Understanding the mechanisms of persistence and dispersal of an insect pathogen and its potential for novel strategic control of African armyworms

非洲粘虫（英语：African armyworm），是非洲的一种主要的迁徙性害虫，类似于蝗虫对农作物和草地的破坏，但更为频繁。在坦桑尼亚这样的国家，粘虫的爆发始于10年中的9年。在每一次爆发中，飞蛾产卵孵化成极其密集的毛虫群，贪婪地吃玉米、水稻、高粱和其他粮食作物。在完成发育后，毛虫形成新的蛾群，迁移到新的地区开始其他的爆发。有些年份，这些疫情可能持续数月，规模不断扩大，从坦桑尼亚蔓延到非洲各地，每年的降雨和破坏数百万公顷的作物。目前对粘虫的控制依赖于喷洒化学杀虫剂。然而，高昂的成本意味着高达70%的粮食生产者遭受损失，通常是作物的全部损失。特别脆弱的是那些依靠家庭玉米地为生的穷人。该项目旨在研究一种全新的生物方法来控制粘虫，通过战略性地使用一种天然粘虫疾病-粘虫核型多角体病毒（NPV）。NPV自然发生，在季节后期，当它在毛虫之间传播时，可以完全消灭粘虫的爆发。然而，在大多数年份，这种疾病在粘虫季节出现得太晚，无法防止大多数暴发和作物损害。先前的研究表明，NPV是粘虫特有的，因此对人类、牲畜和野生动物是安全的。田间试验表明，它可以喷洒到粘虫爆发，开始疾病流行，因此可以用作化学杀虫剂的有效替代品。我们已经证明，这种病毒可以很容易地通过从喷洒的爆发中收获死幼虫来大规模生产，并廉价地配制成生物杀虫剂，准备喷洒新的侵扰。这个新项目将研究粘虫中NPV的自然生物学，重点是它如何在粘虫种群中传播和持续存在。利用这些信息，我们将确定我们是否可以通过操纵病毒种群来防止今年第一次爆发的粘虫传播，从而防止作物损害和昂贵的化学控制的需要。这将涉及对病毒遗传学的重大研究，并需要更深入地了解病毒如何在细胞、个体和群体水平上与宿主相互作用。NPV通常是一种高度传染性的急性疾病，能够迅速传播并在3-4天内杀死昆虫。NPV的一个特别吸引人的方面是，当粘虫在旱季稀少时，病毒可以转变成一种非致命的形式，可以在昆虫体内持续存在，并通过卵传播给它们的后代。后来，当雨水回来，密集的毛毛虫群开始出现时，这种疾病似乎能够重新激活自己，进入高度传染性的致命状态。我们希望，通过了解疾病流行如何发生以及何时可能不发生，我们可以帮助非洲科学家更好地预测作物何时面临风险，以及粘虫爆发的规模何时可能需要控制。此外，我们希望我们可以防止这种严重的作物害虫在非洲的传播和迁移，通过触发病毒的致命和非致命形式之间的转换，和/或通过将NPV应用于最早的爆发，从而将其控制下来。通过研究这种主要害虫的自然生物控制，我们希望减少或消除这种对非洲数百万贫困农民及其家庭粮食供应的威胁，为发展中国家节省数百万美元的控制成本，同时减少在非洲生物多样性和野生动物保护的一些最重要领域使用破坏环境的化学品的需要。

African armyworm, the caterpillar of the African armyworm moth, is a major migratory pest in Africa; similar to locusts in its devastation of crops and grasslands, but much more frequent. In countries like Tanzania, where the outbreaks start, armyworms are a problem in 9 years out of 10. In each outbreak, eggs laid by moths hatch into extremely dense swarms of caterpillars that feed voraciously on maize, rice, sorghum and other grain crops. After completing their development, the caterpillars form new swarms of moths that migrate to new areas to start other outbreaks. Some years these outbreaks can continue for months, growing in size and spreading out from Tanzania across Africa, following the annual rains and damaging millions of hectares of crops. The current control for armyworms relies on spraying outbreaks with chemical insecticides. However, the high cost means that up to 70% of food producers suffer damage and often total loss of crops. Especially vulnerable are the poor who rely on household maize plots for food. This project aims to investigate a radically new biological approach to controlling armyworm, through the strategic use of a natural armyworm disease - armyworm nucleopolyhedrovirus (NPV). NPV occurs naturally and, late in the season, and can completely wipe out armyworm outbreaks as it spreads from caterpillar to caterpillar. However, in most years the disease appears too late in the armyworm season to prevent most outbreaks and crop damage. Previous work has shown that NPV is specific to armyworm and so is safe to humans, livestock and wildlife. Field trials have shown that it can be sprayed onto armyworm outbreaks to start disease epidemics, and so could be used as an effective alternative to chemical insecticides. We have shown that the virus can easily be mass produced by harvesting dead larvae from sprayed outbreaks and cheaply formulated into a biological insecticide ready for spraying new infestations. This new project will study the natural biology of NPV in armyworms, focussing on how it spreads and persists in armyworm populations. Using this information, we will determine whether we can prevent the spread of armyworms from the first outbreaks of the year, by manipulating virus populations and so preventing crop damage and the need for expensive chemical control. This will involve a major study on the genetics of the virus, and will require a deeper understanding of how the virus interacts with its host at the cellular, individual and population level. NPV is normally a highly-infectious acute disease, capable of spreading rapidly and killing insects within 3-4 days. A particularly fascinating aspect of NPV is that, when armyworms are scarce during the dry season, the virus can switch into a non-lethal form that can persist inside insects and be transmitted to their offspring via the eggs. Later, when the rains return and dense swarms of caterpillars start to appear, the disease seems able to re-activate itself into the highly-infectious lethal state. We hope that by understanding how disease epidemics occur, and when they might be absent, we can help African scientists to better predict when crops are at risk, and when the scale of armyworm outbreaks are likely to require control. Also, we hope that we can prevent the spread and migration of this serious crop pest across Africa, either by triggering the switch between the lethal and non-lethal forms of the virus, and/or by applying NPV to the earliest outbreaks, so bringing them under control. By studying this natural biological control of this major pest, we hope to reduce or eliminate this threat to the food supply of millions of poor farmers and their families in Africa, save developing countries millions of dollars in control costs, while at the same time reducing the need to apply environmentally-damaging chemicals to some of the most important areas of biodiversity and wildlife conservation in Africa.