Calag Archive

The five articles included here summarize research to date on the Egyptian alfalfa weevil, Hypera brunneipennis, which poses a major threat to most of the state's 1,140,000 acres of alfalfa, and has already caused losses to growers of more than $6 million on a total alfalfa-hay crop valued at $197,000,000 (in 1970). H. brunnelpennis is not to be confused with its more widely known relative, H. postica, which has been an economically important weevil pest for many years, but has been reduced to minor status in some areas through biological control. The Egyptian alfalfa weevil arrived accidentally through the southeastern corner of California in the late 1930's, and became universally distributed over the south coastal plain. It spread slowly, but in recent years has been identified in many of the lowland alfalfa growing valleys of central and northern California…and is expected to expand over the entire Central Valley. As the studies included here indicate, much further research is necessary before an integrated program involving biological control, resistant alfalfa varieties, cultural control and effective chemical control (or combinations of all) can become practical.

Losses attributable to H. brunneipennis are increasing for a complex of reasons, perhaps the most important of which stem from a lack of detailed information on this species. While many studies have been made on the biology of the species H. postica in California and other states, there is relatively little basic data on the field ecology of H. brunneipennis, which differs significantly from that of H. postica.

Introduction of the larval parasite Bathyplectes curculionis (Thomson) into the Yuma Valley was one of the first counter measures taken against the Egyptian alfalfa weevil (Hypera brunneipennis (Boheman)) following its discovery in the U.S.A. The colonization material was obtained from Utah's Salt Lake Valley where the parasite had been successfully introduced from Southern Europe against the alfalfa weevil (Hypera postica (Gyllenhal)) early in this century.

THE USE OF ALFALFA VARIETIES resistant to the alfalfa weevil (Hypera brunneipennis (Boh.)) offers a sure, inexpensive means of controlling this pest. Results of work conducted at the University of California and elsewhere show that development of varieties resistant to the alfalfa weevil is possible. However, unlike resistance to the spotted alfalfa aphid, where wide differences in reactions were found among plants tested, only relatively small differences in the levels of resistance have been found among plants tested for the alfalfa weevil. These small increments in the levels of resistance will probably have to be combined into a higher level through generations of cross breeding and selection. The number of generations necessary is not known, but a lower number may be possible if use of such varieties is combined with an adequate level of biological control and good crop management.

A research project in southern California originally aimed at seeking resistance to sugar cane mosaic virus in sweet corn has also shown that genetic resistance to air pollution also exists in certain varieties. This was not entirely unexpected, since air pollution resistance is known to exist within other plant species. Tobacco varieties, for example, vary widely in susceptibility to ozone; and petunia varieties vary widely in susceptibility to PAN (a photo-chemical pollutant). Smog-resistant varieties of sweet corn should be of particular interest to growers for the summer and fall harvest periods in Los Angeles, Orange, and western Riverside counties.

Powdery mildew of rose, caused by the fungus, Sphaerotheca pannosa, results in unsightly leaves and flowers and may cause reduced growth. Recently, several new systemic and non-systemic fungicides have become available. These studies were initiated to evaluate these new materials for powdery mildew control in southern California.

The two articles included here are from separate experiments. The first concerns the influence of walnut aphids on production and quality of Payne walnuts, on trees whitewashed in the normal practice for prevention of sunburn. This trial showed a marked loss of both production and quality resulting from aphid infestations. In the second trial, the walnuts were not whitewashed and were subjected to varying degrees of aphid infestation. The resulting evidence showed that most of the sunburn problem was aphid-related. If aphids were controlled, sunburn was reduced to a non-economic level. In orchards where codling moth was not a problem, a recently introduced parasite provided adequate aphid control.

Trials conducted in Tulare County (reported in accompanying article) have demonstrated that the walnut aphid has a profound influence on walnut production and quality. Observations on infested trees during 1969 showed that honeydew accumulations on developing nuts had a phytotoxic effect on husk tissue which resulted in killing surface cells (photo 1). These turn black and together with subsequent sooty mold aggravate sunburn, since dark surfaces have the capacity to absorb more heat. It was also noted that sooty mold developed through the whitewash deposits. This situation of dark husk surfaces due to honeydew (photo 2) compounded by sooty mold accumulation (photo 3), was studied in relation to the sunburn problem of walnuts grown in interior valleys.

UNTIL NEW OR IMPROVED CONTROL procedures can be found, growers of alfalfa in California have little choice but to depend on chemical insecticides for control of both the alfalfa weevil, Hypera postica (Gyllenhal), and the Egyptian alfalfa weevil, H. brunneipennis (Boheman), where these species occur in damaging numbers.