Diurnal periodicity of flight by insects

From the standpoint o f control o f virus diseases, there is perhaps no phase of virology more important than epidemiology The role of weeds in the occurrence and spread o f plant virus diseases is an integral part o f the ecological aspect o f virus transmission L H Bailey (3) has commented that nature knows no plants as weed s Indeed, at the dawn of agriculture some 8,000 years ago, man would have recognized all plants as natural components o f the flora Man altered the earth's floral cover in his first efforts in agriculture, and weeds were born King (61) has pointed out that perhaps the shortest definition of a weed is that attributed to Professor W J Beal, who defined a weed as "a plant out of place" King has summarized some thirty definitions into ten commonly accepted characteristics of a weed: growth in an undesired location; com­ petitive and aggressive habits; wild and rank growth; resistant to control; large p opulations, with abundant, rank and extensive growth; useless, un­ wanted, and undesirable; harmful to man, animals, and crops; spontaneous growth, appearance without being sown or cultivated; of high reproductive capacity, and unsightly with disfigurement of the landscape Thus, weeds are plants, with harmful or objectionable characteristics, that grow where they are unwanted Whether a plant is considered a weed depends not only on its characteristics but also on its relative position with reference to other plants and to man The same plant may be a weed in one environment and an economically important plant in another Thus, in at least one incident, sugar beets were considered to be noxious weeds during certain times of the year in legislation proposed to enforce a beet-free period Numerous reports (5, 14, 32, 118) have indicated a close correlation be­ tween incidence o f beet yellows virus and overwintering beets In Europe, the sources of beet yellows virus include beet seed crops and infected man­ golds in clamps; in the United States, they include escaped beets growing in waste places and beets in overwintering fields The beet mosaic virus (32, 118) has been shown to be even more closely associated with overwintering

Role of Weeds in the Incidence of Virus Diseases

In recent years it has become increasingly apparent that advances in meth­ ods of crop protection have not always kept pace with the progress being made in other aspects of crop production. Losses continue from many well-known diseases or they have become increasingly difficult and expen­ sive to control. Moreover, new problems have been encountered and minor ones have increased in importance to such an extent as to impede progress and decrease the benefits of adopting modern high-yielding varieties and other recent innovations. Pathologists have responded to this situation in different ways. One of the main developments has been the increasing interest in the underlying causes of major epidemics (12a, 62). There has been much emphasis on the fre­ quency of epidemics in crops compared with natural vegetation and this has led scientists in various disciplines to the view that disease epidemics are largely the result of some sort of human interference in "the balance of nature." For example, J. R. Harlan (22) commented: "Man is the direct or indirect cause of most (possibly aU) of the epidemic imbalances we know about" and that "The high virulence and extreme susceptibility of an epi­ demic situation is an unnatural imbalance usually brought about by human disturbance." Similar views have been expressed by ecologists (26): "Epi­ demic diseases . . . are not seen in natural vegetation-except after some major disturbance" and by epidemiologists (75): "Strong epidemics, as far as is known have always followed some identifiable human activity. Epi­ demics are largely man-induced." The concept of man as the direct cause of disease epidemics merits careful consideration, as it heralds a new approach to crop protection and necessi-

Cropping Practices and Virus Spread

The expanding field of plant virus ecology: historical foundations, knowledge gaps, and research directions.

Publisher Summary The British fauna of Hemiptera includes eight families and about 358 species of Homoptera, Auchenorrhyncha. The largest families in Great Britain are Cicadellidae and Delphacidae, here referred to as “leafhoppers.” Relatively little is known about the biology and population ecology of the British grassland leafhoppers and this chapter discusses recent studies made on this neglected group of insects. In numbers of individuals and species, leafhoppers form a most important component of the phytophagus fauna of temperate grasslands. A number of leafhopper species are vectors of plant diseases. Auchenorrhyncha can be serious pests of pasture lands and crops and may cause direct damage to the growing crops by their sucking mouth parts—for example, Saccharosydne saccharivora on sugarcane, Nilaparvata lugens on rice—and by transmitting viruses and mycoplasma to crops. The chapter compares Auchenorrhyncha fauna in acidic grassland with that on calcareous soils.

Studies on grassland leafhoppers (Auchenorrhyncha, Homoptera) and their natural enemies.

Dissemination of the Beet Leafhopper in California

Types of Vegetation in the San Joaquin Valley of California and Their Relation to the Beet Leafhopper

Weeds, Insects, Plant Diseases, and Dust Storms

Describes the results of a scientific study on the effect of summer weeds on crops (mainly sugar beets and tomatoes) and the use of the beet leafhopper to reduce their growth in the San Joaquin Valley in California.

Francis R. Lawson

Papers

Dissemination of the Beet Leafhopper in California

Types of Vegetation in the San Joaquin Valley of California and Their Relation to the Beet Leafhopper

Weeds, Insects, Plant Diseases, and Dust Storms

The Ecology of the Principal Summer Weed Hosts of the Beet Leafhopper in the San Joaquin Valley, California