Resistance gene frequency

The basic concept of resistance management is that resistance gene frequency in a given pest population can be prevented from rising by not selecting so rigorously with insecticides. There may be a number of ways to achieve such an objective ... 

The ease and reliability of monitoring dicofol resistance makes this resistance especially conducive for experimentation that involves tracking changes in resistance gene frequency in populations. 

The difficulty in relating resistance gene frequencies, the presence of resistant phenotypes and the level of resistance to field control indicates a need for monitoring techniques that sample significant numbers of a population and more accurately reflect the field situation (23-24). For example, in many cases, the standard topical assay may not adequately reflect the situation in the field. The efficacy of the pyrethroids seems to depend on many factors other than just contact activity. In cases where topical assays indicate high levels of resistance but no corresponding field failures, contact activity may not be the only factor affecting susceptibility. The repellency and antifeedant attributes of the pyrethroids may also contribute to their efficacy in the field (25-31). These behavioral attributes may also affect the evolution of resistance in field populations (22). The type of exposure in the field to the pyrethroids may also affect field control more than previously considered (33). 

The development of drug resistance is a phenomenon in which a resistance gene potentially present in a pest at low frequency is selected by exposure to a drug and... 

The rate of evolution of resistant weeds is based on several factors, including characteristics of the weed and herbicide, gene frequency, size and viability of the soil seedbank, weed fitness, herbicide potency, frequency and rate of application, and persistence in soil. Various attempts have been made to use modeling to determine the relative importance of these factors and to predict the probability of resistance, as well as to evaluate how to avoid, delay, or solve the problem (Gressel and Segel, 1990). 

Bacteria can obtain the various types of resistance mechanisms described previously by undergoing modifications in their genetic constitution. Many bacteria simply inherit their resistance genes from their forerunners. In addition, genetic mutations can occur that can confer a new trait. For example, it has been estimated that bacteria undergo spontaneous mutation at a frequency of approximately 1 in 10 cells. These mutations can confer resistant traits to the subsequent progeny. Mutations are believed... 

Resistance to insecticides in insects and mites is preadaptive i.e., the genes controlling the resistance mechanism are already present in very low frequencies in the population prior to any use of insecticides. Therefore, the resistant genes are mutants from some earlier era in the insect s history, or were selected by other toxicants in its earlier environment, and... 

The frequency of resistance genes in an unselected population is generally unknown, and there is no efficient way of screening large populations (at least 1 000 000 plants, perhaps much higher) to determine such gene frequencies. Other important information on... 

Few studies have been undertaken of the energetic cost of resistance, the resistance mechanism is often not specified in these studies, and the parameters chosen for measurements vary between studies. Comprehensive information is therefore not available. Still, this kind of information is important for understanding the evolution of resistance and its stability. Most new insecticides, it appears, have a mode of action identical to or extremely similar to one that was used previously. The successful use of new insecticides therefore depends on what kinds and frequencies of resistance genes are already present in insect populations to be controlled. It can be expected that resistance mechanisms that affect fitness will be less stable than those that do not. There is, thus, a need for more studies of the relationships between resistance mechanisms and biotic potential in insects. 

To estimate the relative fitness of the genotypes, dicofol resistance was modelled as a monogenic, recessive trait (22) and it was assumed that any fitness cost associated with the resistance gene would also be recessive. Allelic frequencies for the resistance alleles, R and S, are denoted by q and p, respectively. 

The relationship between monitoring results and field control is still not clearly understood. The presence of resistance genes in a field population does not necessarily lead to field control failures. The frequency of resistance genes in a field population, the size of the pest population and the level of resistance all interact and must be sufficiently high to result in the occurrence of resistance-induced control failures. Conversely, a field control failure does not necessarily imply insecticide resistance. Numerous biological and operational factors interact to affect the evolution of resistance and the level of field control achieved by a pesticide (22.). ... 

Australia. Pyrethroid resistance in Heliothis spp. was first documented in Australia during the 1982-1983 production season, when field control failures occurred against Heliothis armigera (2). Daly and Murray (341, in a thorough evaluation of hypotheses proposed on the evolution of resistance in Australia, concluded that an increase in the frequency of resistance genes in the Emerald area was a result of interactions between high selection pressure, population density, crop phenology and weather, but the evolution of resistance elsewhere was independent of the situation in Emerald. 

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