Mechanisms of Disease Transmission, Variability in Host Susceptibility, and Forest Defoliator Outbreaks

许多森林昆虫爆发，数量激增导致森林落叶，造成广泛的经济损失。当病毒性疾病的流行席卷人群，造成大量死亡时，疫情往往自然结束。为什么这些疾病不能简单地永久地控制人口呢？什么决定流行病何时开始？这些问题的答案将有助于我们在没有杀虫剂的情况下控制害虫，也可能有助于我们了解其他疾病。我们寻找答案的方法是利用舞毒蛾的一种病毒性疾病进行实验。舞毒蛾是一种从美国东北部和加拿大的硬木树上引进的害虫，它正在迅速蔓延。这种病毒性疾病导致舞毒蛾幼虫融化在树叶上，传播病毒颗粒，被其他不幸的幼虫吃掉。我们的工作重点是可能影响这种疾病传播的三个因素。首先，我们正在测试这种昆虫是否会随着时间的推移而进化出抗药性，因为在流行病中幸存下来的个体可能是那些能够更好地抵抗感染的个体。第二，我们正在测试病毒在树叶上的传播模式是否会影响传播，这样当病毒分布更均匀时，传播率就会上升。第三，我们正在测试昆虫赖以为生的树叶中的化合物是否会改变昆虫被感染的几率。每个测试都包括一组实验，在这些实验中，昆虫以自然界中被病毒污染的树叶为食，除了我们依次操纵每个因素；昆虫的遗传背景，病毒在叶子上的模式，以及叶子的化学成分。然后，我们将使用计算机模型综合我们的结果，将数据转化为疾病传播率的预测。

Many forest insects undergo outbreaks, in which soaring populations defoliate forests, leading to extensive economic damage. Outbreaks often come to an end naturally, when epidemics of viral diseases sweep through the population, causing massive mortality. Why do such diseases not simply keep the population in check permanently? What determines when epidemics begin? Answers to these questions would help us control pest insects without insecticides, and might help us understand other diseases as well. Our approach to finding the answers is to carry out experiments, using a viral disease of gypsy moths. The gypsy moth is an introduced pest of hardwood trees in the Northeastern U.S. and Canada, and it is spreading rapidly. The viral disease causes gypsy-moth larvae to literally melt onto foliage, spreading virus particles to be consumed by other hapless larvae. Our work focuses on three factors that may affect the spread of this disease. First, we are testing whether the insect evolves resistance over time, because individuals that survive the epidemic may be those that can better fight off infection. Second, we are testing whether transmission is affected by the pattern of virus on leaves, so that spread rates rise when the virus is distributed more uniformly. Third, we are testing whether chemical compounds in the leaves that the insects feed on may alter the chance that insects become infected. Each test consists of a set of experiments in which insects feed on virus-contaminated leaves in nature, except that we manipulate each factor in turn; the genetic background of the insect, the pattern of virus on the leaves, and the chemical composition of the leaves. We will then synthesize our results using computer models that translate the data into predictions of rates of disease spread.