Pareto-efficient biological pest control enable high efficacy at small costs

Highlights

• We suggest how to find control strategies that optimally trade off between efficacy and profit.

• We demonstrate our methodology on a pest–pathogen–crop system.

• We show that simple control strategies can be efficacious and also near optimal in profit.

Abstract

Biological pest control is increasingly used in agriculture as an alternative to traditional chemical pest control. In many cases, this involves a one-off or periodic release of naturally occurring and/or genetically modified enemies such as predators, parasitoids, or pathogens. As the interaction between these enemies and the pest is complex and the production of natural enemies potentially expensive, it is not surprising that both the efficacy and economic viability of biological pest control are debated. Here, we investigate the performance of very simple control strategies. In particular, we show how Pareto-efficient one-off or periodic release strategies, that optimally trade off between efficacy and economic viability, can be devised and used to enable high efficacy at small economic costs. We demonstrate our method on a pest–pathogen–crop model with a tunable immigration rate of pests. By analyzing this model, we demonstrate that simple Pareto-efficient one-off and periodic release strategies are efficacious and simultaneously have profits that are close to the theoretical maximum obtained by strategies optimizing only the profit. When the immigration rate of pests is low to intermediate, one-off control strategies are sufficient, and when the immigration of pests is high, periodic release strategies are preferable. The methods presented here can be extended to more complex scenarios and be used to identify promising biological pest control strategies in many circumstances.

Introduction

Pests are major concerns in agriculture. Local outbreaks cause financial losses and regional outbreaks threaten the food security of entire populations. This is of particular concern in developing nations where agriculture constitutes a larger share of the economy but in which agricultural practices have not yet reached the same technical and procedural standards as in developed nations. In India, for example, the “Army worm” Spodoptera litura (Fabr.) has defoliated many economically important crops including cotton, sunflower, and soybean (Dhaliwal et al., 2010). Farmers have traditionally resorted to pesticides to prevent and mitigate pest outbreaks, but their use may have unwanted consequences including insect resistance, resurgence, outbreak of secondary pests, and pesticide residues affecting human health and the environment. Indeed, heavy usage of synthetic pesticides has been linked to pest resistance, pest resurgence, health risks from exposure, and food contamination (Khooharo et al., 2008, Yadav, 2010).

Biological pest control is an alternative to chemical pest control in which naturally occurring enemies such as predators, parasitoids, or pathogens rather than pesticides are used to control the pests. The use of naturally occurring enemies to suppress insect pests has several advantages over chemical pest control, in particular safety for farmers, consumers, and non-targeted organisms. Biological pest control can potentially be efficacious at low cost and should not normally pose any danger for either farmers or consumers. They can be host-specific, they preserve natural enemies, and they may beneficially impact biodiversity (Lacey et al., 2001). Unlike the use of pesticides, there is little consensus on how to apply biological control for maximal efficiency. One reason for this is the complex interplay of non-linear interactions between the crop, the pest, and the natural enemy. The potential benefits of improved biological pest-control strategies are particularly large for inundative and augmentative applications, in which large numbers of natural enemies are released, as the timing of the release may significantly affect the total cost and efficacy.

To the authors knowledge, a handful of studies have explored design of biological pest-control strategies from the perspective of mathematical analysis and/or optimal control theory. These studies have considered problems of bioeconomic equilibrium, demographic stability, and optimal-release strategies (Getz and Gutierrez, 1982, Grasman et al., 2001, Bhattacharyya and Bhattacharya, 2006, Rafikov et al., 1993, Cardoso et al., 2009). While these studies have furthered our understanding of biological pest control, the proposed pest-control strategies may not easily be communicated to agriculture professionals as they typically lack a regular pattern and sometimes require continuous release of natural enemies. Moreover, with Cardoso et al. (2009) as an important exception, only single-objective optimization is usually considered. Finally, to the authors knowledge, the studies to date have not explicitly modeled the crop, which as a third dynamic state variable could potentially impact the results. Developing simple but efficient rules for biological pest control in agricultural systems with crop–pest–enemy interactions thus remains an important challenge from both a theoretical and applied perspective.

In this paper, we suggest a simple method for developing strategies for biological pest control that are easy to apply, efficacious, and simultaneously near optimal in terms of profit. The strategies are Pareto efficient in that they optimally trade off between profit and efficacy. We demonstrate our method on a dynamic model of the Spodoptera litura worm defoliating soybean crops while being controlled by a natural enemy, the Spodoptera polyhedrosis virus (Cherry et al., 1997, Fuxa, 2004). Specifically, we investigate one-off control strategies and periodic-control strategies. Using our measures of efficacy and profit, we find Pareto-efficient one-off and periodic control strategies that are close to optimal in the sense of profit and simultaneously not sensitive to perturbations. We show that one-off control strategies are preferable when immigration of pests is relatively low to intermediate. We also show that, for high immigration rates, one-off control can be replaced by simple periodic controls to achieve even better results.