Insects of the order Hemiptera have mouthparts specialized for piercing and sucking, and within the suborder Homoptera of this order two groups, the Auchenorhyncha and Sternorhyncha, specifically feed on plants. As their general name implies these insects feed on the sap of plants. This can be the sap of individual mesophyll or palisade cells of leaves or the translocating elements of plants, in particular phloem. In feeding on phloem sap not only has these insects access to a more continuous supply of food but they can inject disease-causing organisms and saliva containing physiologically active chemicals, which are then translocated throughout a plant. In addition by telescoping generations aphids have overcome the developmental constraint and for their size achieved prodigious rates of increase. As a consequence aphids often become very abundant and so in addition to any indirect damage they can be extremely damaging because of the nutrient drain they impose on plants. That is, many phloem feeders in particular are such serious pests of trees that they threaten their survival, e.g., the scale insects Carulaspis minima and Lepidosaphes newsteadi attacking Bermuda cedar on Bermuda and Orthezia insignis attacking the native gumwood on St Helena.
[...] Second, they generally have symbiotic bacteria in bacteriosomes within their haemocoel, which increase the efficiency of their nitrogen metabolism by converting the nonessential amino acids in phloem sap into the essential amino acids the insects need to sustain their growth. In addition the symbionts may also recycle some of the insect's nitrogenous waste. In this way the aphids in particular are able to sustain a prodigious rate of growth on what is a very poor quality diet. That is, they are able to process quickly very large quantities of phloem sap and upgrade the quality of the amino-nitrogen component of their diet. [...]
[...] Although aphids do not affect the number of leaves borne by lime (Tilia spp.), oak (Quercus spp.), or sycamore, sycamore produces smaller leaves, which contain more nitrogen, when heavily infested in spring. However, the leaf area equivalent to the energy removed by sycamore aphids only accounts for a small proportion of the observed diminution in leaf area. If the drain imposed is expressed in terms of nitrogen rather than energy then aphids again remove far less nitrogen than expected from the reduced size of the leaves. [...]
[...] Since ants are largely regarded as predominantly predacious, this pattern challenges the usually accepted pattern of energy flow, in which the biomass of predators should only constitute a proportion of their prey. This has led authors to hypothesize that the availability of homopteran (Coccidae and Membracidae) honeydew provides a key resource for ants. As in temperate regions the homoptera on a particular tree are mostly monopolized by a single ant colony. In summary, the few data that are available tend to indicate that there are more species of sapsucker in the tropics but they are less abundant than aphids such as the lime or sycamore aphids in temperate regions. [...]
[...] In oak this is a consequence of a 25% increase in the quantity of both chlorophyll A and B. Associated with this is an increase in dry matter production per unit area of leaf, which in sycamore can be 1.7 times greater in infested than in uninfested saplings. Following years of heavy aphid infestations lime and sycamore break their buds later than usual, and in the case of lime the leaves are smaller and a darker green, and have a net production 1.6 times greater than the leaves of previously uninfested saplings. [...]
[...] Because phloem sap is rich in simple sugars it creates an osmotic problem for the insects, which is overcome by converting the simple sugars into complex sugars, which effectively reduces the osmolality of the phloem sap as it passes through the insect. In addition, these insects are very effective at assimilating and utilizing the low levels of amino-nitrogen in their food. First, they are able to process rapidly relatively large volumes of phloem sap and so fuel their total requirements for amino-nitrogen. [...]
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