Available online at www.sciencedirect.com
Pest management challenges for biofuel crop production Linda J Thomson and Ary A Hoffmann New biofuel crops, whether derived from existing food crops such as maize or representing second-generation biomass crops, will be attacked by a range of pests despite claims that they might be immune from pests. Biofuel crops grown on marginal land will impact on ecosystem services provided by these areas, and biofuel crops can also act as a reservoir for pests of food crops. Moves towards sustainable pest control in traditional crops need to be extended to cover biofuel crops. This includes focusing on less toxic pesticides rather than broad spectrum pesticides and conserving landscape features that contribute to effective biological control. Where crops are planted into new regions, there is the possibility of predicting likely pest pressures based on distribution modeling and through assessing the status of pests on related crops. Address Centre for Environmental Stress and Adaptation Research (CESAR), Zoology Department, University of Melbourne, Bio21, 30 Flemington Rd., Parkville, Vic. 3010, Australia Corresponding author: Hoffmann, Ary A (
[email protected]) Current Opinion in Environmental Sustainability 2011, 3:95–99 This review comes from a themed issue on Terrestrial systems Edited by Andy W Sheppard, S Raghu, Cameron Begley and David M Richardson Received 9 July 2010; Accepted 9 November 2010 Available online 3rd December 2010 1877-3435/$ – see front matter Crown Copyright # 2010 Published by Elsevier B.V. All rights reserved. DOI 10.1016/j.cosust.2010.11.003
Introduction Biofuel production from dedicated crops could play a role in mitigating climate change with potential to decrease net carbon emissions from combustion of petroleum by 0.775 Tg in the US alone [1]. Existing biofuel markets are dominated by US ethanol production based on corn, Brazilian ethanol production based on sugarcane, and European biodiesel production based on rapeseed oil. Use of existing crops for biofuel production need not be the only source — dedicated ‘biomass crops’ have energy advantages over these crops [2]. In establishing biomass crops, consideration must be given both to environmental gains made in agriculture and in ensuring biosecurity of current food crops especially the cereals wheat, rice and corn which together currently provide 60% of human food energy (UN Food and Agriculture Organisation http://www.fao.org/docrep/u8480e/u8480e07.htm). The www.sciencedirect.com
more significant contribution to world energy is predicted to be derived from biomass crops such as switchgrass Panicum virgatum L., Miscanthus and giant reed Arundo donax L. [3]. Switchgrass is one of the dominant species of the central North American tallgrass prairie. Miscanthus giganteus is a perennial, sterile hybrid derived from a common grass, Miscanthus spp., native to Asia and now occurring throughout the United States. Giant reed (Arundo donax L.) is a fast growing perennial grass, native to East Asia, and widespread throughout the Mediterranean area. Biomass crops have energy advantages when compared to diversion of current food crops such as maize to biofuel production and are expected to constitute the bulk of biofuel production into the future despite their potential to become environmental weeds [4]. Limited attention pest issues have received so far in debates around the production of biofuel crops indicates they will not be immune [5,6,7]. As pest control is an important component of managing agricultural yields, potential pest problems in biofuel crops need to be identified, along with ways in which these might differ from problems in traditional arable crops. The introduction or expansion of crops as biofuels in current production systems and into new regions including areas marginal for farming may generate novel issues around pest control. If the biofuel industry is to maintain an environmentally responsible image, it will need to develop and adopt sustainable pest control practices.
Biofuel crops and pest control: what challenges are likely to arise? To reduce environmental impacts of dedicated biofuel crops, proponents have focused on perennial and highly (water and nutrient) efficient crops, tolerant to infertile soils, that rapidly establish highly competitive monocultures [8] and harbor few potential pests or diseases [9]. The leading candidates are rhizomatous perennial grasses [2], which require minimal human intervention for establishment and survival of productive stands. For example, three decades of breeding switchgrass have generated dozens of cultivars and varieties, many of which produce dense stands, tolerate infertile soils, and readily regenerate from vegetative fragments [10]. A focus on crops that maximize yield while allowing cultivation on less productive, marginal lands has the admirable aim of minimizing agricultural (food production) impacts. However, developing these lands for biofuel production has other potential consequences, including loss of areas currently providing other ecosystem services [11] and the potential for the Current Opinion in Environmental Sustainability 2011, 3:95–99
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crops to become weeds in environmentally sensitive areas. Marginal lands, monocultures and biological services
It has been suggested that all arable lands unsuitable for high value agriculture might be used to grow energy crops [12]. However many of the drivers of biodiversity loss are associated with intensification of agriculture. Conservation of intact or restored habitat spared from conversion to agriculture has become an important issue in reconciling sustainable production with biodiversity [13] and maintenance of ecosystem services [14]. The latter include pest suppression [15]. Populations of natural enemies can be reduced by activities within the crop (application of chemicals, mowing, harvesting, etc.), but their persistence also depends on reinvasion which in turn can depend on locally available populations from relatively undisturbed vegetation [16,17]. As a consequence, noncrop habitats often serve as reservoirs of natural enemies which can colonize and suppress herbivore populations [18]. The abundance of natural enemies within crops is reduced as diverse habitat is lost [19]. Although the use of perennial crops may reduce the importance of reinvasion potential [20] due to increased survival of beneficials within these less disturbed crops [21], there is still likely to be an overall decrease in natural enemy abundance if marginal noncrop habitat is lost to biofuel production. This will be exacerbated if biomass crops are established as monocultures to reduce crop handling costs. Overall, the value of biological control provided by diverse natural enemies has been estimated at 1994 US$ 23 ha 1 year 1 in grass/rangelands or $24 in cropland [22]. Aphid parasitoids provide an example of lost value of pest control in a food crop due to increased biofuel production. Aphids (Hemiptera: Aphididae) are economically important pests of a range of crops and their hymenopteran parasitoids play an important role in population suppression [23]. Adult parasitoids depend on sugar rich food sources such as nectar or honeydew for survival and egg maturation [24], and enhanced suppression of a variety of aphid pests has been associated with landscape heterogeneity [25,26]. Alteration of landscapes due to increased demand for maize as a source of ethanol production has led to acreage increasing 19% nationally in 2006–2007, with an estimated cost due to loss of natural control of the soybean aphid across four US States at $33 ha 1 or $239 million/year [11]. Enhancing pest numbers in adjacent food crops
Biofuels may enhance pest numbers in existing food crops. Plantings of nonfood biofuels within landscapes in marginal areas interspersed with food crops can potentially sustain pests on alternative hosts [5]. The relationship Current Opinion in Environmental Sustainability 2011, 3:95–99
between maize and Miscanthus is a case in point. The area selected for Miscanthus planting in the US includes the Corn Belt, giving rise to a potential increase in abundance of Western corn rootworm (WCR) Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), a destructive maize pest responsible for US$ 1 billion in annual yield losses and control costs in the US Corn Belt (http:// www.plantmanagementnetwork.org/pub/php/review/ 2004/rootworm/). Strong selection for egg-laying in crops outside maize due to several decades of control by crop rotation has changed the cornfield specialist WCR into a pest that will lay eggs in the soil around almost any crop in the landscape. Recent host specificity research shows that WCR can complete egg-to-adult development on Miscanthus. Given relaxed egg-laying fidelity among the crop rotation-resistant population, there is a potential for WCR to become a significant pest of Miscanthus in addition to corn. Increased maize production in response to biofuel initiatives will increase the likelihood that founding populations of high dispersal ability WCR become established [27]. WCR presence is also associated with reduced populations of predatory mites and predatory thrips possibly because feeding (silk clipping) by WCR adults destroys this microhabitat of the beneficial organisms [27]. WCR larval damage to corn plants can also change the structure and characteristics of the whole field by increasing the number of underdeveloped plants, reducing parasitism of another significant corn pest, European corn borer Ostrinia nubilalis (Hu¨bner) (Lepidoptera: Pyralidae) [28]. This indirect impact may occur because Macrocentrus grandii (Goidanich) (Hymenoptera: Braconidae), a specialist parasitoid of O. nubilalis, prefers tall and dense maize stands. There are other pests which may attack Miscanthus and enhance pest abundance in relation to food crops including the yellow sugarcane and corn leaf aphids Sipha flava (Forbes) and Rhopalosiphum maidis (Fitch) [6]. Like the WCR, both pests may affect the biofuel crop, and abundance of these pests may then increase in adjacent food crops. Both aphids are pests of sorghum, maize, wheat and corn and are also vectors of viruses in these crops. Crops attacked by S. flava and R. maidis contributed to more than $6.4 billion of the 2007 US sugar and grain production value (about 4% of the 2007 total US crop production value) (http://usda.mannlib.cornell.edu/usda/nass/CropValuSu// 2000s/2009/CropValuSu-02-13-2009.pdf). Breaking links with natural enemies
When the range of current crops is expanded or new crops are introduced, pest problems may arise because pest species are no longer subjected to control by existing communities of natural enemies; the final outcome will depend on whether natural enemies follow the pest or newly establish from a resident community of natural enemies [29]. Hitchhiking on exotic plants, on foliage or in soil, provides a potential pathway for new pests and exotic plants selected as biomass crops may bring with www.sciencedirect.com
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them new pests with few local natural enemies. Native natural enemies typically have a limited impact on newly invading pests, such as the lack of parasitoid impact on populations of the European corn borer in North America even 100 years after its invasion. In this case imported parasitoids and other enemies from the native range provide relatively more effective biocontrol agents (http://entnemdept.ifas.ufl.edu/creatures/field/e_corn_ borer.htm#natural). Within the last 5 years, problems with pests of maize in Europe including Mediterranean corn borer Sesamia nonagrioides Lefe`bvre, cotton bollworm Helicoverpa armigera Hu¨bner and cutworms Agrotis spp. (all Lepidoptera: Noctuidae) have increased as populations expand because of warmer climatic conditions [30]; between 2 and 4 million ha maize in Europe now suffer economic damage due to these corn boring pests. Though native to North America, WCR has only recently invaded a wide geographic range of USA and Canada and first appeared in Europe in 1992. Its rapid spread has resulted in a large and growing infested area (http://entm.purdue.edu/wcr) spreading at an average rate of 40 km per year [27]. The diapause strategy of WCR allows it to survive dry conditions in its native range, providing a pre-adaptation to survive cold seasons in temperate regions. Chemical impact of pest control on biofuel crops
A claimed benefit of biomass crops is their lack of pests and hence reduced requirement for chemical pest control [31], with Miscanthus and switchgrass suggested to be low input crops requiring little to no management for insect pests [10,21]. However, several orders of herbivorous insects feed on Miscanthus [32]; this suggests that rather than being pest-free, the identity of insect pests and their effects on harvestable biomass are simply not yet known [7,33]. Recent research [6] suggests aphids including yellow sugar cane and corn leaf have the potential to damage young Miscanthus plants. When biofuel crops are attacked by pests, there may be less incentive for sensitive chemical use (lower toxicity chemicals or less concern with pre-harvest and postharvest intervals) than in the case of high value food crops. Because of tight profit margins, there is likely to be a tendency to spray pests with cheap broad spectrum pesticides, as happens in many broad-acre crops [34]. In the absence of effective pest monitoring, which is expensive and requires a high level of skill, farmers are likely to apply ‘insurance’ sprays, increasing the risk of resistance, secondary pests, disruption of natural predators, altered pest dynamics for existing crops and environmental pollution. Increased chemical applications can lead to changes in pest complexes [35] and species with relatively high levels of tolerance or the ability to evolve resistance to chemicals can increase in frequency. Widespread resistance to multiple chemicals has developed in various pest aphid species [36] and WCR has evolved www.sciencedirect.com
resistance to some chemicals [27]. There are several examples from broad acre crops where development of tolerance to currently registered pesticides is implicated in increased pest abundance, for example red legged earth mites Halotydeus destructor (Tucker) (Acarina: Penthaleidae) in Australia due to resistance development against synthetic pyrethroids [37]. If biofuel crops were to be sprayed with broad spectrum pesticides, this would counter progress made in sustainable pest control through the use of low impact chemicals, monitoring and more effective control via natural enemies. When selective chemicals with lower overall toxicity are applied, this can promote a diverse community of enemies that exerts more effective pest control [38,39]. Increased use of broad spectrum insecticides on nonfood crops will harm naturally occurring predators and parasitoids which often exert effective biological control in the field.
Conclusion Plantings of biofuel crops including Miscanthus are increasing rapidly, raising issues around sustainable pest control. Biofuel crops are not likely to be immune to pest issues, their management will be a key issue for ensuring that biofuel crop production is profitable. As in the case of food crops, it will be a challenge to grow biofuels while minimizing negative impacts on the environment and food production. Much progress has been made in developing ways of reducing the environmental impact of agricultural production over the last two decades. Diverse initiatives have all contributed to this, and include reducing the impacts of chemicals used for pest control and soil nutrition, developing targeted and more effective water use, and recognizing the role of diverse landscapes in providing ecosystem services. In establishing biofuel crops, the challenge is to ensure that these gains are maintained. This means that pest solutions based only on broad spectrum pesticides need to be avoided, that the value of marginal lands in providing reservoirs of beneficial organisms and for biodiversity protection need to be appreciated, and that plantings in large scale monocultures need to be avoided particularly in the case of invasive weedy biofuel crops. Biofuel production systems could play a positive role in mitigating climate change, but pests need to be controlled sustainably, maintaining landscape heterogeneity is a key component of this effort.
Acknowledgements Our research on pest control is supported by the Grape and Wine Research and Development Corporation, the Grains Research and Development Corporation and a Fellowship from the Australian Research Council.
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