bug 指小昆虫，尤指吸血的臭虫，也可指细菌。
insect 含义广泛，多指蚂蚁、黄蜂、蝉等等。
pest 指有害的昆虫或有害的小动物。
worm 侧重指蚯蚓、蚕、毛虫等小而无骨的虫。
参考例句
1. Pest control is one of the major areas of concern in intensive agriculture. 病虫害防治是集约型农业所关心的主要方面之一。
2. The first genetically modified animal to leave the lab will probably not be one designed to control a pest. 第一只离开实验室进入野外的转基因动物也许不是设计好去控制害虫的。
3. The creatures, which normally prey on pest insects, had been altered in a first step toward making them more effective against pests. 那些正常捕食害虫的动物，首先是改变基因使其朝更有效的捕食害虫能力方面发展了。
英英解释
n.
1. a serious (sometimes fatal) infection of rodents caused by Yersinia pestis and accidentally transmit
2. any epidemic disease with a high death rate
3. a persistently annoying person
4. any unwanted and destructive insect or other animal that attacks food or crops or livestock etc.
