The alfalfa weevil, Hypera postica, is an example of a highly successful case of the "classical" approach to biological control in the Midwest. The alfalfa weevil is known as a serious pest of alfalfa in the Midwest and throughout the U.S. The weevil damages alfalfa by chewing and skeletonizing the foliage. Both adults and larvae feed on the plant, but the larvae are by far the more injurious stage. The alfalfa weevil typically has just one generation per year, with larvae present during the spring, and thus the weevil is a pest mainly of the first alfalfa crop. However, following the first cutting, surviving larvae and adults can significantly delay alfalfa regrowth by feeding on the new growth of the second crop.
In the Midwest, alfalfa weevil adults emerge from pupae during late spring or early summer, feed for several weeks, and then spend the remainder of the summer in a quiescent state known as "aestivation." Aestivation is completed by late summer or fall, and the adults become active, feed and begin to lay eggs during the fall (Fig. 1). Fall oviposition is substantial in the southern parts of the region but is usually inconsequential in the north. Both eggs and adults are capable of overwintering.
|Figure 1. Alfalfa weevil seasonal
Figure courtesy of
Both the alfalfa plant and the alfalfa weevil are native to the Near East and Central Asia. Alfalfa was introduced into the Midwest (Minnesota) in the 1850's. Alfalfa weevil populations were accidentally introduced into the U.S. three times: the "western strain" of the alfalfa weevil into Utah in 1904, the "Egyptian" alfalfa weevil into Arizona in 1939, and the "eastern strain" of the alfalfa weevil into Maryland in 1951. (Egyptian alfalfa weevil is actually a different species, H. brunneipennis, but some authorities now think it and the eastern strain are more closely related than are the eastern and western strains.) It is primarily the eastern strain weevil that occurs in the Midwest, although western strain weevils can be found on the western fringe of the region. This report will focus on biological control efforts against the eastern strain.
As an exotic pest, the alfalfa weevil was considered a good candidate for importation biological control, and a program of introducing natural enemies against the eastern strain weevil was initiated by the Agricultural Research Service of the U.S. Department of Agriculture (USDA) in 1957. A number of parasitoid species (parasitic wasps) were purposely introduced from Europe and the Middle East and released in the eastern U.S., of which six became established. However, of these only one, Bathyplectes curculionis, was able to move effectively with the weevil as it spread westward from the eastern seaboard. A second alfalfa weevil biological control program was thus initiated in 1980, this time by the Animal and Plant Health Inspection Service of the USDA, to redistribute the established parasitoid species throughout the eastern U.S. This second phase lasted until 1988, and resulted in five parasitoid species becoming widely established in the Midwest (Table 1).
|Table 1. Alfalfa weevil natural enemies established in the Midwest|
|Name||Type||Weevil Stage Attacked/Killed|
|Anaphes luna||parasitic wasp||egg/egg|
|Tetrastichus incertus||parasitic wasp||larva/larva|
|Bathyplectes curculionis||parasitic wasp||larva/prepupa|
|Bathyplectes anurus||parasitic wasp||larva/prepupa|
|Erynia phytonomi||fungal pathogen||larva & pupa/larva & pupa|
|Microctonus colesi||parasitic wasp||larva/adult|
|Microctonus aethiopoides||parasitic wasp||adult/adult|
Included also in Table 1 are two natural enemies that occur in the Midwest but were not introduced through the USDA programs. Anaphes (=Patasson) luna, an egg parasitoid, was probably accidentally introduced into the eastern U.S. Erynia (= Zoophthora) phytonomi, a fungal pathogen that infects and kills alfalfa weevil larvae and pupae, is a more interesting story. This fungus first appeared in Ontario, Canada in 1973 and now commonly occurs with the weevil throughout the Midwest, but it is not known to occur outside North America. Although there are several theories about where the fungus might have come from, its origin remains a mystery.
Impact on Alfalfa Weevil Populations. A problem many classical biological control programs share is lack of follow-up analysis to rigorously assess the impact of newly established natural enemies on the pest and thus document the success or failure of the program. This has not been the case with alfalfa weevil biological control in the Midwest. Two excellent studies have provided information on and insight into the impact natural enemies are having on alfalfa weevil populations in the Midwest.
The first study was a series of alfalfa weevil life tables initiated in Ontario, Canada in the early 1970's. Although this study was not initially intended to document the impact of natural enemies on weevil populations, during the course of the study Erynia phytonomi appeared for the first time, and several parasitoids became established in Ontario as a result of the USDA programs. Thus, this study was able to examine alfalfa weevil population dynamics "before and after" the establishment of natural enemies. Among other things, analyses of the life tables demonstrated that E. phytonomi and Microctonus aethiopoides plus M. colesi were important regulating factors acting to suppress and stabilize weevil populations at subeconomic levels, and also that sensitivity of the fungal pathogen to environmental conditions (this pathogen, like most entomopathogenic fungi, needs a moist environment for spore germination and infection of host larvae) can lead to reduced effectiveness of the pathogen in dry years, which destabilizes the system and can lead to weevil outbreaks.
The second study was conducted specifically to assess the impact of the USDA parasitoid redistribution program on alfalfa weevil populations and the economics of alfalfa production. This analysis indicated that, by any measure, the program was a success. Following redistribution, parasitism of larvae (primarily by B. anurus) and adults (by M. aethiopoides) increased, alfalfa weevil population densities declined, and fewer alfalfa fields were sprayed with insecticide for alfalfa weevil control. Moreover, a detailed economic evaluation concluded that the benefit:cost ratio accruing from the biological control program was an almost unthinkable 91:1!
In addition to these studies, data on parasitism and disease in alfalfa weevil populations from various states in the region have indicated that in terms of magnitude or abundance, the most important alfalfa weevil natural enemies are the two Bathyplectes spp., E. phytonomi, and M. aethiopoides. Several years ago when I was preparing a presentation on biological control in alfalfa, I conducted an informal poll of alfalfa research and extension entomologists in the Midwest to determine their views on alfalfa weevil biological control, and I received the following responses.
Q: How do you rate biological control of alfalfa weevil in your state (complete, substantial, or partial)?
A: Complete (1), Substantial (5), Partial (2)
Q: Rank the following for their contribution to alfalfa weevil control in your state: Bathyplectes curculionis, B. anurus, Erynia phytonomi, Microctonus aethiopoides, and other natural enemies.
A: No. 1 rank: Microctonus (4), Erynia (3), and a 3-way tie between Erynia, Microctonus and B. anurus.
Clearly, the consensus view in the Midwest is that biological control is an important component in the management of the alfalfa weevil. Interestingly, the species perceived to be the dominant natural enemy tended to vary geographically, which is perhaps a reflection of climatic differences. Microctonus tended to be rated the most effective in the northern and eastern parts of the region, whereas Erynia tended to be rated most effective in the western and southern parts.
Finally, why has biological control of the alfalfa weevil proven to be such a success in the Midwest? I think attributes of the crop, the pest and the natural enemies have all contributed to this.
Crop: Alfalfa is a perennial and thus provides a relatively stable habitat for natural enemies. In addition, flowering weeds in the crop may provide a nectar source for parasitoid adults, and alfalfa fields are generally treated with insecticide infrequently if at all. Although frequent harvesting of the crop can be destabilizing, the successful natural enemies are not unusually harmed by this practice (see below).
Pest: Life history characteristics of the alfalfa weevil --narrow host range (essentially only alfalfa in North America), limited dispersal ability, and long generation time-- suggests that this species should be vulnerable to attack by specialist natural enemies and thus well suited for biological control.
Natural Enemies: The natural enemies that have proven to be important in alfalfa weevil biological control share two attributes that have been keys to their success. First, their seasonal histories are well synchronized with the alfalfa weevil seasonal history. Second, the successful natural enemies tend to be "resistant" to harmful effects of alfalfa harvesting.
- Dave Hogg, University of Wisconsin-Madison
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