Distribution and establishment of introduced parasitoids of the alfalfa weevil in Tennessee

The alfalfa weevil, Hypera postica (Gyllenhal), is the most important pest of alfalfa, Medicago sativa L., in North America since its introduction into Utah in 1904. By the mid-1950s, the alfalfa weevil had spread into the northeastern United States. In Tennessee, the alfalfa weevil was first detected in 1959, and within five years, the weevil had spread throughout the entire state.

In 1994, a two-year research project was initiated to study the alfalfa weevil and its biological control agents. One objective of this research was to initiate an intensive study to determine seasonality and life history of the alfalfa weevil in Tennessee. Additional objectives of this research included monitoring the establishment and distribution of introduced parasitoids of the alfalfa weevil, determining parasitism levels of the alfalfa weevil, and monitoring disease incidence of the alfalfa weevil. A state wide survey was established to monitor the alfalfa weevil and its biological control agents in 27 alfalfa fields in nine counties. A questionnaire was developed and distributed to all agricultural extension agents in Tennessee to evaluate alfalfa production and the status of the alfalfa weevil.

Alfalfa fields in the intensified study were sampled each week from January to December in Giles and McMinn Counties. Alfalfa fields in the state-wide study were sampled twice in 1994 and twice in 1995. In Tennessee, alfalfa weevils overwintered in both the egg and adult stages. Densities of alfalfa weevil adults and larvae peaked in April during 1994 and 1995.

During this two-year study, approximately 11.39% of all field-collected larvae (n=6,215) and only about 0.43% of all field-collected adults (n=460) were parasitized. Bathyplectes anurus (Thomson) and Bathyplectes curculionis (Thomson), parasitoids of alfalfa weevil larvae, were recovered in all counties sampled. Bathyplectes anurus was the predominant larval parasitoid, suggesting B. anurus has displaced B. curculionis in Tennessee. Percent parasitism of alfalfa weevil larvae by B. anurus was 7.12% (n=6,215), and 4.12% (n=6,215) by B. curculionis. State-wide, Microctonus aethiopoides (Loan) was recovered in low numbers, suggesting poor establishment of alfalfa weevil adult parasitoids in Tennessee.

Erynia sp., a fungal pathogen of alfalfa weevil larvae, was detected from early March to late April. The number of field-collected alfalfa weevil larvae infected by Erynia sp. was greater in 1994 [436 infected larvae (n=2,439)] than in 1995 [45 infected larvae (n=3,320)].

As a result of the alfalfa management questionnaire, agricultural extension agents provided solid information concerning alfalfa production and the impact of the alfalfa weevil in Tennessee. The alfalfa weevil is responsible for 15 % reduction in alfalfa yield, and 78% of the extension agents listed the alfalfa weevil as the major pest of alfalfa. The number one limiting factor of alfalfa production in Tennessee is expenses. Much of these expenses are attributed to insecticide applications for the alfalfa weevil. Growers in 96% of the alfalfa-producing counties applied insecticides to control the alfalfa weevil.

The absence of a successful adult weevil parasitoid allows alfalfa weevils to produce at much greater numbers, and larval parasitoids will not sufficiently reduce alfalfa weevil densities below the economic threshold level. Parasitism of alfalfa weevil adults is greater in northern states than in southern states, and the low parasitism of alfalfa weevil adults in Tennessee indicates that M. aethiopoides may not be adapted to the southern climate. Therefore, data from this research will provide information needed to develop and implement effective biological control methods for the alfalfa weevil in Tennessee.