Bioeconomic synergy between tactics for insect eradication in the presence of Allee effects

Insect population dynamics under Wolbachia-induced cytoplasmic incompatibility: Puzzle more than buzz in Drosophila suzukii

In theory, the introduction of individuals infected with an incompatible strain of Wolbachia pipientis into a recipient host population should result in the symbiont invasion and reproductive failures caused by cytoplasmic incompatibility (CI). Modelling studies combining Wolbachia invasion and host population dynamics show that these two processes could interact to cause a transient population decline and, in some conditions, extinction. However, these effects could be sensitive to density dependence, with the Allee effect increasing the probability of extinction, and competition reducing the demographic impact of CI. We tested these predictions with laboratory experiments in the fruit fly Drosophila suzukii and the transinfected Wolbachia strain wTei. Surprisingly, the introduction of wTei into D. suzukii populations at carrying capacity did not result in the expected wTei invasion and transient population decline. In parallel, we found no Allee effect but strong negative density dependence. From these results, we propose that competition interacts in an antagonistic way with Wolbachia-induced cytoplasmic incompatibility on insect population dynamics. If future models and data support this hypothesis, pest management strategies using Wolbachia-induced CI should target populations with negligible competition but a potential Allee effect, for instance at the beginning of the reproductive season.

Optimizing the use of suppression zones for containment of invasive species

Despite efforts to prevent their establishment, many invasive species continue to spread and threaten food production, human health, and natural biodiversity. Slowing the spread of established species is often a preferred strategy; however, it is also expensive and necessitates treatment over large areas. Therefore, it is critical to examine how to distribute management efforts over space cost-effectively. Here we consider a continuous-space bioeconomic model and we develop a novel algorithm to find the most cost-effective allocation of treatment efforts throughout a landscape. We show that the optimal strategy often comprises eradication in the yet-uninvaded area, and under certain conditions, it also comprises maintaining a "suppression zone," an area between the invaded and the uninvaded areas, where treatment reduces the invading population but without eliminating it. We examine how the optimal strategy depends on the demographic characteristics of the species and reveal general criteria for deciding when a suppression zone is cost effective.

Forest Insect Biosecurity: Processes, Patterns, Predictions, Pitfalls

The economic and environmental threats posed by non-native forest insects are ever increasing with the continuing globalization of trade and travel; thus, the need for mitigation through effective biosecurity is greater than ever. However, despite decades of research and implementation of preborder, border, and postborder preventative measures, insect invasions continue to occur, with no evidence of saturation, and are even predicted to accelerate. In this article, we review biosecurity measures used to mitigate the arrival, establishment, spread, and impacts of non-native forest insects and possible impediments to the successful implementation of these measures. Biosecurity successes are likely under-recognized because they are difficult to detect and quantify, whereas failures are more evident in the continued establishment of additional non-native species. There are limitations in existing biosecurity systems at global and country scales (for example, inspecting all imports is impossible, no phytosanitary measures are perfect, knownunknowns cannot be regulated against, and noncompliance is an ongoing problem). Biosecurity should be a shared responsibility across countries, governments, stakeholders, and individuals.

Approaches for estimating benefits and costs of interventions in plant biosecurity across invasion phases

Nonnative plant pests cause billions of dollars in damages. It is critical to prevent or reduce these losses by intervening at various stages of the invasion process, including pathway risk management (to prevent pest arrival), surveillance and eradication (to counter establishment), and management of established pests (to limit damages). Quantifying benefits and costs of these interventions is important to justify and prioritize investments and to inform biosecurity policy. However, approaches for these estimations differ in (1) the assumed relationship between supply, demand, and prices, and (2) the ability to assess different types of direct and indirect costs at invasion stages, for a given arrival or establishment probability. Here we review economic approaches available to estimate benefits and costs of biosecurity interventions to inform the appropriate selection of approaches. In doing so, we complement previous studies and reviews on estimates of damages from invasive species by considering the influence of economic and methodological assumptions. Cost accounting is suitable for rapid decisions, specific impacts, and simple methodological assumptions but fails to account for feedbacks, such as market adjustments, and may overestimate long-term economic impacts. Partial equilibrium models consider changes in consumer and producer surplus due to pest impacts or interventions and can account for feedbacks in affected sectors but require specialized economic models, comprehensive data sets, and estimates of commodity supply and demand curves. More intensive computable general equilibrium models can account for feedbacks across entire economies, including capital and labor, and linkages among these. The two major considerations in choosing an approach are (1) the goals of the analysis (e.g., consideration of a single pest or intervention with a limited range of impacts vs. multiple interventions, pests or sectors), and (2) the resources available for analysis such as knowledge, budget and time.

Combining multiple tactics over time for cost-effective eradication of invading insect populations

Because of the profound ecological and economic impacts of many non-native insect species, early detection and eradication of newly founded, isolated populations is a high priority for preventing damages. Though successful eradication is often challenging, the effectiveness of several treatment methods/tactics is enhanced by the existence of Allee dynamics in target populations. Historically, successful eradication has often relied on the application of two or more tactics. Here, we examine how to combine three treatment tactics in the most cost-effective manner, either simultaneously or sequentially in a multiple-annum process. We show that each tactic is most efficient across a specific range of population densities. Furthermore, we show that certain tactics inhibit the efficiency of other tactics and should therefore not be used simultaneously; but since each tactic is effective at specific densities, different combinations of tactics should be applied sequentially through time when a multiple-annum eradication programme is needed.

Multiple agents managing a harmful species population should either work together to control it or split their duties to eradicate it

The management of harmful species, including invasive species, pests, parasites, and diseases, is a major global challenge. Harmful species cause severe damage to ecosystems, biodiversity, agriculture, and human health. In particular, managing harmful species often requires cooperation among multiple agents, such as landowners, agencies, and countries. Each agent may have incentives to contribute less to the treatment, leaving more work for other agents, which may result in inefficient treatment. A central question is, therefore, how should a policymaker allocate treatment duties among the agents? Specifically, should the agents work together in the same area, or should each agent work only in a smaller area designated just for her/him? We consider a dynamic game-theoretic model, where a Nash equilibrium corresponds to a possible set of contributions that the agents could adopt over time. In turn, the allocation by the policymaker determines which of the Nash equilibria could be adopted, which allows us to compare the outcome of various allocations. Our results show that fewer agents can abate the harmful species population faster, but more agents can better control the population to keep its density lower. We prove this result in a general theorem and demonstrate it numerically for two case studies. Therefore, following an outbreak, the better policy would be to split and assign one or a few agents to treat the species in a given location, but if controlling the harmful species population at some low density is needed, the agents should work together in all of the locations.

Novel insights on population and range edge dynamics using an unparalleled spatiotemporal record of species invasion

Quantifying the complex spatial dynamics taking place at range edges is critical for understanding future distributions of species, yet very few systems have sufficient data or the spatial resolution to empirically test these dynamics. This paper reviews how data from a large-scale pest management programme have provided important contributions to the fields of population dynamics and invasion biology. The invasion of gypsy moth (Lymantria dispar) is well-documented from its introduction near Boston, Massachusetts USA in 1869 to its current extent of over 900,000 km² in Eastern North America. Over the past two decades, the USDA Forest Service Slow the Spread (STS) programme for managing the future spread of gypsy moth has produced unrivalled spatiotemporal data across the invasion front. The STS programme annually deploys a grid of 60,000-100,000 pheromone-baited traps, currently extending from Minnesota to North Carolina. The data from this programme have provided the foundation for investigations of complex population dynamics and the ability to examine ecological hypotheses previously untestable outside of theoretical venues, particularly regarding invasive spread and Allee effects. This system provides empirical data on the importance of long-distance dispersal and time-lags on population establishment and spatial spread. Studies showing high rates of spatiotemporal variation of the range edge, from rapid spread to border stasis and even retraction, highlight future opportunities to test mechanisms that influence both invasive and native species ranges. The STS trap data have also created a unique opportunity to study low-density population dynamics and quantify Allee effects with empirical data. Notable contributions include evidence for spatiotemporal variation in Allee effects, demonstrating empirical links between Allee effects and spatial spread, and testing mechanisms of population persistence and growth rates at range edges. There remain several outstanding questions in spatial ecology and population biology that can be tested within this system, such as the scaling of local ecological processes to large-scale dynamics across landscapes. The gypsy moth is an ideal model of how important ecological questions can be answered by thinking more broadly about monitoring data.

A PREDATOR-PEST MODEL WITH ALLEE EFFECT AND PEST CULLING AND ADDITIONAL FOOD PROVISION TO THE PREDATOR — APPLICATION TO PEST CONTROL

The harmful effects of insect pests on human health and agricultural output are a major global concern. Frequent use of chemical pesticides as a means of pest control can have detrimental effects on the environment, resulting in water and soil pollution, food toxicity, resistance to pesticides, etc. As a result, there is an urgent need to develop a biological pest-control approach that would mitigate these harmful effects. The main purpose of the present study is to explore the interaction between strong Allee effects in the pest with other biological control mechanisms, such as providing additional food to the predator and pest culling as a means of proposing an efficient pest-control policy. To achieve this goal, local stability analysis around the equilibria, possible bifurcation and some basic dynamical features of the system was performed. Our work focuses on the basin of stability in multiple stable regions of the model, which yields the probability of convergence of each equilibrium for a given set of different initial conditions. The system exhibits bi-stability and tri-stability of the equilibria. Our findings indicate that providing additional food to the predator can be an efficient stand-alone pest control strategy, which can, if needed, be combined with other methods.

Allee effects in tritrophic food chains: some insights in pest biological control

Release of natural enemies to control pest populations is a common strategy in biological control. However, its effectiveness is supposed to be impaired, among other factors, by Allee effects in the biological control agent and by the fact that introduced pest natural enemies interact with some native species of the ecosystem. In this work, we devise a tritrophic food chain model where the assumptions previously raised are proved correct when a hyperpredator attacks the introduced pest natural enemy by a functional response type 2 or 3. Moreover, success of pest control is shown to be related to the release of large amounts (i.e., inundative releases) of natural enemies.

Is the Combination of Insecticide and Mating Disruption Synergistic or Additive in Lightbrown Apple Moth, Epiphyas postvittana?

Pest suppression from combinations of tactics is fundamental to pest management and eradication. Interactions may occur among tactical combinations and affect suppression. The best case is synergistic, where suppression from a combination is greater than the sum of effects from single tactics (AB >> A+B). We explored how mating disruption and insecticide interacted at field scale, additively or synergistically. Use of a pheromone delivery formulation (SPLAT^™) as either a mating disruption treatment (i.e. a two-component pheromone alone) or as a lure and kill treatment (i.e. the two-component pheromone plus a permethrin insecticide) was compared for efficacy against the lightbrown apple moth Epiphyas postvittana. Next, four point-source densities of the SPLAT^™ formulations were compared for communication disruption. Finally, the mating disruption and lure and kill treatments were applied with a broadcast insecticide. Population assessment used virgin female traps and synthetic pheromone in replicated 9-ha vineyard plots compared with untreated controls and insecticide-treated plots, to investigate interactions. Lure and kill and mating disruption provided equivalent suppression; no additional benefit accrued from including permethrin with the pheromone suggesting lack of contact. The highest point-source density tested (625/ha) was most effective. The insect growth regulator methoxyfenoxide applied by broadcast application lowered pest prevalence by 70% for the first ten weeks compared to pre-trial. Pheromone addition suppressed the pest further by an estimated 92.5%, for overall suppression of 97.7% from the treatment combination of insecticide plus mating disruption. This was close to that expected for an additive model of interactivity between insecticide and mating disruption (AB = A+B) estimated from plots with single tactics as 98% suppression in a combination. The results indicate the need to examine other tactical combinations to achieve the potential cost-efficiencies of synergistic interactions.

Mating Disruption as a Suppression Tactic in Programs Targeting Regulated Lepidopteran Pests in US

Mating disruption, the broadcast application of sex-attractant pheromone to reduce the ability of insects to locate mates, has proven to be an effective method for suppressing populations of numerous moth pests. Since the conception of mating disruption, the species-specificity and low toxicity of pheromone applications has led to their consideration for use in area-wide programs to manage invasive moths. Case histories are presented for four such programs where the tactic was used in the United States: Pectinophora gossypiella (pink bollworm), Lymantria dispar (gypsy moth), Epiphyas postvittana (light brown apple moth), and Lobesia botrana (European grapevine moth). Use of mating disruption against P. gossypiella and L. botrana was restricted primarily to agricultural areas and relied in part (P. gossypiella) or wholly (L. botrana) on hand-applied dispensers. In those programs, mating disruption was integrated with other suppression tactics and considered an important component of overall efforts that are leading toward eradication of the invasive pests from North America. By contrast, L. dispar and E. postvittana are polyphagous pests, where pheromone formulations have been applied aerially as stand-alone treatments across broad areas, including residential neighborhoods. For L. dispar, mating disruption has been a key component in the program to slow the spread of the infestation of this pest, and the applications generally have been well tolerated by the public. For E. postvittana, public outcry halted the use of aerially applied mating disruption after an initial series of treatments, effectively thwarting an attempt to eradicate this pest from California. Reasons for the discrepancies between these two programs are not entirely clear.

Eradication of tephritid fruit fly pest populations: outcomes and prospects

BACKGROUND

The number of insect eradication programmes is rising in response to globalisation. A database of arthropod and plant pathogen eradications covers 1050 incursion responses, with 928 eradication programmes on 299 pest and disease taxa in 104 countries (global eradication database b3.net.nz/gerda).

METHODS

A subset of the database was assembled with 211 eradication or response programmes against 17 species of fruit flies (Tephritidae) in 31 countries, in order to investigate factors affecting the outcome.

RESULTS

The failure rate for fruit fly eradication programmes was about 7%, with 0% for Ceratitis capitata (n = 85 programmes) and 0% for two Anastrepha species (n = 12 programmes), but 12% for 13 Bactrocera species (n = 108 programmes). A number of intended eradication programmes against long-established populations were not initiated because of cost and other considerations, or evolved during the planning phase into suppression programmes. Cost was dependent on area, ranged from $US 0.1 million to $US 240 million and averaged about $US 12 million (normalised to $US in 2012). In addition to the routine use of surveillance networks, quarantine and fruit destruction, the key tactics used in eradication programmes were male annihilation, protein bait sprays (which can attract both sexes), fruit destruction and the sterile insect technique.

CONCLUSIONS

Eradication success generally required the combination of several tactics applied on an area-wide basis. Because the likelihood of eradication declines with an increase in the area infested, it pays to invest in effective surveillance networks that allow early detection and delimitation while invading populations are small, thereby greatly favouring eradication success.

Eradication of Invading Insect Populations: From Concepts to Applications

Eradication is the deliberate elimination of a species from an area. Given that international quarantine measures can never be 100% effective, surveillance for newly arrived populations of nonnative species coupled with their eradication represents an important strategy for excluding potentially damaging insect species. Historically, eradication efforts have not always been successful and have sometimes been met with public opposition. But new developments in our understanding of the dynamics of low-density populations, the availability of highly effective treatment tactics, and bioeconomic analyses of eradication strategies offer new opportunities for developing more effective surveillance and eradication programs. A key component that connects these new developments is the harnessing of Allee effects, which naturally promote localized species extinction. Here we review these developments and suggest how research might enhance eradication strategies.

Integrated Pest Management in a Predator-Prey System with Allee Effects

A commonly used biocontrol strategy to control invasive pests with Allee effects consists of the deliberate introduction of natural enemies. To enhance the effectiveness of this strategy, several tactics of control of invasive species (e.g., mass-trapping, manual removal of individuals, and pesticide spraying) are combined so as to impair pest outbreaks. This combination of strategies to control pest species dynamics are usually named integrated pest management (IPM). In this work, we devise a predator-prey dynamical model in order to assess the influence of the intensity of chemical killing on the success of an IPM. The biological and mathematical framework presented in this study can also be analyzed in the light of species conservation and food web dynamics theory.

From integrated pest management to integrated pest eradication: technologies and future needs

BACKGROUND

With growing globalization and trade, insect incursions are increasing worldwide. A proportion of incursions involve pests of major economic crops (e.g.Mediterranean fruit fly), conservation value (e.g. tramp ants) or health significance(e.g.mosquitoes), and maybe the targets of eradication programmes. Historically, such responses have included the use of broad spectrum insecticides. However, with increasing public awareness of the negative aspects of pesticides, new environmentally friendly and effective techniques are needed. Here, we review and evaluate a range of selective to broad-spectrum tactical options for suppression which either have, or show potential for, integration within arthropod eradication programmes.

RESULTS

Most of the available technologies have their roots in pest management, but higher efficacy is required. Further refinement may be needed for use in eradication. Integration of several tactics is usually needed, as compatible tools can be used simultaneously to target different parts of the pest life cycle. However, not all technologies are fully compatible; for example, the simultaneous use of mass trapping and the sterile insect technique (SIT) may be suboptimal, although sequential application may still be effective.

CONCLUSIONS

Broad-spectrum insecticides are generally incompatible with some biologically based technologies such as the SIT, but may be used to reduce the population so that density-dependent tactics can be used. Several novel technologies with fewer nontarget impacts have been proposed in recent years, and need to be properly evaluated for their applicability to insecteradication. Overall, there are still major gaps in surveillance and selective eradication technologies for most insects.

Estimating Allee dynamics before they can be observed: polar bears as a case study

Allee effects are an important component in the population dynamics of numerous species. Accounting for these Allee effects in population viability analyses generally requires estimates of low-density population growth rates, but such data are unavailable for most species and particularly difficult to obtain for large mammals. Here, we present a mechanistic modeling framework that allows estimating the expected low-density growth rates under a mate-finding Allee effect before the Allee effect occurs or can be observed. The approach relies on representing the mechanisms causing the Allee effect in a process-based model, which can be parameterized and validated from data on the mechanisms rather than data on population growth. We illustrate the approach using polar bears (Ursus maritimus), and estimate their expected low-density growth by linking a mating dynamics model to a matrix projection model. The Allee threshold, defined as the population density below which growth becomes negative, is shown to depend on age-structure, sex ratio, and the life history parameters determining reproduction and survival. The Allee threshold is thus both density- and frequency-dependent. Sensitivity analyses of the Allee threshold show that different combinations of the parameters determining reproduction and survival can lead to differing Allee thresholds, even if these differing combinations imply the same stable-stage population growth rate. The approach further shows how mate-limitation can induce long transient dynamics, even in populations that eventually grow to carrying capacity. Applying the models to the overharvested low-density polar bear population of Viscount Melville Sound, Canada, shows that a mate-finding Allee effect is a plausible mechanism for slow recovery of this population. Our approach is generalizable to any mating system and life cycle, and could aid proactive management and conservation strategies, for example, by providing a priori estimates of minimum conservation targets for rare species or minimum eradication targets for pests and invasive species.

Host-parasitoid dynamics and the success of biological control when parasitoids are prone to allee effects

In sexual organisms, low population density can result in mating failures and subsequently yields a low population growth rate and high chance of extinction. For species that are in tight interaction, as in host-parasitoid systems, population dynamics are primarily constrained by demographic interdependences, so that mating failures may have much more intricate consequences. Our main objective is to study the demographic consequences of parasitoid mating failures at low density and its consequences on the success of biological control. For this, we developed a deterministic host-parasitoid model with a mate-finding Allee effect, allowing to tackle interactions between the Allee effect and key determinants of host-parasitoid demography such as the distribution of parasitoid attacks and host competition. Our study shows that parasitoid mating failures at low density result in an extinction threshold and increase the domain of parasitoid deterministic extinction. When proned to mate finding difficulties, parasitoids with cyclic dynamics or low searching efficiency go extinct; parasitoids with high searching efficiency may either persist or go extinct, depending on host intraspecific competition. We show that parasitoids suitable as biocontrol agents for their ability to reduce host populations are particularly likely to suffer from mate-finding Allee effects. This study highlights novel perspectives for understanding of the dynamics observed in natural host-parasitoid systems and improving the success of parasitoid introductions.

The biology of small, introduced populations, with special reference to biological control

Populations are introduced into novel environments in different contexts, one being the biological control of pests. Despite intense efforts, less than half introduced biological control agents establish. Among the possible approaches to improve biological control, one is to better understand the processes that underpin introductions and contribute to ecological and evolutionary success. In this perspective, we first review the demographic and genetic processes at play in small populations, be they stochastic or deterministic. We discuss the theoretical outcomes of these different processes with respect to individual fitness, population growth rate, and establishment probability. Predicted outcomes differ subtly in some cases, but enough so that the evaluating results of introductions have the potential to reveal which processes play important roles in introduced populations. Second, we attempt to link the theory we have discussed with empirical data from biological control introductions. A main result is that there are few available data, but we nonetheless report on an increasing number of well-designed, theory-driven, experimental approaches. Combining demography and genetics from both theoretical and empirical perspectives highlights novel and exciting avenues for research on the biology of small, introduced populations, and great potential for improving both our understanding and practice of biological control.