A mysterious malady that has been killing honeybees en masse for several years appears to have expanded drastically in the last year, commercial beekeepers say, wiping out 40 percent or even 50 percent of the hives needed to pollinate many of the nation's fruits and vegetables. Many beekeepers suspect the biggest culprit is the growing soup of pesticides, fungicides and herbicides that are used to control pests. Beekeepers and some researchers say there is growing evidence that a powerful new class of pesticides known as neonicotinoids, incorporated into the plants themselves, could be an important factor. The explosive growth of neonicotinoids since 2005 has roughly tracked rising bee deaths. Neonics, as farmers call them, are ... systemic pesticides, often embedded in seeds so that the plant itself carries the chemical that kills insects that feed on it. Neonicotinoids persist for weeks and even months. A coalition of beekeepers and environmental and consumer groups sued the E.P.A. last week, saying it exceeded its authority by conditionally approving some neonicotinoids. The European Union has proposed to ban their use on crops frequented by bees. Some researchers have concluded that neonicotinoids caused extensive die-offs in Germany and France. Neonicotinoids are hardly the beekeepers' only concern. Herbicide use has grown as farmers have adopted crop varieties, from corn to sunflowers, that are genetically modified to survive spraying with weedkillers.