Under natural conditions in California, there are two generations a year for Aphrophora permutata Uhler and Clastoptera brunnea Ball, and one generation for six varieties of Philaenus leucophthalmus (Linnaeus). The molting of last-instar nymphs of A. permutata is described in detail. All of these spittle insects transmitted the virus of Pierce’s disease of grapevines from diseased to healthy vines; their efficiencies (in single-insect tests) varied from 12 per cent for Clastoptera brunnea to 65 per cent for Philaenus leucophthalmus var. leucophthalmus. P. leucophthalmus and the two species of Aphrophora occasionally transmitted the virus from diseased grapevines to healthy alfalfa, and from alfalfa plants infected with alfalfa dwarf to healthy grapevines, but C. brunnea did not; and only P. leucophthalmus transmitted the virus from healthy to diseased alfalfa. Two varieties of P. leucophthalmus were the only ones among these insects found to be naturally infected. Grapevines proved to be an unfavorable food plant for all except P. leucophthalmus, and alfalfa was unfavorable for Clastoptera brunnea. None of these vectors have been collected on grapevines, nor in alfalfa fields in California, though P. leucophthalmus was once collected on volunteer alfalfa growing among weeds. Hence none of these insects appear to be of economic importance in spreading the virus to grapevines or alfalfa under natural conditions in California, except that P. leucophthalmus may possibly infect perennials, such as herbs, which may serve as reservoirs of the virus. The minimum latent period of the virus in adults of four varieties of P. leucophthalmus ranged from 2 to 7 hours. The virus was retained by P. leucophthalmus from 29 to 76 days.
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Henry H. P. Severin (1950) studied this question.