Natural Enemy Densities Research Articles

Keragaan Hama, Penyakit, dan Musuh Alami pada Budidaya Padi Organik

Organic rice cultivation intents to minimize the use of inorganic fertilizers and chemical pesticides, but in fact not apart from the disruption of pests and diseases. The study aimed to determine the pests, diseases, and natural enemies on organic rice cultivation. The research was conducted at Screen Field of ICRR at WS 2015/2016 and DS 2016, using Split Plot research design with three replications. The main plot was varieties of: 1). Inpari 25, 2). Inpari 33, and 3). Sintanur. The sub-plot was application of leaf extract of: 1). Mindi, 2). Soursop, 3). Mahogany, and 4). Control. Applications were performed on 4, 6, 8, and 10 weeks after planting (WAP). Observations of pests, diseases, and natural enemies on 5, 7, 9, and 11 WAP as many as 20 clumps/plots. The results showed that varieties and leaf extract applications significantly affected on severity of stem rot disease at 5 and 7 WAP in WS 2015/2016. In the rainy season found Yellowing on Inpari 25 and Inpari 33 varieties. However, Sintanur was more resistant to Yellowing with LCC value on two growing seasons was highest compared to the other varieties. Sintanur had the lowest Cercospora Leaf Spot disease severity compared to the other varieties. The severity of disease generally increased with the age of rice increase, while the pest population density and natural enemies were fluctuate. The density of the pest was quite high in the control plot, while the natural enemy density was high enough in the application plots of Mindi, Mahogany, and Soursop extracts.

Landscape‐scale interactions of spatial and temporal cropland heterogeneity drive biological control of cereal aphids

Summary Agricultural landscapes are characterised by dynamic crop mosaics changing in composition and configuration over space and time. Although semi‐natural habitat has been often shown to contribute to pest biological control, the effects of increasing landscape heterogeneity with cropland have been disregarded. Here, we examine how cereal aphids, their enemies and biological control are affected by the composition and configuration of the crop mosaic and its inter‐annual change due to crop rotation. We studied the abundance of cereal aphids, natural enemies and aphid parasitism over 2 years on 51 winter wheat fields. Arthropods were monitored at three distances (0, 10, 30 m) from field borders. Fields were embedded in landscapes of 1‐km diameter selected along orthogonal gradients of compositional crop heterogeneity (crop diversity), configurational heterogeneity (field border and grassy field boundary length) and inter‐annual change in cover of aphid host habitats (cereal, maize and grassland). We aimed to disentangle spatial and temporal heterogeneity effects through these independent landscape gradients. Aphid densities were lower in landscapes with smaller field size (more field borders) coupled with high amounts of grassy field boundaries. Aphid densities decreased also in landscapes with higher crop diversity when the cover of aphid host habitat had decreased from the year before. Aphid natural enemy densities decreased with smaller field size and high amounts of grassy field boundaries. Aphid parasitism decreased with the inter‐annual expansion in aphid host habitat, but only in landscapes with small field sizes. Synthesis and applications. Our study shows for the first time that cereal aphid numbers can be reduced by optimising the composition, configuration and temporal heterogeneity of the crop mosaic. We highlight the value of maintaining small field sizes in agricultural landscapes and high densities of grassy boundaries for reducing aphid abundance. Landscape‐wide crop diversification can reduce aphid densities as well.

Manipulating field margins to increase predation intensity in fields of winter wheat (Triticum aestivum)

AbstractThe effectiveness of natural enemies to control pests can be enhanced through habitat manipulation. However, due to the differences in their ecology, generalist and specialist species may respond differently to the same manipulation. Moreover, interactions among natural enemies (i.e. cannibalism, intraguild predation, hyperparasitism) may complicate the assumption that a higher density of natural enemies would increase the level of biological control. We investigated the natural enemy guild composition and the predation rate along flower vs. grass margins at the edge of winter wheat (Triticum aestivum) fields in Denmark. Natural enemies were sampled by pitfall trapping and by suction sampling; predation intensity was measured using two different sentinel prey methods: artificial caterpillars made of plasticine, and sentinel aphid colonies. Specialist and generalist species responded differently to the two margin types: specialists (mostly parasitic wasps) were attracted by the flower margins, while generalists (ground beetles, rove beetles and spiders) were more active in grass margins. The number of artificial caterpillars attacked was significantly greater in grass margins (mean = 48.9%, SD = 24.3) than in flower margins (mean = 30.7%, SD = 17.4). We found a significant positive relationship between the number of artificial caterpillars attacked by chewing insects, and activity density for large (≥15 mm) ground beetles. Predation of sentinel aphids in wheat fields did not vary significantly in relation to margin type. Our results suggest that flowering margins may be beneficial for canopy‐active specialist natural enemies, but grassy margins are more useful for ground‐active generalist predators.

Alyssum flowers promote biological control of collard pests

Collard greens Brassica oleracea (L.) are often attacked by various pests including whiteflies, aphids and diamondback moth. Hitherto, the main method used to manage these pests in Brazil has been the application of a limited number of registered insecticides. The search for more sustainable pest management strategies is therefore warranted. In this context, the conservation biological control stands out as an appealing alternative. Conservation biological control is achieved, at least in part, by strip-cultivating and/or conserving flowering plants within the agroecosystem. The present study investigates how alyssum flowers Lobularia maritima (L.) could contribute to the attraction of natural enemies and to the management of collard pests. Two field experiments were conducted in different years. Each experiment consisted of two treatments and three replicates, which were set up in a completely randomized design. The treatments were (1) collards alone, and (2) collards + alyssum. We evaluated weekly the population density of natural enemies and pests on both treatments. The results show that the alyssum flowers attractiveness contributed to increase the abundance of generalist predators during both experiments, which in turn translated into a significant reduction of collards pests, especially aphids. Some of the main predators attracted/harbored by alyssum flowers were spiders, coccinellids, syrphids and Orius sp. Finally, strip intercropping alyssum with collards can be an important strategy to manage brassica pests and cope with the limited availability of insecticides registered for this vegetable crop.

Impact of Insecticide Applications on Arthropod Predators and Plant Feeders in Cambodian Rice Fields

Rice (Oryza sativa) is the staple crop of Cambodia. The heavy use of pesticides has caused significant financial losses to local farmers. This study examined the necessity and economic viability of insecticide use for rice cultivation, and the multiple interactions between the populations of both insect pests and natural enemies in dry-season rice fields. The outcomes of calendar-based insecticide-sprayed and non-sprayed rice fields were compared at three provincial research stations in Cambodia in terms of rice growth, economic returns, damage caused by insect pests, and population dynamics of insect pests and arthropod natural enemies. The results showed that rice fields without insecticide application did not suffer significant damage by insect pests or by diseases and had similar yields to those treated with insecticide application in two provinces. Although the occurrence of caseworm was observed in the third province, this was successfully controlled by an integrated pest management (IPM) treatment without using insecticides. Insufficient densities of natural enemies appeared to correlate with the caseworm outbreak in this location, whereas high densities of natural enemies relative to those of insect pests contributed to control potential pest expansion in the other two provinces. Natural enemy populations were significantly decreased by insecticide applications. Reduced insecticide input in the insecticide-free control and IPM plots resulted in higher net profits than those in insecticide-applied plots, except for plots in the region with a caseworm outbreak (insecticide-free). Overall, this study provides encouraging insight into cost effective and environmentally friendly pest management in Cambodian rice fields.

Structure, function and management of semi-natural habitats for conservation biological control: a review of European studies.

Different semi-natural habitats occur on farmland, and it is the vegetation's traits and structure that subsequently determine their ability to support natural enemies and their associated contribution to conservation biocontrol. New habitats can be created and existing ones improved with agri-environment scheme funding in all EU member states. Understanding the contribution of each habitat type can aid the development of conservation control strategies. Here we review the extent to which the predominant habitat types in Europe support natural enemies, whether this results in enhanced natural enemy densities in the adjacent crop and whether this leads to reduced pest densities. Considerable variation exists in the available information for the different habitat types and trophic levels. Natural enemies within each habitat were the most studied, with less information on whether they were enhanced in adjacent fields, while their impact on pests was rarely investigated. Most information was available for woody and herbaceous linear habitats, yet not for woodland which can be the most common semi-natural habitat in many regions. While the management and design of habitats offer potential to stimulate conservation biocontrol, we also identified knowledge gaps. A better understanding of the relationship between resource availability and arthropod communities across habitat types, the spatiotemporal distribution of resources in the landscape and interactions with other factors that play a role in pest regulation could contribute to an informed management of semi-natural habitats for biocontrol. © 2016 Society of Chemical Industry.

Nontarget effects of orchard pesticides on natural enemies: Lessons from the field and laboratory

The nontarget effects of insecticide programs used to control codling moth, Cydia pomonella L. (Lepidoptera: Tortricidae), were studied in large-plot field trials in apples, pears, and walnuts in the western United States. We assessed the health of the natural enemy community by sampling the abundance of natural enemies and by monitoring for outbreaks of secondary pests. The insecticides used in the field tests overlapped those tested in laboratory bioassays. Using these parallel lab and field studies, we examined two hypotheses: 1) pesticides found to have negative effects on natural enemy fitness in laboratory bioassays will predict reductions in natural enemy densities in the field, and 2) reductions in natural enemy densities in the field will result in outbreaks of secondary pests. We found only one clear instance, Forficula auricularia (L.) (Dermaptera: Forficulidae), where laboratory results documenting negative effects corresponded to a significant reduction in field studies (apple). This same instance was the only case where a reduction in a natural enemy population was associated with a significantly increased density of a secondary pest, Eriosoma lanigerum (Hausmann) (Hemiptera: Aphididae). There were several instances where secondary pest outbreaks were associated with unchanged or even increased natural enemy densities. The limited number of field trials, variability in field trial conditions among years and sites, duration of the negative effect relative to sampling interval, sampling efficiency, plot size/inter-plot movement, and compensation by other natural enemies attracted to a large host/prey resource may all have contributed to the poor predictive success. Overall, the laboratory bioassays predicted a far greater negative impact than was found in the field trials.

Characterization of the natural enemy community attacking cotton aphid in the Bt cotton ecosystem in Northern China.

Planting Bt cotton in China since 1997 has led to important changes in the natural enemy communities occurring in cotton, however their specific effect on suppressing the cotton aphids (being notorious in conventional cotton ecosystem) has not been fully documented yet. We observed strong evidence for top-down control of the aphid population, e.g. the control efficiency of natural enemies on cotton aphid increased significantly in open field cages compared to exclusion cages, accounted for 60.2, 87.2 and 76.7% in 2011, 2012 and 2013 season, respectively. The cotton aphid populations peaked in early June to late July (early and middle growth stages) in open field cotton survey from 2011 to 2013. The population densities of cotton aphids and natural enemies were highest on middle growth stage while lowest densities were recorded on late stage for aphids and on early plant stage for natural enemies. Aphid parasitoids (Trioxys spp., Aphidius gifuensis), coccinellids and spiders were key natural enemies of cotton aphid. Briefly, natural enemies can suppress aphid population increase from early to middle plant growth stages by providing biocontrol services in Chinese Bt cotton.

Effects of agroforestry on Phytoseiidae communities (Acari: Mesostigmata) in vineyards. A synthesis of a 10-year period of observations

Several species of the predatory mite family Phytoseiidae are of major economic importance for biological pest control in crops, including grapevines. Plant diversification in agrosystems is reported to enhance useful biodiversity and provide ecosystem services. Thus, agroforestry, which consists in co-planting trees and crops, is assumed to be a possible way to ensure regulation of pest outbreaks by phytoseiid mites. This paper investigates the effect of trees (Pinus pinea and Sorbus domestica) within vineyards on Phytoseiidae communities. Five experimental plots were considered, two where vines were co-planted with P. pinea and S. domestica, and three monoculture plots: vines, P. pinea and S. domestica. Sampling was carried out on vines and trees in 2003, 2004, 2005, 2010 and 2012. A higher Phytoseiidae diversity was observed in agroforestry plots than in monoculture plots. Kampimodromus aberrans (replacing T. [ T.] exhilaratus) seemed to colonize these plots 15 years after its plantation, especially when vines were co-planted with P. pinea. Factors affecting these faunistical changes are discussed. Effects on Phytoseiidae densities differed depending on grape cultivartree species associations. Factors affecting these interactions are discussed; plant diversity does not simply lead to a higher density and diversity of natural enemies.

Effects of agroforestry on Phytoseiidae communities (Acari: Mesostigmata) in vineyards. A synthesis of a 10-year period of observations

Several species of the predatory mite family Phytoseiidae are of major economic importance for biological pest control in crops, including grapevines. Plant diversification in agrosystems is reported to enhance useful biodiversity and provide ecosystem services. Thus, agroforestry, which consists in co-planting trees and crops, is assumed to be a possible way to ensure regulation of pest outbreaks by phytoseiid mites. This paper investigates the effect of trees (Pinus pinea and Sorbus domestica) within vineyards on Phytoseiidae communities. Five experimental plots were considered, two where vines were co-planted with P. pinea and S. domestica, and three monoculture plots: vines, P. pinea and S. domestica. Sampling was carried out on vines and trees in 2003, 2004, 2005, 2010 and 2012. A higher Phytoseiidae diversity was observed in agroforestry plots than in monoculture plots. Kampimodromus aberrans (replacing <I>T.</I> [ <I>T.</I>] exhilaratus) seemed to colonize these plots 15 years after its plantation, especially when vines were co-planted with P. pinea. Factors affecting these faunistical changes are discussed. Effects on Phytoseiidae densities differed depending on grape cultivartree species associations. Factors affecting these interactions are discussed; plant diversity does not simply lead to a higher density and diversity of natural enemies.

Absence of windbreaks and replanting citrus in solid sets increase density of Asian citrus psyllid populations

Densities of an herbivorous pest may be impacted by landscape and orchard architecture. We present two orchard experiments where the densities of the Asian citrus psyllid (Diaphorina citri) were compared depending on: (1) the presence or absence of a windbreak and (2) if the orchards consisted of a solid set re-planting or an orchard with a mixture of mature and reset-replacement trees. Psyllid abundance was measured on the edges of five orchards. The factor investigated was the presence or absence of a windbreak. We observed significantly fewer psyllids on the edges of orchards with windbreaks as compared to those without windbreaks. We found no significant difference in the number of natural enemies between the edges with or without windbreaks, suggesting that windbreaks do not affect densities of psyllid natural enemies. Also, during two consecutive years, we compared the densities of psyllids on young trees less than 3 years of age in a solid set re-planting versus on resets (trees planted in replacement of dead or huanglongbing-infected trees) present randomly within mature orchards. This was conducted in four orchards and among three citrus varieties. More psyllids were found in the solid set re-plantings as compared with on the resets within mature orchards. To our knowledge, this is the first report to demonstrate that the planting strategy of new trees in orchards may impact the populations of a horticultural pest. Overall our data suggest that establishment and conservation of windbreaks might be beneficial to protect orchards from D. citri. The data also suggest that D. citri populations increase more within uniform landscapes of seedling trees as compared with mature orchards with randomly interspersed young seedlings.

ANALISIS KORELASI DAN REGRESI DINAMIKA POPULASI HAMA DAN MUSUH ALAMI PADA BEBERAPA VARIETAS UNGGUL PADI SETELAH PENERAPAN PHT DI KABUPATEN BONE PROVINSI SULAWESI SELATAN

&lt;div data-canvas-width="788.5133333333328"&gt;Different types of rice pests, have been reported. The explosions and a potential pests are Rice Stem Borer (Tryporiza&lt;/div&gt;&lt;div data-canvas-width="802.6483333333334"&gt;sp.), Brown Planthoppers (Nilaparvata lugens), Green Leafhoppers (Nephotettix virescens), and Rats (Rattus sp.), wich&lt;/div&gt;&lt;div data-canvas-width="802.7016666666667"&gt;an obstacle in creating stability of rice production in Bone. In rice ecosystem, natural enemies of arthropod predators&lt;/div&gt;&lt;div data-canvas-width="802.703333333333"&gt;are most instrumental in suppressing the pest population. Fluctuation in population density of natural enemies, affecting&lt;/div&gt;&lt;div data-canvas-width="802.6466666666668"&gt;pests presence. Efforts to suppress pest proportions through IPM, is a strategy of lowering the level of pests and damage&lt;/div&gt;&lt;div data-canvas-width="802.6933333333333"&gt;to rice crops. This study aims to identify and measure the relationship strength between pests and natural enemies&lt;/div&gt;&lt;div data-canvas-width="802.6733333333332"&gt;populations in some rice varieties after IPM, using correlation and regression analysis. Observations conducted in Bone,&lt;/div&gt;&lt;div data-canvas-width="802.6600000000001"&gt;from May to August 2012, with 2 observed variables, pests and natural enemies populations in Inpari 6, Inpari 10, Inpari&lt;/div&gt;&lt;div data-canvas-width="802.6633333333332"&gt;16, Ciherang, Cigelis, and Mekongga. Data analysis were performed using correlation and regression analysis. The&lt;/div&gt;&lt;div data-canvas-width="802.6833333333332"&gt;results showed a high degree of correlation between pest and natural enemies (predators) populalions, characterized by&lt;/div&gt;&lt;div data-canvas-width="802.7066666666666"&gt;a positive correlation coefficient (0.957) and significant at the level of 0.01, where each increase in the intensity of pest&lt;/div&gt;&lt;div data-canvas-width="564.7166666666665"&gt;populations, will be followed by increase in the number of natural enemies captured.&lt;/div&gt;

Natural vegetation benefits synergistic control of the three main insect and pathogen pests of a fruit crop in southern Africa

Most studies of the potential for natural habitat to improve agricultural productivity have been conducted in transformed, temperate regions, but little is known of the importance of agroecosystem services in biodiverse developing countries. Natural vegetation may promote the density and/or diversity of natural enemies of crop pests, but the strength of the effect varies, and few studies directly measure concurrent impacts on pest density. Considering multiple pest species within the same agroecosystem may help explain why some pests are more affected than others by landscape complexity. Here, we investigated multiple pest species (three species of Tephritidae fruit fly, leaf galling flies and pathogenic fungi Fusarium spp.) and their enemies in cultivated mango Mangifera indica, in north-eastern South Africa. The density of generalist Tephritidae fruit flies increased with distance from natural vegetation during harvesting months, and predation rate of pupae sharply decreased from ~50% at the edge with natural vegetation to 0% at 250 m into the crop. Parasitism rates of the cryptic, gall-forming fly increased with proximity to natural vegetation, but pest density was unrelated to distance from natural vegetation. Incidence of the fungal pathogen disease increased with distance from natural vegetation, possibly due to decreased predation of commensal mites. Although the relationship with distance to natural vegetation was significant for all species considered, the strength of this relationship varied across pest species and type of natural enemy studied, suggesting the benefits of natural vegetation depend on each natural enemy species' ability to disperse into the agricultural environment. Synthesis and applications. Our results suggest that natural vegetation is a net source of natural enemies in a region of South Africa that still contains much of its natural biodiversity. However, the decline in natural enemies, and increase in pests, with distance from natural habitat indicates that this biocontrol is limited by natural enemy dispersal. In landscapes like these that are still dominated by natural habitat, conservation biocontrol can still be improved by management aimed at providing corridors of key plants and habitat elements into the crops, to facilitate natural enemy dispersal.

Temporal change in vegetation productivity in grain production landscapes: linking landscape complexity with pest and natural enemy communities

1. Strategies to enhance the ecosystem service of pest control in agricultural landscapes often rely on manipulating the structure of the landscape to reduce pest population build‐up or facilitate natural enemy activity. For highly mobile and polyphagous invertebrate pest species the agricultural landscape represents a continually changing mosaic of resources and habitats that at certain points in time may be conducive to high population productivity and therefore pest outbreaks. Deciding which features of the landscape we need to measure and monitor to understand this process is the first step in developing intervention strategies. Previous studies have focused on static measures of landscape composition and have generally ignored landscape configuration and change across time.2. In this study we sampled invertebrate pests and predatory natural enemies repeatedly over two years from crop, pasture and semi‐natural habitats (up to 36 sites per landscape) in multiple agricultural landscapes (14km diameter circles) across Australia's grain production regions. The temporal pattern in vegetation productivity was quantified (using remote‐sensed Normalized Differential Vegetation Index) at the landscape‐scale and related to the pest and natural enemy density data.3. Metrics derived from the vegetation show variation across time and highlight transition points in the landscape that might represent important shifts in resources for higher trophic levels. The landscape metrics that were related to pest density were different in each landscape, but generally characterized connectivity of vegetation classes (more than configuration or composition). How connected or dis‐aggregated vegetation productivity classes were appears important for pest dynamics in these landscapes.

Density-dependent benefits in ant-hemipteran mutualism? The case of the ghost ant Tapinoma melanocephalum (Hymenoptera: Formicidae) and the invasive mealybug Phenacoccus solenopsis (Hemiptera: Pseudococcidae).

Although density-dependent benefits to hemipterans from ant tending have been measured many times, few studies have focused on integrated effects such as interactions between ant tending, natural enemy density, and hemipteran density. In this study, we tested whether the invasive mealybug Phenacoccus solenopsis is affected by tending by ghost ants (Tapinoma melanocephalum), the presence of parasitoids, mealybug density, parasitoid density and interactions among these factors. Our results showed that mealybug colony growth rate and percentage parasitism were significantly affected by ant tending, parasitoid presence, and initial mealybug density separately. However, there were no interactions among the independent factors. There were also no significant interactions between ant tending and parasitoid density on either mealybug colony growth rate or percentage parasitism. Mealybug colony growth rate showed a negative linear relationship with initial mealybug density but a positive linear relationship with the level of ant tending. These results suggest that benefits to mealybugs are density-independent and are affected by ant tending level.

Combining Repellent and Attractive Aromatic Plants to Enhance Biological Control of Three Tortricid Species (Lepidoptera: Tortricidae) in an Apple Orchard

Non-host plants that are intercropped with crops can either repel or attract pests, and in some cases achieve pest management. Three aromatic plant species, ageratum (Ageratum houstonianum L.; Asterales: Asteraceae), French marigold (Tagetes patula L.; Asterales: Asteraceae) and summer savory (Satureja hortensis L.; Lamiales: Lamiaceae), were intercropped in ratios of 1:1 or 1:1:1 in an organic apple orchard to investigate the community characteristics and dynamic changes in densities of the Tortricidae species Adoxophyes orana Fisher, Spilonota lechriaspis Meyrick and Acleris fimbriana Thunberg and their natural enemies in 3 vertical strata of the orchard. Intercropping aromatic plants in the orchard increased the densities of natural enemies belonging to the Trichogrammatidae, Ichneumonidae and Braconidae and reduced the rates of increase and the densities of the tortricid species relative to the control. A correlation analysis of the densities of pest species and the densities of parasitoids indicated that the reduction in some tortricid species was related to the densities of certain natural enemy species. We concluded that intercropping with aromatic plants in apple orchards can increase the biological control of tortricid pests.

Functional response of Euseius concordis to densities of different developmental stages of the cassava green mite

Both prey density and developmental stage of pests and natural enemies are known to influence the effectiveness of biological control. However, little is known about the interaction between prey density and population structure on predation and fecundity of generalist predatory mites. Here, we evaluated the functional response (number of prey eaten by predator in relation to prey density) of adult females and nymphs of the generalist predatory mite Euseius concordis to densities of different developmental stages of the cassava green mite Mononychellus tanajoa, as well as the fecundity of adult females of the predator. We further assessed the instantaneous rate of increase, based on fecundity and mortality, of E. concordis fed on eggs, immatures and adults of M. tanajoa. Overall, nymphs and adults of E. concordis feeding on eggs, immatures and females of M. tanajoa had a type III functional response curve suggesting that the predator increased prey consumption rate as prey density increased. Both nymphs and adult females of the predator consumed more eggs than immatures of M. tanajoa from the density of 20 items per leaf disc onwards, revealing an interaction between prey density and developmental stage in the predatory activity of E. concordis. In addition, population growth rate was higher when the predator fed on eggs and immatures in comparison with females. Altogether our results suggest that E. concordis may be a good candidate for the biological control of M. tanajoa populations. However, the efficiency of E. concordis as a biological control agent of M. tanajoa is contingent on prey density and population structure.

Development of biological control of Tetranychus urticae (Acari: Tetranychidae) and Phorodon humuli (Hemiptera: Aphididae) in Oregon hop yards.

The temporal development of biological control of arthropod pests in perennial cropping systems is largely unreported. In this study, the development of biological control of twospotted spider mite, Tetranychus urticae Koch, and hop aphid, Phorodon humuli (Schrank), in a new planting of hop in Oregon is described over a period of 9 yr (2005-2013). Both the abundance and diversity of natural enemies increased over time. Known predators of hop aphid (Coccinellidae and Anthocoridae) were present in all years; however, stable biological control of hop aphid was not achieved in most years and aphicides were required to suppress populations at commercially acceptable levels in 5 of 9 yr. Populations of aphidophagous coccinellids developed synchronously with hop aphid populations, and temporal correlations indicated these are the primary predatory insect associated with hop aphid regulation. However, sampling methods did not assess levels of aphid parasitoids and hyperparasitoids and their contribution to biological control was unquantified. Spider mite biological control was associated primarily with predatory mites (Phytoseiidae) and Stethorus spp. (Coccinellidae). The magnitude of temporal correlations of abundance of these predators with spider mites was found to be greatest on the same sampling dates and at lags of 7-14 d. Stable biological control of spider mites occurred after four field seasons, suppressing spider mites to levels similar to those commonly achieved with chemical control. A survey of 11 commercial hop yards in Oregon documented pest and natural enemy densities under commercial management practices over a period of 4 yr (2008-2011). Natural enemy abundance in commercial hop yards was similar to that of a 2- to 3-yr-old hop yard with limited disturbance. Whereas total reliance on biological control for hop aphid is unlikely to be successful, there appears to be unrealized potential for biological control of spider mites in commercial production. Dynamic action thresholds that consider the value of natural enemies are needed for both pests.

Effects of land use and insecticides on natural enemies of aphids in cotton: First evidence from smallholder agriculture in the North China Plain

Studies conducted in the USA and Europe have shown that diverse landscapes in general support greater natural enemy abundance. No quantitative evidence on the relationship between land use diversity and natural enemies has been reported from developing countries, where fields and farms are much smaller than in modernized agriculture in the west, and where insecticide use is often high and indiscriminate. This paper examines the effects of land use and farmers’ insecticide application on natural enemies of aphids in cotton production, based on a unique dataset that links household and cotton field surveys to a detailed assessment of land uses in the landscapes surrounding the cotton fields in the North China Plain (NCP), a major grain and cotton production region in China. Our results show that, in the NCP where farms are small and landscape is dominated by a few crops, Shannon or Simpson land use diversity index is not a good indicator for explaining the relationship between land use and densities of aphid natural enemies. Instead, the types and proportions of cropland habitat mattered. Landscapes with more maize and grassland have higher ladybeetle populations in cotton fields. Farmers’ pest management practices such as the amount and timing of insecticide use significantly affect ladybeetle densities. These results imply that there is a need to recognize the potential positive role of cropland use in pest management and call for more judicious insecticide use strategies by smallholder farmers in the North China Plain.

Relative Influence of Plant Quality and Natural Enemies on the Seasonal Dynamics of &lt;I&gt;Bemisia tabaci&lt;/I&gt; (Hemiptera: Aleyrodidae) in Cotton

The abundance and distribution of insect herbivores is determined by, among other things, plant quality and natural enemies. These two factors vary temporally and spatially, subsequently affecting seasonal population dynamics. The relative influence of plant quality and natural enemies on the seasonal dynamics of Bemisia tabaci (Gennadius) was investigated in a 3-yr field study in cotton. Plant quality was manipulated through varying irrigation regimes: irrigations done at 20, 40, and 60% soil water depletions; and natural enemy densities were manipulated using broad spectrum insecticide applications that reduced their densities compared with unsprayed controls. In each year, densities of B. tabaci eggs, large nymphs and adults were consistently higher when natural enemy densities were reduced compared with when they were left unaltered, regardless of irrigation regime. In contrast, effects of plant quality on densities of all whitefly stages were weak and inconsistent. In addition, natural enemy densities and predator:prey ratios also were not generally affected by plant quality. Interactions between natural enemies and plant quality on whitefly dynamics were rare. In general, whitefly densities were elevated two-thirds of the time and increased two- to sixfold when natural enemy densities were reduced compared with plant quality effects which influenced whitefly densities about one-third of the time and were expressed inconsistently over the years. This indicates that natural enemies exert a comparatively greater influence on seasonal dynamics of B. tabaci in cotton than plant quality, as manipulated by differential irrigation.