Diet and Feeding
Although many species of insects may take advantage of different types of diet at different stages of their life cycle, diets may be classified as saprophytic, phytophagous or carnivorous.
Saprophytic
Saprophytic insects consume plant or animal remains. They are important in recycling nutrients.
- Plant remains are consumed by many species of fly and beetle larvae and all life stages of many of the Collembola and Protura. These animals are commonly seen in compost heaps.
- Animal corpses are similarly broken down by many species of fly and beetle larvae. Some of these are important in forensic entomology.
- Animal faeces are broken down by dung beetles which collect or bury dung for their larvae. Native Australian beetles are unable to cope with the dung or introduced animals. CSIRO, from 1960, introduced large numbers of African dung beetles. This resulted both in partial control of the bush fly and more productive pastures.
Phytophagous
Phytophagous insects attack living plants
- Polyphagous insects (eg grasshoppers) consume many species of plants
- Oligophagousinsects (eg wander butterfly larvae) consume a few species of usually closely related plants.
- Monophagous insects (eg citrus butterfly, white cedar moth) consume only a single species of plant.
Type of feeding on plant
Insect mouthparts are primitively of the chewing type, but many specialised insects have piercing and sucking and/or lapping mouthparts.
Chewing - of leaves, stems, roots, fruit, wood, flowers, pollen. May form galls, leaf rolls, silk and frass nest, leaf blisters or be leaf miners, or seed gatherers.
Piercing/sucking - of leaves, roots, stems (either phloem or xylem) or nectar. The insects may be under scales, wax covers or froth.
Sucking or lapping - of nectar or saps by flies, butterflies, bees.
Factors involved in feeding on plants
Desiccation
Plants have high moisture content but humidity decreases away from the plant. This necessitates:
- drinking water
- sheltering under leaves
- modification of breathing system (see respiration)
Attachment
It can be difficult to hang onto leaves. This has led to
- structural modifications such as Velcro-like surfaces under prolegs
- waxy secretions by scale insects
- living inside the plant (borers, gall insects etc)
Nutritional content
Plants, (comprising mainly CHO) differ in chemical composition from insects (mainly protein). The level of nitrogen in plants is critical, determining rates of growth, survival and reproduction.
(Higher N levels do not always improve growth rates. This may be a response to an imbalance of amino acids, lack of other nutrients or the ratio of N to water.
Allelochemicals
These are secondary plant compounds such as canavanine, oxalic acid, cyanide, cyanogenic glycosides, alkaloids, terpenoids and lignins. It is thought that they may have a metabolic role in the plant or perhaps defend the plant against desiccation, cold, UV or salinity. Some allelochemicals are excretory products. All are a defence against insects but are not effective against all insects. Some allelochemicals are used by insects by being incorporated into the body of the insect for its own protection against predators (e.g. wanderer butterfly larva).
Allelochemicals may act in either a qualitative or quantitative manner:
Qualitative allelochemicals act as poisons in small quantities (e.g. cyanides, alkaloids). These usually have a high metabolic cost to the plant.
Quantitative allelochemicals act in proportion to their concentration (e.g. tannins, resins, silica). These chemicals are usually of lower metabolic cost to the plant.
Apparency of plants
Apparent plants are easier for insects to find. They are large, long-lived trees that grow in clumps. Theirlong life enables them to accumulate high-cost, quantitative poisons.
Unapparent plants are harder for insects to find. They include small, scattered annuals that produce low-cost, qualitative allelochemicals.
Unapparent plants are often turned into apparent plants by monoculture.
Use of insects by plants
Insects and flowering plants have been evolving together for about 200 million years. More than 65% of flowering plants are pollinated by animals - predominantly insects - mostly bees, wasps, flies, beetles, butterflies and moths.
Animal pollination, using a sticky pollen, is much more effective than wind pollination.
How do plants attract pollinating insects?
- Reward of pollen. Nectar in special nectaries
- Attraction by
- flower colours - insects are attracted to yellow, blue, purple and UV. (Red attracts birds)
- smell e.g. some plants smell like rotten meat to attract blowflies
- mimicry of female insect e.g. some plant species resemble a female wasp which attracts the male to mate with it
Carnivorous
Carnivorous insects consume animal tissues. Predators, parasites and parasitiods may be used as biological control agents.
Predators eat other insects or animals. They may capture by speed in pursuit (e.g. robber fly, dragonfly), trapping (antlion larva) or by use of modified appendages (e.g. raptorial legs of mantid, extendable labium of dragonfly nymph).
Parasites live off a host but do not kill it. Ectoparasites live externally on the host; endoparasites live inside the host.
Blood sucking insects may be ectoparasitic for a long period, (e.g. fleas, lice) or is with the host only while feeding (e.g. mosquitoes, bed bugs).
Parasitoids kill the host.
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