting for this pest should be directed to the field edges, with sample points at least 20 m apart from each other for independent insect counts. © 2020 Society of Chemical Industry.

Reforestation provides a foraging habitat for brown bears (Ursus arctos) by increasing cicada Lyristes bihamatus density in the Shiretoko World Heritage site

Reforestation, which converts abandoned farmland back into forestland by planting woody species, can provide habitat for wildlife, including the brown bear (Ursus arctos Linnaeus, 1758). In the Shiretoko World Heritage site, northern Japan, where brown bears occur at high density, conifers have been planted since the 1970s to reforest abandoned farmland. In this area, brown bears were first observed digging for cicada Lyristes bihamatus Motschulsky, 1861 = Auritibicen bihamatus (Motschulsky, 1861) nymphs from 2000. Our preliminary observations suggested that the emergence of digging behavior might be associated with reforestation. We examined whether reforestation provided a foraging habitat for brown bears. We found that digging occurred only within the restored conifer forests, but not within the natural forest. The densities of cicada nymphs in the restored forests were higher than in the natural forest. These results indicate that the reforestation of abandoned farmland provides a foraging habitat for brown bears by increasing the availability of cicada nymphs in the Shiretoko World Heritage site.

Age-dependent functional response of Aphidius matricariae (Hymenoptera: Braconidae) on tobacco aphid, Myzus persicae nicotianae (Hemiptera: Aphididae)

• Functional response of A. matricariae was affected by the parasitoid’s age. • The searching efficiency of A. matricariae was declined with increasing age. • The highest efficiency of A. matricariae gained in the first three days of life. This study was performed to investigate the age-dependent functional response of Aphidius matricariae Haliday (Hymenoptera: Braconidae) at varying densities of tobacco aphid, Myzus persicae nicotianae Blackman (Hemiptera: Aphididae). Six densities (2, 4, 8, 16, 32, and 64 numbers) of third instar nymphs of M. persicae nicotianae were daily exposed to one pair of 1-day-old A. matricariae until the parasitoid female died. Each host aphid density was replicated 10 times. The experiments were conducted at constant environmental conditions (25 ± 1 °C, 70 ± 5% RH and 16:8h L:D photoperiod). The logistic regression model demonstrated that the functional response of A. matricariae was type III in the second day and type II in the first day and from third to the end of adult parasitoid lifetime. The handling time ( T h ) was lowest in the first day (0.2305 h) and highest in the sixth day (2.9296 h) of the parasitoid’s life. The searching efficiency ( a ) values were 0.0573, 0.0649, 0.0503, 0.0229, and 0.0077 h −1 in the first, third, fourth, fifth, and sixth days of the parasitoid’s life, respectively. Also, the b value for type III functional response was 0.0147 in the second day. The values of maximum attack rate ( T / T h ) ranged from 104.12 nymphs/24 h in the first day to 19.8 nymphs/24 h in the sixth day of parasitoid’s life. Based on the results, A. matricariae showed a high potential to control the population of tobacco aphid and the highest efficiency gained in the first half of the female parasitoid life.

Landscape factors and how they influence whitefly pests in cassava fields across East Africa

ContextAfrican production landscapes are diverse, with multiple cassava cultivars grown in small patches amongst a diversity of other crops. Studies on how diverse smallholder landscapes impact herbivore pest outbreak risk have not been carried out in sub-Saharan Africa.ObjectivesBemisia tabaci is a cryptic pest species complex that cause damage to cassava through feeding and vectoring plant-virus diseases and are known to reach very high densities in certain contexts. However, the factors driving this phenomenon are unclear.MethodsBemisia density data in cassava across a large number of sites representing a geographic gradient across Uganda, Tanzania and Malawi were collected. We tested whether in-field or landscape factors associated with land-use patterns underpinned Bemisia density variability and parasitism.ResultsWe found the B. tabaci SSA1 species dominated our study sites, although other species were also common in some cassava fields. Factors associated with the surrounding landscape were unimportant for explaining variability in adult density, but the in-field variables of cassava age and cultivar were very important. The density of nymphs and the parasitism of nymphs was heavily influenced by a diversity of landscape factors surrounding the field, including the size of focal cassava field, and area of cassava in the landscape. However, unlike the trend from many other studies on drivers of natural enemy populations, this pattern was not solely related to the amount of non-crop vegetation, or the diversity of crops grown in the landscape.ConclusionsOur findings provide management options to reduce whitefly abundance, including describing the characteristics of landscapes with high parasitism. The choice of cassava cultivar by the farmer is critical to reduce whitefly outbreak risk at the landscape-scale.

Intraspecific competition counters the effects of elevated and optimal temperatures on phloem-feeding insects in tropical and temperate rice.

The direct effects of rising global temperatures on insect herbivores could increase damage to cereal crops. However, the indirect effects of interactions between herbivores and their biotic environment at the same temperatures will potentially counter such direct effects. This study examines the potential for intraspecific competition to dampen the effects of optimal temperatures on fitness (survival × reproduction) of the brown planthopper, Nilaparvata lugens [BPH] and whitebacked planthopper, Sogatella furcifera [WBPH], two phloem-feeders that attack rice in Asia. We conducted a series of experiments with increasing densities of ovipositing females and developing nymphs on tropical and temperate rice varieties at 25, 30 and 35°C. Damage from planthoppers to the tropical variety was greater at 30°C compared to 25°C, despite faster plant growth rates at 30°C. Damage to the temperate variety from WBPH nymphs was greatest at 25°C. BPH nymphs gained greater biomass at 25°C than at 30°C despite faster development at the higher temperature (temperature-size rule); however, the effect was apparent only at high nymph densities. WBPH survival, development rates and nymph weights all declined at ≥ 30°C. At about the optimal temperature for WBPH (25°C), intraspecific crowding reduced nymph weights. Temperature has little effect on oviposition responses to density, and intraspecific competition between females only weakly counters the effects of optimal temperatures on oviposition in both BPH and WBPH. Meanwhile, the deleterious effects of nymph crowding will counter the direct effects of optimal temperatures on voltinism in BPH and on body size in both BPH and WBPH. The negative effects of crowding on BPH nymphs may be decoupled from resource use at higher temperatures.

Sampling scale and season influence the observed relationship between the density of deer and questing Ixodes ricinus nymphs

BackgroundThe relationship between environmentally transmitted tick parasites, Ixodes spp., and their main reproductive host, deer, is generally thought to be positive. However, measuring host abundance and density directly can be challenging and indirect methods are often used. The observed relationship between the parasite and host may be affected by sampling scale and season, which could lead to different inferences being made. Here, we aimed to test the effect of sampling scale and season on the relationship between density of deer and the density of questing Ixodes ricinus nymphs.MethodsThe density of deer (primarily Dama dama) was estimated using line transect distance sampling of deer dung quantified in different seasons (winter and summer) and measured at three different nested scales (site, transect and observation level). Questing nymph density was measured using blanket drag methods and estimates were calculated at the same scales as deer density estimates. General linear models were used to evaluate the relationship between questing nymphs, deer density and other environmental variables at each sampling scale and each season deer density was measured at.ResultsWhile a positive relationship between deer density and questing nymph density was detected at the site and transect scale, no relationship was apparent at the observation level. This was likely due to increased variation and reduced precision of deer dung counts at the finest sampling scale. Seasonal changes in deer populations were observed likely reflecting seasonal shifts in habitat usage. The summer estimates of deer density explained questing nymph density whereas winter estimates did not.ConclusionsOur results show that the scale of sampling can affect the detectability of the positive association between host and vector species. Furthermore, such associations can be obscured if hosts exhibit seasonal changes in habitat use. Thus, both sampling scale and season are important to consider when investigating the relationship between host and vector species.