Modifying Soil Nutrient Level Affects Soybean Insect Predators

The relationship between predator density, community composition, and field predation of Colorado potato beetle eggs

Does the “Enemies Hypothesis” operate by enhancing natural enemy evenness?

The ecological investigation was carried out on the predaceous Coleoptera in various citrus orchards in the collective pest-controlled area, Chiziwa-machi, Nagasaki Prefecture, Japan, and the following results were obtained. In either controlled or uncontrolled citrus orchards, the population densities of Pseudoscymnus hareya, Hyperaspis japonica, Stethorus japonicus, Scymnus rectus, Scymnus ishidai and Telsimia nigra were relatively high. Only in the controlled orchard, Pseudoscymnus hareja and Stethorus japonicus were found comparatively in large numbers. Comparison of the predator density between the controlled and uncontrolled orchards revealed that the density in the former was much lower than that in the latter, being as low as only about 1/10. In the controlled area, the larger number of species and a little higher predator density were found in the adult tree orchards (tree age above 10 years) whereas in the young tree orchards (tree age below 10 years) the predators of the scale insects were not found at all but only the predators of aphids and citrus red mites. Thus, there was a difinite relation between the predator and pest densities. The predator density was low immediately after the spray of pesticides, and showed a gradual increase with the lapse of days. Comparison of the number of all insects other than the major pests between the controlled and uncontrolled orchards showed that relatively large numbers of both Coleoptera and Hymenoptera were found in the uncontrolled orchards.

Augmentative release of predators is used in agriculture to suppress pest populations and minimise crop damage. However, predators affect prey populations not only through consumption but by inducing changes to prey morphology, physiology, behaviour and life history. These risk‐induced trait responses can increase prey fitness, potentially reducing the efficacy of certain predators as biocontrol agents. We investigated how non‐consumptive effects contribute to the overall influence of predators by modelling the effect of the predator Podisus maculiventris on the population dynamics of a major agricultural pest, the Colorado potato beetle ( Leptinotarsa decemlineata ). We developed a demographic model incorporating both consumption and non‐consumptive Podisus ‐induced shifts in beetle reproductive strategy. Our model combines data from multiple mechanistic experiments showing that predator‐exposed beetles produce fewer offspring but increase offspring provisioning, and that provisioned larvae have increased survival when predators are present. We simulated beetle densities across multiple beetle generations and a range of predator densities. The influence of non‐consumptive effects depended on predator density. At low predator densities, non‐consumptive effects strengthened the negative effect of predators by up to 45%, and risk‐induced changes to offspring provisioning had a stronger effect on prey densities than reduced offspring quantity. In contrast, at high predator densities, non‐consumptive effects changed the effect of predators by <5%, likely because beetle densities were very low both with and without non‐consumptive effects. Our results suggest that even adaptive responses to predation risk by prey do not necessarily reduce the ability of predators to suppress pest populations.

Landscape‐level crop diversification is a promising tool for ecological intensification, whereby biodiversity and ecosystem services are enhanced, and pesticide applications reduced. Yet evidence for the effects of crop diversity at multiple scales and in different landscape contexts is lacking. Here, we investigate the potential benefits and context dependencies of multiscale crop diversity on natural enemies and overall biological control in winter wheat. Simultaneously, we examine the mediating effects of bird predation on aphid regulation in this system. Eighteen conventional winter wheat fields were selected along two independent gradients of crop diversity and semi‐natural habitat cover (SNH). We assessed biological control using a natural enemy exclusion experiment (“Open Treatment,” “Bird Exclosure,” “Full Exclosure”). Biological control, predator and parasitoid densities within cages were analysed as functions of landscape (crop diversity × SNH), bird predation (yes/no) and temporal change (three surveys) on six spatial scales (100–3,000 m). Crop diversity rather than SNH enhanced aphid regulation in our study system. Biological control in fields with high landscape‐level crop diversity was 8%–33% higher than in low‐diversity landscapes, with main effects observed on scales <500 m. Predator and parasitoid densities increased with crop diversity on small (100–250 m) and large (2,000–3,000 m) spatial scales respectively. Nevertheless, our findings suggest that natural enemies other than birds, parasitoids and aerial arthropods facilitated biological control. Synthesis and applications. Our study shows that landscape‐level crop diversification can improve biological control in agroecosystems. Therefore, increased crop diversity could lower dependence on insecticides while enhancing yield stability through ecological intensification of farming. We also highlight the need to assess biological control rather than natural enemy abundances to avoid bias due to sampling artefacts or species interactions. Finally, simple measures of crop diversity (e.g. “effective number of crop types”) help in science communication and the development of farm management guidelines.

Natural enemies have not been shown previously to play a significant role in regulating thrips in field crops. Most thrips are r-selected with population attributes that are believed to result in thrips outstripping the capacity of natural enemies to regulate thrips populations. In replicated field experiments conducted in 1995, insecticide applications to peppers were observed to increase rather than decrease populations of Frankliniella spp. thrips in flowers. We conducted experiments in 1996 and 1997 to determine if this phenomenon resulted from exclusion of predators. Species of thrips abundant in the pepper flowers were Frankliniella occidentalis (Pergande), F. tritici (Fitch), and F. bispinosa (Morgan). The only common predator in the flowers was Orius insidiosus (Say). Different treatments of biological and synthetic insecticides were used which altered the population densities of prey and predator. Covariate analyses over time were used each year to separate insecticide and predator effects on thrips populations. The predator significantly suppressed populations of all three thrips species. F. occidentalis and F. tritici reproduced in pepper flowers, but most of the larval thrips in untreated plots were F. occidentalis. Near extinction of F. occidentalis adults and larvae occurred within days after predator:prey ratios reached 1:40. Exclusion of the predator with synthetic insecticides resulted in continued abundance of thrips. We conclude that O. insidious was an effective predator that suppressed populations of Frankliniella spp. in field peppers during a period when thrips were rapidly colonizing and developing in the flowers. Species of Orius are abundant predators nearly worldwide, and the role of density dependence in regulation of thrips populations should be reevaluated.

The southern green stink bug, Nezara viridula is one of the most serious pests of tomatoes in Florida. During 2014 and 2015 tomato cropping seasons, three cultivars of sweet alyssum, Lobularia maritima (Brassicales: Brassicaceae) were evaluated in north Florida to determine their effectiveness in conserving the natural enemies of the southern green stink bug in open field conditions. The experimental refuge crops were cultivated on two outer rows and one center row of the tomato crop. Each section of the rows of trap crops and refuge crops was 20 × 3 ft. (6 × 0.91 m) in size. Pest insects and their natural enemies were collected weekly and identified. On refuge crops, big‐eyed bug, Geocoris punctipes (Hemiptera: Geocoridae); minute pirate bug, Orius insidiosus (Hemiptera: Anthocoridae), ladybird beetles (Coleoptera: Coccinelidae), assassin bugs (Hemiptera: Reduviidae), damsel bugs (Hemiptera: Nabidae), hover flies (Diptera: Syrphidae) and spiders (Arachnida) were collected. All cultivars of L. maritima attracted the natural enemies of the southern green stink bug; however ‘carpet of snow’ was the most effective cultivar followed by ‘tall white’ in attracting G. punctipes in the tomato crop. N. viridula population first peaked in July and declined in week 11 and then had a sharp increase in weeks 12 and 13 when G. punctipes population was comparatively high. Data on the availability of suitable refuge crops which provide nectar and shelter to natural enemies are useful for integrated pest management of the southern green stinkbug, because these plants provide necessary resources to conserve beneficial species.

Population fluctuations in a continuous predator-prey system consisting of the spider mite,Tetranychus urticae Koch and its predator,Typhlodromus occidentalis Nesbitt, were assessed for almost 2 years (ca. 50 prey generations), on six mini-orchards of young apple trees in a climatically controlled glasshouse. During the first half of the experiment the plants were either ‘connected’ to each other with dowel rods or ‘unconnected’ and separate. In the second half, the plants were all ‘connected’. Population densities of both prey and predator on the unconnected mini-orchards were always higher than on the connected ones. Within the mini-orchards (=local scale), prey and predator populations go through a pattern of large amplitude cycles which continued throughout the experimental period. At the regional scale (mean over all mini-orchards) two types of fluctuations were observed. Large amplitude fluctuations associated with synchrony of the local cycles and small amplitude fluctuations associated with periods when local population cycles are proceeding out of phase. Transition from synchronous to asynchronous cycles took place very fast (within a few weeks), suggesting a mechanism generating asynchrony and possibly also partial refuges, which in turn trigger stabilising mechanisms at a regional scale. Causes of the synchrony are unclear. They may relate to the application of a pesticide (pirimicarb) used to control aphids, but there may just as well be an indirect form of causation, e.g. whereby the factors that promote aphid outbreaks also promote spider-mite population growth and temporary escape from control by the predators. Despite the synchronising factors the predator-prey system persisted, without external inputs, at a fairly small spatial scale, the size of a small glasshouse. Assessing the spatial scale and heterogeneity whereby decoupling of local cycles is sufficient for the cycles to proceed out of phase, remains an important area of experimental research. The experiment reported here provides some first clues and may form a standard for forthcoming experiments to assess the mechanisms promoting asynchrony, persistence and possibly also stability.

Ironically, pesticide applications in agriculture may result in the opposite of the intended reduction of the pest population. Following an initial treatment, the target population may resurge rapidly to greater than pre-treatment levels. In addition, populations of non-target species that were previously far below economic thresholds may increase greatly after applications and develop into pests of economic importance. Pesticide-induced outbreaks have been reported for very different groups of phytophagous arthropods and for several types of pesticides (Ripper 1956, Barbosa and Schultz 1987, Penman and Chapman 1988, Roush and Tabashnik 1990). This raises the question whether there are general principles underlying these outbreaks. One explanation for pesticide-induced outbreaks is that, for reasons rooted in their evolutionary past, phytophagous pest species are less susceptible to pesticides than their natural enemies. Hence, pesticide treatment will have a more drastic effect on natural enemies than on pests, resulting in outbreaks. However, this explanation ignores population dynamical effects on predator-prey balances. Furthermore, measurements of susceptibility to direct pesticide doses reveal that natural enemies are generally not more vulnerable to pesticides than their prey species (Hoy 1990) and may even show a trend towards lower predator susceptibility (Croft and Brown 1975). Thus, empirical support is lacking and (as we argue below) there is no good reason to expect that predatory arthropods would have been exposed to a narrower range of toxicants than their prey. A more sound explanation follows from consideration of the coupled population dynamics of predator and prey (May 1985, May and Dobson 1986). After pesticide application the densities of phytophagous arthropods are reduced. As soon as the harmful effects of the pesticide abate, conditions are favourable for herbivores: they suffer less predation and possibly experience reduced competition. Hence the numbers of pests and potential secondary pests will increase at a rate close to their maximum growth rate. Even if natural enemy densities are not directly affected, their densities will decrease after pesticide application since they are largely deprived of their main or only food source: phytophagous arthropods. Populations of predators can only increase after prey have attained sufficiently high densities. The increase in predator numbers therefore typically lags behind that of the pests. By the time the predators have returned to the pre-application densities, the pest populations have had the opportunity to increase unchecked for some time. The importance of food limitation to differential rates of recovery after pesticide application has been confirmed in a number of simulation studies (Tabashnik 1986 and 1990); even when the immediate mortality is similar for predator and prey, predator populations are more severely suppressed by pesticides than are pest populations (Waage et al. 1985). Pesticide induced outbreaks are often considered as transient events (Ripper 1956); after pesticide use is stopped, the ecosystem is expected to recover and predatorprey balances to restore. However, this need not be the case. Persistent outbreaks can result from the simultaneous existence of two stable states; one in which the predator controls the pest, and another in which the predator exists at very low levels, or is absent, and the pest is not controlled. After a small disturbance the system will return to its original state. For larger disturbances, however, the system may stabilize at either state, depending

The complex influence of rice variety, fertilization timing, and insecticides on the population dynamics of Sogatella furcifera (Horvath) and Nilaparvata lugens (Stal), and their natural enemies was studied in rice in Hangzhou, China. In the early season, S. furcifera adults immigrated to japonica Xiushui 11 (XS11) in a much higher density than to indica-japonica hybrid Xieyou 413 (XY413), which resulted in a higher density of the egg population of successive generations in XS11, while the peak density of the population peak of the 2nd generation in XY413 was higher due to a higher growth rate. In both of the rice varieties, spraying organic phosphorous triazophos in mid-season (during the 1st generation of the planthoppers) induced a population resurgence of the 2nd generations of S. furcifera and N. lugens. The main mechanism for the resurgence was the stimulation of reproduction of the 1st-generation adults and reduction of the density of predators, which in turn increased the survival of the planthoppers. Plots that were fertilized early attracted more N. lugens immigrants, but did not necessarily attract S. furcifera. Reproduction of the S. furcifera immigrants was enhanced by early fertilization of the fields of both rice varieties. The negative correlation between population growth rate from the 1st to the 2nd generation of S. furcifera and the density of spiders during the same period demonstrates the potential of predators in the natural control of rice planthoppers.

We consider a Rosenzweig–MacArthur predator-prey system which incorporates logistic growth of the prey in the absence of predators and a Holling type II functional response for interaction between predators and preys. We assume that parameters take values in a range which guarantees that all solutions tend to a unique limit cycle and prove estimates for the maximal and minimal predator and prey population densities of this cycle. Our estimates are simple functions of the model parameters and hold for cases when the cycle exhibits small predator and prey abundances and large amplitudes. The proof consists of constructions of several Lyapunov-type functions and derivation of a large number of non-trivial estimates which are of independent interest.

Vegetation texture (i.e., plant density, species diversity, and structural complexity) may influence the abundance of a herbivore by affecting its movement, altering the suitability of host plants, and by changing the herbivore's vulnerability to attack by natural enemies. We investigated these effects on the Mexican bean beetle (Epilachna varivestis) on snap bean (Phaseolus vulgaris) growing in high—and low—density monocultures and intercropped with short (0.5 m) or tall (1.2 m) corn plants (Zea mays). Beetle populations consistently reached higher densities on beans in monocultures than on beans intercropped with tall corn. Neither bean plant density nor plant species diversity alone significantly affected beetle abundance. Instead, the presence of tall corn plants contributed most to lower bean beetle density in the tall corn—bean intercrop, primarily by reducing adult colonization. However, the tall corn also increased the suitability of intercropped bean plants to the beetles because adults preferred to feed on and larvae developed faster on beans in the tall corn—bean habitat. Host plant effects were independent of the influence of microclimate and risk of predation. Abundance of natural enemies of the bean beetle was low and overall, predator density and bean beetle larval survival did not differ significantly among habitats. However, it is difficult to predict the effect or mortality factors, such as natural enemies, on bean beetle populations because larval mortality over time was affected by habitat type (i.e., there was a significant interaction between habitat type and time). Our results were partially consistent with the resource concentration hypothesis. Whereas Mexican bean beetles colonized bean monocultures more readily than they did tall corn—bean diculture, the presence of more suitable host plants in the diculture acted to increase, rather than decrease, bean beetle density in the tall corn—bean habitat. However, higher adult feeding preference and faster larval development on beans in the tall corn—bean habitat did not offset lower adult colonization in this habitat.

Effects of a disease affecting a predator on the dynamics of a predator–prey system

Biological control has been attracting an increasing attention over the last two decades as an environmentally friendly alternative to the more traditional chemical-based control. In this paper, we address robustness of the biological control strategy with respect to fluctuations in the controlling species density. Specifically, we consider a pest being kept under control by its predator. The predator response is assumed to be of Holling type III, which makes the system's kinetics "excitable." The system is studied by means of mathematical modeling and extensive numerical simulations. We show that the system response to perturbations in the predator density can be completely different in spatial and non-spatial systems. In the nonspatial system, an overcritical perturbation of the population density results in a pest outbreak that will eventually decay with time, which can be regarded as a success of the biological control strategy. However, in the spatial system, a similar perturbation can drive the system into a self-sustained regime of spatiotemporal pattern formation with a high pest density, which is clearly a biological control failure. We then identify the parameter range where the biological control can still be successful and describe the corresponding regime of the system dynamics. Finally, we identify the main scenarios of the system response to the population density perturbations and reveal the corresponding structure of the parameter space of the system.

Integrated Pest Management (IPM) is often described as a knowledge‐intensive approach to invertebrate pest management, requiring information on the biology, ecology and phenology of a pest combined with an understanding of the interactions between crop growth and pests and between pests and their natural enemies. We conducted a systematic quantitative literature review to summarise what is known about pest and natural enemy species common to Australian grain production systems, based on 1513 published and unpublished research studies. Drawing on this information, we address three issues: what are the knowledge gaps in relation to grain pests and their natural enemies, do these knowledge gaps limit the development of an IPM package for grain growers in Australia and what further ecological or biological information might growers require to enhance the use of IPM approaches for managing pests? The main gaps identified include a lack of understanding around specific factors that lead to pest outbreaks or factors that could be useful for predicting when and where pest outbreaks will occur in the future. Monitoring techniques for many pests are not well developed, and therefore, it is difficult to link the density recorded in a field with crop damage and yield loss and to develop economic thresholds that can be linked with intervention decisions. For most natural enemies, the impact in terms of reduction in pest numbers has not been quantified, with very few studies including both pests and natural enemies together. There is large variability in the level of control provided by natural enemies between years and regions, and the factors leading to this variability are not well understood. Finally, the lack of taxonomic resolution for individual species within groups is identified as a critical knowledge gap. We suggest that a more comprehensive fundamental knowledge base is required across the invertebrate community in grain systems aimed at reducing insect pest outbreaks, combined with a greater depth of understanding in monitoring strategies for pests that contribute to pesticide‐use decisions.