Arthropod pests can be divided into two categories, direct and indirect pests. Direct pests attack fruit and fruit buds, causing immediate injury. In some cases, damage is cosmetic, not affecting nutritional value or flavor but diminishing aesthetic quality for marketing purposes. However, injury by direct pests can reduce storability of fruit for processing and increase likelihood of secondary invaders. This type of damage is extremely important economically because fruit can be destroyed outright or rejected for fresh-market sale or for processing. Although damage by direct pests does not adversely affect tree vigor, indirect pests are extremely important to tree vigor, as they attack foliage, roots, limbs, or other woody tissues. Foliar-feeding arthropods reduce photosynthesis due to a reduction in either quality (e.g., mites) or quantity (e.g., leafminers) of photosynthetic leaf surface area. This, in turn, leads to a reduction in the amount of carbohydrates manufactured, ultimately resulting in future production problems, such as poor flower bud formation, reduced fruit set, and decreased fruit, shoot, or root growth. The feeding of insects on surfaces of woody tissue or within the cambial layer of roots, trunks, or limbs leads to declines in tree vigor and yield. Furthermore, this type of feeding facilitates secondary damage by opportunistic insects and diseases. Finally, some insects and arthropods may be important economically not because of any particular feeding damage they create but because of their ability to transport or transmit pathogenic or disease-causing organisms such as bacteria and viruses to deciduous fruit trees. For example, many species of blossom-visiting and flying insects transport the bacterium that causes fire blight, Erwinia amyl-ovora, from infected to uninfected apple or pear trees. Such disease transmission may occur as the result of blossom visitation by insects carrying bacteria on their outer body surfaces (e.g., pollinating bees). Diseases, especially viruses, are readily transmitted by insects with specialized needlelike mouthparts (e.g., leafhoppers and aphids) used to probe plant tissue.

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