Pest density

Diseases, insects, and weeds are important constrains to crop production their combined effect has been "questimated" at 25-45%, on a world-wide basis. Pesticides comprise one of the major means of pests. In the U.S., insecticides, herbicides, or fungicides are used on more than 90 million hectares of crop land. Percentages of crop area treated with pesticides range from <1 to >90% for crops ranging from pasture to apples. The amount of chemicals used and the seriousness of plant pests mandate that pesticide usage be based on sound biological data and principles. Pest control decisions ideally are based on 1) precise estimates of pest density and... 

At a much smaller scale, pest density can vary tremendously within a given region. This is particularly true with soil-borne fungi and arthropods. Two adjacent fields may have densities that vary by several orders of magnitude. With pests that move in the air, there will be less aggregation than this, but differences can still be large. Unfortunately, it is still not a wide-spread practice to assess pest densities in fields in order to properly determine the need to use a pesticide. There are obvious exceptions, however, which fall under the concept of Integrated Pest... 

An implicit assumption of IPM is that pesticides should only be used when necessary. Absence, or anticipated absence, of a given pest is a situation in which a pesticide application is not necessary. Even when a pest is present, control decisions can be made on the anticipated pest increase and the relationship between pest numbers and yield (see sections below). In practice, trained scouts can sample a given field by counting, measuring, or assessing pest density at selected locations. Growers themselves can also make the assessment. 

The combined effects of plant pests will result in measurable direct primary loss if a threshold pest density is surpassed. Below this level (injury or disease damage level), crops theoretically are capable of "compensating" for the injurious effects of the pests. 

Such thresholds, however, are difficult to determine experimentally. For instance, at low pest density (where the threshold is likely to be) there is great within-field variation and pest aggregation. 

Above the threshold, whether precisely known or not, there is a proportionate reduction in yield with increases in pest densities. Eventually, a minimum yield could be reached in which further increases in pests do not produce additional yield losses. More complicated aspects of the yield/pest relationship are discussed by Teng (16). Ideally, one attempt to maintain pest density below the threshold level, provided that the threshold exists and is known. 

In essence, age-structured populations of this insect are reared continuously in cotton-field simulation chambers, and plants are sprayed from overhead at threshold pest densities with available or candidate insecticides applied either singly, together or sequentially at various application rates. The effects of these treatments are assessed by accurately monitoring changes in adult numbers over several generations without interfering with insects or plants (29). 

Extension of the research into field trials is necessary to test the products under more realistic pest densities and biting rates. As highlighted in previous studies, testing repellents against Aedes aegypti, while considered a recommended methodology for botanical repellents, " may underestimate the repellent activity provided by these products. 

Every continent, except Antarctica, grows com 40% of the present world crop is produced in the United States. In the 1987—1988 crop year, 12 states (Iowa, HI., Nebr., Minn., Ind., Ohio, Wis., Mo., S. Dak., Mich., Kans., and Tex. in order of production) produced 157.5 million metric tons (6.2 biUion bushels) that was 88% of the United States and 36% of the world s crop (66). Yield is influenced by many factors, including climate, pest control, planting density, and fertilization. Yield in the United States has increased from about 1.5 metric tons /hectare in the 1930s to about 7.5 metric tons /hectare. In 1985, a test plot produced 23.2 metric tons /hectare and yields approaching 40 metric tons /hectare are considered possible com is the most productive of the principal food crops. 

New cocoa hybrids and selections have been developed in Malaysia and other countries that produce significantly higher yields in select soil and climate conditions. In addition, high density plantings have demonstrated higher and earlier yield in Malaysia and the Philippines. Low or no shade cocoa has also proven to increase yields. However, both high density and reduced shade cocoa production requires additional inputs of management and nutrition. Additional inputs to control pests and diseases also maybe required. 

The development of criteria for sound economic thresholds in order to determine when-to-treat is highly complex. Establishing the guidelines for an effective economic threshold for a single pest must take into account the following interacting factors 1) density of the pest, 2) densities of its parasites and predators,... 

Crop Rotations and Diversification. For thousands of years farmers have known that planting their crops in a new location about every 2 years helped reduce Insect, disease, and weed problems. Indeed, crop rotations are effective in reducing many pest problems and are often highly cost effective (15, 16). Unfortunately for many crops, the trend has been toward abandonment of rotations and increased monocultures. Where rotations are not practiced, certain pests tend to multiply as the crops are cultured on the same land year after year (23). As a result, the density of pest populations increases to levels that necessitate heavy pesticide applications. 

PEST. This code ( 3) was developed within the framework of Rensselaer Polytechnic Institute s CLEAN (Comprehensive Lake Ecosystem Analyzer) model. It includes highly elaborated algorithms for biological phenomena, as described in this volume (44). For example, biotransformation is represented via second-order equations in bacterial population density (Equation 5) in the other codes described in this section PEST adds to this effects of pH and dissolved oxygen on bacterial activity, plus equations for metabolism in higher organisms. PEST allows for up to 16 compartments (plants, animals, etc.), but does not include any spatially resolved computations or transport processes other than volatilization. 

As a result, organic fruit growers have to make maximum use of preventative or indirect plant protection measures, some of which were already described in terms of their effect on fruit quality (e.g. lower planting densities, see sections above). However, there remains an increased risk for pests and diseases causing stress, decreased photosynthetic activity and capacity. Also in certain sensitive cultivars, the application of permitted plant protection products (such as lime sulphur) can result in phytotoxic effects or latent stress in trees (Palmer et al, 2002). The decrease in photosynthetic activity by both pest/disease attack and pesticides may, in turn, result in reduced fruit quality. Both long-term latent and short-term acute side effects of organic plant protection products have not yet been sufficiently investigated. 

Figure 5.3 shows the phase diagram of tin, and clearly shows the transition from tin(white) to tin(grey). Unfortunately, the tin allotropes have very different densities p, so p(tm, grey) = 5.8 gem-3 but p(tm, white) = 7.3 g cm-3. The difference in p during the transition from white to grey tin causes such an unbearable mechanical stress that the metal often cracks and turns to dust - a phenomenon sometimes called tin disease or tin pest . 

Direct behavioral evidence of how stored-product insects move among patches is limited, but what is available shows that stored-product pests readily leave patches of food, can find and exploit multiple patches, and that these processes are influenced by a variety of endogenous and exogenous factors. The time Cryptolestes ferrugineus spent in refugia has been shown to be influenced by strain, sex, and age (Cox and Parish, 1991 Cox et al., 1989, 1990). A variety of factors have been shown to influence the decision by red flour beetles to leave food patches, including insect density... 

There is evidence, moreover, that higher population densities and increased survival of some insect pests follow chemical treatments designed to control them. This is because such treatments result in the destruction of predator insects that naturally control pest populations. Killing the predators only increases the needs for future insecticide treatments. ... 

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