Showing papers in "Annual Review of Entomology in 1961"

Principles of Insect Cold-Hardiness

Abstract: Cold is a relative term which for present purposes will be considered to encompass temperatures too low to support normal development of the in­ sects concerned. The effects of cold on insects may be manifold, but a pri­ mary distinction must be made on the hasis of whether freezing does or does not result. The dividing line, however, is seldom at the freezing point of the tissues but rather at the supercooling point, for insects almost invariably supercool to some extent, usually many degrees. Cooling without freezing has been termed "chilling" (9), particularly when the emphasis is on temperatures above O°C. Such investigations usually deal with i nsects that do not normally encounter cold, e.g., household pests. Unaccustomed to cold, they tolerate it poorly. On the other hand, insects that hibernate in temperate and colder climates must tolerate a wide range of low temperatures, often for long periods. Some species improve their toler­ ance through processes of cold-acclimation and cold-hardening. All insects supercool to some extent, regardless of need; in fact, some that need to do so least supercool below -40°C. Most hibernating insects do need to supercool, however, to avoid freezing, which would be fatal; the remainder have not this need because freezing is not injurious to them. The former group are called freezing-susceptible and the latter freezing-resistant or freezing-tolerant (34). The principles of insect cold-hardiness will be discussed under three main headings : (a) chilling and cold-acclimation, (b) avoidance of freezing by supercooling, and (c) freezing-tolerance.

The Theoretical and Practical Study of Natural Insect Populations

Abstract: The first five volumes of the Annual Review of Entomology contained four articles on essentially the same topic as is discussed below. In the same period, many other papers have been published elsewhere on this subject. The authors are far from reaching agreement on general principles, let alone conclusions. It may be that some authors are more interested in proving their opponents wrong than in providing evidence for alternative theories. But fundamentally, apart from the fact that the subject is a very difficult one, disagreement probably arises from the absence of sufficient evidence of the right kind. Laboratory experiments are, in general, inconclusive, since the detailed methods of culturing insects are an all-important part of the population problem and determine the nature of the results obtained. This is not to say that special problems cannot be usefully studied in the labora­ tory; it is possible, for instance, to show that predatory mites alone are cap­ able, under some conditions, of controlling the fruit tree red spider, Meta­ tetranychus ulmi (Koch), a fact that might take many years to establish under field conditions [Collyer (8)]. Most population theories, so far as they are not purely inductive, are based on imperfect field data that are not derived from planned population studies in which all the relevant factors were measured simultaneously. In 20 or 30 years time, when perhaps more of such fundamental studies will be available, we may be able to discuss our theories with more light and less heat. The present article is a review of the difficulties and an attempt to show how some of them can be solved and is not a review of the whole literature. One of the difficulties in the theoretical discussions of insect population dynamics has been the lack of agreement about the meanings of terms. At least four terms must be agreed on, and I shall give my own definitions or explanations without insisting that they are the only possible ones, though I believe they may be the most convenient. The term \"population\" means all those individuals of one species whose lives are sufficiently integrated to have an influence on one another. A population will normally occupy a spatially fairly well-defined area even though the distribution of a species is hardly ever strictly continuous. Al­ though populations so defined will often tend to have somewhat woolly edges, it is only to such populations that one can correctly attribute such features as birth rates and death rates.

Ecological aspects of plant virus transmissions.

Abstract: The movement of insects from one host plant to another is the common­ place activity upon which all transmission of plant virus diseases by arth ro­ pods depends. This movement is conditioned' by many factors, the normal life-hi story of the in sect, its host range and host preferences, the availability and condit ion of these hosts, and their status as virus reservoirs. Superim­ posing their determi nativ e influence on all these biotic factors are the physi­ cal factors of the environment. This discussion is concerned with the int er­ relationships of these factors. MOVEMENT, DISPERSAL, AND MIGRATION

Arthropod vectors as reservoirs of microbial disease agents.

Abstract: The subject of arthropod vectors as reservoirs of agents of animal diseases is difficult to review because of divergent viewpoints of the definition of a "reservoir." Some investigators would reserve this term for vertebrate hosts and the term "vector" for intermediate hosts (141). Others would consider vectors to be reservoirs if they participate in an essential phase of the disease cycle, or show cyclic propagation or transovarial transmission of the patho­ gen [a comprehensive, tabular review of ticks in this role is supplied by Neitz (126)]. Other factors also complicate precise definition of the word "reservoir." These include: (a) length of life and types of metamorphosis of the arthropods; (b) method of transmission by biting or by contamination from fresh or old feces; and (c) "premunition" (latent persistence of organ­ isms after initial infection) in the vertebrate host and the too often specula­ tive role this plays in providing a source of infection for ectoparasites. In this review, the reservoir mechanism is envisioned as a flexible bio­ logical adjustment of the vertebrate and arthropod hosts to pathogens, in which the relative importance of either host as a reservoir varies in different diseases. Space permits only restricted selection of evidence that emphasizes the role of arthropods beyond that of simply being vectors (e.g., yellow fever virus and mosquitoes). The terms "transovarial" (or "hereditary") and "transstadial" refer to passage of disease agents through ova and between active stages of a given vector, respectively.