Seeking Alfalfa Resistance to a Rhizophagous Pest, the Clover Root Curculio (Sitona hispidulus F.)

Since the cancellation of broad-spectrum soil-active insecticides in alfalfa (Medicago sativa L.) production, clover root curculio (Sitona hispidulus F.) (CRC) larval root damage has increased. Current CRC management practices are limited in their ability to suppress larval feeding belowground. First, we field screened developmental alfalfa populations for CRC damage. Subsequently, we developed a soil-less arena to observe nodule feeding and development (head capsule width) of larvae in the lab. This method was used to evaluate five alfalfa populations (two CRC-susceptible (control) and three CRC-resistant populations) against larvae. Further, one CRC-resistant population paired with its genetically similar susceptible population were tested against adult leaf consumption and oviposition in the greenhouse. Field screening revealed that the alfalfa populations selected for little or no larval root feeding damage were more resistant to CRC larval feeding than their corresponding unselected cultivars and significantly more resistant than populations selected for susceptibility. The development of a soil-less arena provided a useful method for evaluation of root-larva interactions. Although larval development was similar across susceptible and resistant alfalfa populations, one CRC-resistant population (NY1713) displayed overall increased nodulation and, thus, had a significantly lower proportion of nodules consumed by larvae. Adult feeding and oviposition aboveground were similar across all populations tested. These results provide possible candidates and screening method for the development and evaluation of alfalfa cultivars that may reduce the impacts of larval feeding and that offer an additional option for CRC management.

Biology and Management of Clover Root Curculio (Coleoptera: Curculionidae)

Clover root curculio (Sitona hispidulus (F.)) (CRC) has become an economically important pest of alfalfa since the regulatory phase-out of carbofuran, methyl bromide, and other synthetic soil fumigants. Native to temperate Europe, the Middle East, and Asia, CRC was introduced to North America in the early 1900s and is now widespread across the U.S. Adult foliar herbivory is not typically economically important. However, severe root damage by larvae can result in secondary plant pathogen infections, leading to losses in forage quality, yield, and stand life. Monitoring efforts include sweep netting adults and soil sampling around the crown and roots of plants for eggs and larvae. Relationships between CRC densities and root damage as well as economic thresholds have yet to be established. Although several pyrethroids have been registered for use against CRC adults, they are not recommended due to potential non-target effects. Chemical controls have not been registered for use against the cryptic larval stage. This review discusses CRC identification, life history, biology, and ecology. Potential ecological management strategies (e.g., crop rotation, planting dates, resistant host varieties), biological control agents, and insecticide options are also described as key components of an integrated pest management program against CRC in U.S. alfalfa.

MULTIVARIATE ANALYSIS OF THE POPULATION STRUCTURE OF SITONA HISPIDULUS (COLEOPTERA: CURCULIONIDAE) IN ALFALFA FIELD SOIL

The multivariate statistical techniques of canonical correlation and canonical redundancy analyses were used to assess the population structure of larvae of Sitona hispidulus (F.) in alfalfa field soil. A series of rhizosphere variables was correlated with a series of insect variables that represented the developmental stages of the insect to identify relationships between the insect and rhizosphere. Results indicated that 1st and 2nd-instar larvae were correlated with small root nodules and soil moisture, but not with taproot biomass. Third- and 4th-instar larvae and pupae were not correlated with any of the rhizosphere components measured. Fifth-instar larvae were associated with taproot biomass.