Aphids, called "plant lice", are among the most feared parasites in the plant world. It is a species of insects belonging to the Aphidoidea family and to the Rhynchota genus. Their danger derives from the fact that the females of these insects have a high reproductive capacity combined with a certain polymorphism. This term means a reproductive and morphological differentiation that allows the adaptation of new generations even to adverse environmental conditions. The aphids they can be both oliphagous and polyphagous, that is, they can feed on a single species or several plant species, but not only, they can also attack only specific parts of the same plant or different parts. In the first case we will have the so-called homotope cycle, while in the second, the heterotope one.
The aphids they are about 4 millimeters in size, with a stocky green, black, yellow, pink body and a head and back smaller than the abdomen. Their main feature of polymorphism is that they can be both winged and wingless. Wingless individuals are called wingless. These have even smaller and stockier bodies than the winged ones. When present, the wings are thin, elongated and covered with a slight vein in the shape of branches. Aphids, depending on the polymorphic species to which they belong, can feed on both the plant parenchyma and the sap. Their larval and juvenile form is represented by the nymphs that can take on different genetic characteristics based on the way they were generated. The reproductive cycle of the aphids can take place, in fact, both by sexual coupling between two individuals of the opposite sex, and by unisexual way, that is, only by the females. The unisexual reproduction of some living species is called "parthenogenesis". Female aphids, born from a fertilized egg, are called founders, are wingless and reproduce through parthenogenesis. From this can be born both males and females, parched or winged. The latter are also called migrants because they allow the population of insects to be propagated on other species of host plants, creating the heterotopic reproductive cycle. With parthenogenesis, female aphids can generate even more than 5 nymphs per day. The latter, even before birth, may contain other developing embryos. The nymphs develop within seven days and immediately begin to feed on the plant sap.
Aphids affect both woody and shrub species and herbaceous ones. Herbaceous plants are, however, secondary guests, because the species preferred by aphids are precisely those arboreal and with woody bark. Shrubs allow aphids to better resist low temperatures. These insects develop, in fact, in dry and temperate climates. Unfortunately, the defense and reproduction mechanisms of the aphids are allowing these insects to adapt even to adverse climatic conditions. Just think that unisexual reproduction occurs mainly in temperate climates, while sexual or amphigonic reproduction allows to lay eggs particularly resistant to low temperatures. The plants infested by aphids are most of the agricultural ones and the vast majority, or rather, almost all of the ornamental garden and house plants, including potted plants. It should be emphasized that aphid infestations are also accompanied by those of ants, which they protect aphids and favor its reproduction due to the large amount of honeydew (sugary substance) produced by the metabolic processes of these insects. The parts of the plant that are attacked by aphids are the bark and leaves. The wood of the bark acts as a protection for the fertilized eggs, while the leaves can be affected during the migration of winged insects.
Aphids are equipped with a sucking, stinging and inhaling mouth apparatus that acts in two ways: by removing lymph and nutrients from the plant and by inhaling saliva inside the plant tissue (plant parenchyma). After injecting the saliva, the affected plant undergoes a sort of systemic infection that leads it to also receive viruses contained in the insect's own saliva. These viruses are not pathogenic for the aphid, but only for the plant. Indeed, the insect has a kind of mutualistic and symbiotic bond with viruses since they stimulate the production of amino acids that favor the multiplication and reproduction of aphids.
The removal of the plant sap takes energy away from the plant, which will appear weak and withered. Lymphatic deficiency prevents the plant itself from carrying out photosynthesis, a factor that also damages the color of the leaves, causing them to turn yellow, wither and then fall, until the final death of the affected plant species. The subtraction of the sap represents the main direct damage of aphid infestations. The indirect damage of aphids, or those caused by the inhalation of saliva and viruses within the plant tissue, is also economically and aesthetically serious. The symptoms of indirect damage are manifested by deformations, leaf rolling and the formation of galls. The latter are real plant tumors caused by infestations of insects and parasites. The toxic substances absorbed by the plant tissue create an excessive reproduction of the plant cells, leading to the formation of growths that damage the productivity and quality of crops. Other indirect damage is caused by honeydew (the sugary substance secreted by aphids), which is deposited on the surface of the plant and causes further infestations by fungi, including fumaggini.
Environmental causes of the infestation
Aphids are now widespread insects on any species of plant. Their high infestation capacity is, in part, caused by climate changes which have recorded a constant and systematic rise in temperatures over the years. Aphids, in fact, prefer temperate and dry climates and find their ideal location in the Mediterranean areas and in Southern Italy, where there are mild winters and an arid climate tending to drought. Aphids have also learned to adapt to unfavorable conditions, such as severe winters, in which they deposit fertilized eggs on the bark of trees and in which sexual reproduction prevails. In temperate climates, aphids mainly infest those plant species that develop precisely with mild temperatures, such as artichokes, roses and cucurbits. In roses, for example, the eggs hatch in spring, while in cucurbits (melon, pumpkin, cucumbers, etc.) the attack takes place in the first days of July, by winged females. These also attack the cotton plant. Other factors that stimulate the reproduction of aphids are the excess of nitrogenous nutrients present in the spring sap. In fact, nitrogen favors the asexual reproduction of females. A further condition that helps the development of aphids is excessive chemical treatments, which cause the birth of new generations of aphids that are increasingly resistant to the active ingredients of the pesticides used.
The prevention of attacks by aphid it's not always easy to do. However, especially for vegetable and garden plants, some useful tricks can be used to limit the appearance of this annoying insect. It is well known that ants protect aphids and transport them from one plant to another as they feed on their sugary substances, so if ants appear in the garden, it is likely that sooner or later aphids will also arrive. The strategy to avoid the consequent attack of these parasites consists in eliminating the ants, which can be kept away with natural nettle-based pesticides. Another preventive tactic against aphids is mulching, ie covering the ground with a little straw or a plastic sheet in order to cause it to overheat, which prevents pest attacks and the growth of weeds. This technique is to be avoided if the temperature is already high because it could cause the plants to wilt. In agricultural cultivation systems, the modification of temperatures and the reduction of nutrients to the plant in the summer period can halve or eliminate the colonies of aphids.
The control of aphids takes place both with biological and chemical control. The latter is not always effective due to the high resistance developed by the generations of aphids that are gradually brought into the world. The chemical pesticides used for aphids are Thiaclorid,
Acetamiprid and Thiamethoxam. From some experimental studies conducted by the manufacturers of pesticides and pesticides, it seems that only Thiaclorid has proved effective in preventing the reappearance of aphids in lightly attacked plants, while in those with severe infestation, the product was found to be effective for 85%. of aphids. The other two pesticides did not have the same results, fighting only 60% of the pest aphids. Better results are found with biological control that uses natural predators of aphids, such as ladybugs, chrysopa, diptera larvae, neuroptera, hymenoptera, varieties of bedbugs and some species of birds, such as swallows and blackcaps. Aphids defend themselves from predators by using tubes called siphons that secrete a fluid substance capable of paralyzing the predator's limbs and mouth. The most effective fight against aphids is therefore the integrated one, using pesticides and natural predators in combination. Pesticides must contain selective active ingredients, that is specific for aphids, and should never be used preventively, but only after the onset of the infestation.