Insecticide For Corn Rootworm

1. Introduction

The western corn rootworm, Diabrotica virgifera virgifera LeConte (Dvv) is a galerucerine Chrysomelid beetle (Figure 1A) that is one of the most significant insect pests of maize (Zea mays L.) in the United States (U.S.). Annually, this species is responsible for over $1 billion in control costs and yield losses [1,2]. Similar to other Diabrotica species, the larvae are root feeders and adults feed on above-ground plant tissues [3,4]. Dvv larvae survive only on a small number of grass species [3,4,5,6,7,8] while adults feed primarily on pollen and reproductive tissues of a variety of plants [9,10]. Maize is the primary Dvv host in modern agroecosystems [10,11] and is strongly attractive to wild-type adult Dvv [12,13,14]. The initial Dvv species description was made in 1868 from collections made in what today is Wallace Co., KS [15] but phylogenetic research points toward a species origin in Mexico or Central America [16,17]. It has been hypothesized that Dvv may have survived at low densities on native grasses such as western wheatgrass, Pascopyrum smithii (Rydb.) at the time of initial discovery [7]. The pheromone of Dvv is very efficient at ultra-low levels, which supports the low Dvv density hypothesis before adaptation to maize monocultures [11]. It is unclear when Dvv arrived in what today is the southwestern U.S. but records document that Dvv did not occur east of western Kansas, Colorado, and southwestern Nebraska, U.S. prior to the 1920s [18].

Dvv was first recorded feeding on maize roots at Ft. Collins, Co. in 1909 [19]. Annual rotation from maize to a crop that would not support Dvv larval survival was the recommended Dvv management tactic as early as 1930 in southwestern Nebraska [20,21] but the profitability of maize led some growers to start planting continuous maize (maize planted for ≥2 years in one location). As agriculture developed in western areas of Nebraska during the 1930s sporadic reports of larval Dvv injury were reported in continuous maize [20,21,22]. By the 1940s, Dvv injury to continuous maize (Figure 1B) in central Nebraska was common and was facilitated in part by the introduction of irrigation systems and synthetic fertilizer. This agricultural system became very profitable and helped meet the demand for maize from a growing confined livestock industry. Continuous maize provided optimal conditions for the build-up of Dvv densities, increasing larval injury and greater adult dispersal from infested fields. The large monocultures of continuous maize may have been the bridge needed to jump-start the fairly rapid range expansion that progressed through the U.S. Corn Belt reaching NJ, USA by the 1980’s.

The emergence of Dvv as a major insect pest coincided with the post-World War II emergence of agrochemical companies that synthesized and manufactured synthetic organic insecticides. The need for Dvv control in continuous maize created a niche that developed into a large insecticide market in the U.S. Corn Belt. Evaluation of insecticide efficacy targeting larvae (soil insecticides) or adults (foliar applications) was a major focus of both industry and academia in the 1940s–1990s [25,29,30,31,32,33,34,35,36,37,38]. Seed treatments and transgenic plants that express proteins that are toxic to Dvv (plant incorporated protectants) were introduced and widely adopted in the 2000s, which reduced the role of insecticides as management tactics in continuous maize [26,39,40,41]. This paper reviews the history of insecticide use in Dvv management programs, Dvv adaption to insecticides (Figure 2), i.e., field-evolved resistance and associated mechanisms of resistance, plus the current role of insecticides in the transgenic era.