University of California

The apparent climatic limitations of the alfalfa weevil in California


A. E. Michelbacher
John Leighly

Authors Affiliations

A. E. Michelbacher was Junior Entomologist in the Experiment Station, Division of Entomology and Parasitology, Berkeley, California; John Leighly was Associate Professor of Geography, University of California, Berkeley.

Publication Information

Hilgardia 13(3):101-139. DOI:10.3733/hilg.v13n03p101. April 1940.

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The alfalfa weevil, Hypera postica Gyll.,4 was first discovered in lowland middle California in 1932. The extent and direction of its spread from its original areas are shown on the map in figure 1. The expansion of the area of infestation has been slight in view of the rapidity of spread of the insect that has been reported from other parts of the United States. Along some parts of the periphery of the area of infestation a slow rate of spread may possibly be the result of absence of suitable and abundant host plants. But in one sector of the periphery of the infested area—in the northwestern San Joaquin Valley—the boundary cuts across a continuous area devoted to the cultivation of alfalfa, where any insect preying on alfalfa might be expected to be disseminated rapidly and continuously from field to field. As may be seen from figure 1, no such spread has been observed. There appears to be no good reason why the southward spread of the insect is checked here unless it encounters a climatic barrier. Southward in the San Joaquin Valley summer temperatures become steadily higher; and all investigations of the alfalfa weevil indicate that high temperatures check the activity of the adult weevil and eventually inhibit its activity altogether.

The existence of an apparent climatic limit to the southward spread of the weevil in the San Joaquin Valley, and the reasonable conclusion that the climatic barrier encountered is high summer temperature, are the considerations that have prompted the investigation reported in this paper. Since the weevil is of rather recent introduction in California, it appeared that some light might be thrown on this question by a study of the temperatures obtaining in its original habitat in the Old World, with particular attention to the southern limit of its distribution there.

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Michelbacher A, Leighly J. 1940. The apparent climatic limitations of the alfalfa weevil in California. Hilgardia 13(3):101-139. DOI:10.3733/hilg.v13n03p101

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