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

非洲粘虫是非洲粘虫蛾的毛虫，是非洲主要的迁徙害虫；类似于蝗虫对庄稼和草原的破坏，但更频繁。在像坦桑尼亚这样疫情爆发的国家，粘虫每10年有9年是一个问题。在每次爆发中，飞蛾产下的卵孵化成极其密集的幼虫群，它们贪婪地以玉米、水稻、高粱和其他谷物作物为食。幼虫发育完成后，形成新的飞蛾群，迁徙到新的地区，开始其他疫情。有些年份，这些疫情可以持续数月，规模不断扩大，并在每年的降雨之后从坦桑尼亚蔓延到非洲各地，破坏了数百万公顷的农作物。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，往往是农作物的全部损失。尤其脆弱的是那些依靠家庭玉米田获取食物的穷人。本项目旨在通过战略性地利用一种天然粘虫疾病——粘虫核多角病毒（NPV），研究一种全新的控制粘虫的生物学方法。NPV在季节后期自然发生，当它在毛毛虫之间传播时，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，它可以喷洒到爆发的粘虫身上，从而引发疾病流行，因此可以作为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疫情中收集死亡的幼虫来大量生产，并廉价地配制成一种生物杀虫剂，准备喷洒新的虫害。这个新项目将研究粘虫体内NPV的自然生物学，重点研究它是如何在粘虫种群中传播和持续存在的。利用这些信息，我们将确定我们是否能够通过控制病毒种群，从而防止作物受损和需要昂贵的化学控制，从今年的第一次爆发开始防止粘虫的传播。这将涉及对病毒遗传学的重大研究，并将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，当旱季粘虫稀少时，病毒可以转变成一种非致命的形式，这种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，当雨季来临，密集的毛虫群开始出现时，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，通过了解疾病流行是如何发生的，以及何时可能没有流行，我们可以帮助非洲科学家更好地预测作物何时处于危险之中，以及何时可能需要控制粘虫爆发的规模。此外，我们希望我们能够通过触发致命和非致命形式的病毒之间的转换，以及/或通过在最早的爆发中应用NPV，从而使它们得到控制，从而防止这种严重作物害虫在非洲的传播和迁移。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，同时减少在非洲一些最重要的生物多样性和野生动物保护领域使用对环境有害的化学品的需求。

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.