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For educational purposes:--

Information on the basics of Entomology

 

Introduction                                                                                                                                                                                                                                                                           Contents

 

An Introduction To The Study of Entomology 1

Kingdom:  Animalia, Phylum: Arthropoda

Subphylum: Hexapoda: Class: Insecta: Order: Homoptera

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Pteragota:  Paurometabola

  Order:  Homoptera (15 Families)

    General Summary

    Characteristics

    Virus Diseases

    Biological Control Projects

    References   Citations  Examinations

    Families of Homoptera

      Cicadidae. --

      Membracidae. --

      Cercopidae. --

      Cicadellidae. --

      Aphidae. --

      Coccidae. --

      Phylloxeridae

 

General Summary of Homoptera

 

          The Homoptera includes a large number of different forms ranging in size from the usually microscopic Coccidae to the large tropical lantern bugs (Fulgoridae) and the cicadas, which may attain 5 cm. In length and with a wing expanse of 10 cm. With the cicadas are the leafhoppers, treehoppers and froghoppers, all active insects.   DNA evidence shows a close relationship to the Hemiptera, so that eventually the Homoptera may be once again grouped together with the Hemiptera as they have been under Heteroptera earlier.

 

          Sometimes the life cycle may be greatly prolonged, Cicada septendecim, ie., having a 17-year cycle. The eggs are injected into holes in twigs of trees, and the nymphs hatching from them fall to the ground into which they burrow to feed on the roots. After about 17 years of nymphal growth a stage resembling a pupa is passed through before emergence of the adult (derived and updated from Borradaile & Potts, 1958).


          Scale insects (Coccidae) also belong to the Homoptera. Pseudococcus is the mealy bug, Tachardia lacca the lac insect of

Commerce and Aspidiotus perniciosus the San Jose scale insect that attacks citrus trees.


          Plant lice (Aphididae) are noted for their wide distribution and for their rapid reproduction, and transparent wings. The tarsus is 2-jointed, that of the Coccidae being 1-jointed. Wax-secreting cornicles are located dorsally on the abdomen.

 

          In the Coccidae the reproductive phenomena are of much economic importance. A comparatively simple life cycle is that of Aphis rumicis. The winter is passed on the spindle tree Euonymus as eggs, which are laid in the autumn by fertilized females. In spring these eggs give rise to wingless viviparous parthenogenetic females. A variable number of these parthenogenetic generations is passed through in the summer, then winged parthenogenetic females appear that migrate to another host plant (the bean Vicia or other plants) and there reproduce, giving rise to generations of parthenogenetic females which eventually produce winged females that return to the primary host plant Euonymus. From these there now appear oviparous females to copulate with winged males, that are migrants from the secondary host plant, the bean.

 

          The life cycles are complex.  Fertilized eggs are laid in autumn that give rise to wingless viviparous parthenogenetic females.  These then give rise to winged migrant parthenogenetic females and wingless parthenogenetic viviparous females.  Some of these give rise to winged viviparous females that in turn give rise to wingless oviparous females.  Others give rise to winged males that mate with the wingless oviparous females.  Eggs are laid in the autumn.


          In other groups, such as Phylloxera vastatrix, the notorious pest of vineyards, the life cycle is more complicated and involves migrations between root and stem of the host plant. The reproductive capacity of these insects is extraordinary but is kept down by the number of enemies they possess.

 

          The cyclical reproductive phenomena in aphids leads to important questions relating to the differences between sexual and parthenogenetic individuals, and to the environmental conditions determining the occurrence of these phases in any life cycle.


          Fertilized eggs produce only parthenogenetic females. These multiply by diploid parthenogenesis, i.e. the eggs retain the full complement of chromosomes and are not capable of fertilization. Eventually there come individuals capable of bearing sexual forms, sexuparae. The sexual forms arising from these produce haploid germ cells that have undergone normal reduction. Fertilization then will restore diploid parthenogenesis. Sexual differences are indicated in the chromosomes; the female of Aphis saliceti has six, of which two are sex chromosomes; the male only five, one only being a sex chromosome. However, sexual reproduction leads only to the production of parthenogenetic females and not to males and females in equal numbers, as might be expected. This appears to be due to the fact that in the maturation of sperms, those with only two chromosomes die. Fertilization is always between sperms and ova each with three chromosomes, of which in each case two are normal chromosomes (autosomes) and one is a sex chromosome X. The capacity of females with six chromosomes to produce male offspring with only five is due to the fact that in the maturation of male-producing parthenogenetic eggs, reduction in the number of chromosomes only affects the sex (X) chromosomes, one remaining in the egg, the other going to the polar body. In this way a parthenogenetic female with six chromosomes, i.e. 4+ XX, gives rise to males with only five, i.e. 4+ X (derived and updated from
Borradaile & Potts, 1958).

 

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Characteristics of Homoptera

 

          All Homoptera are exclusively plant feeders and some species have been used in the biological control of weeds.  They have membranous or leathery wings, but it is possible to trace venation to the base.  Apterous forms are also present.  The mouthparts are beak-like and appear to arise from the front legs.

 

          Two large groups of Homoptera are (1) the cicadas and leafhoppers, and (2) the aphids, scale insects and mealybugs.  The size ranges from very minute to larger forms.  There are a great variety of habits although they are all plant feeding.  The cicada has the longest life cycle of any insect, and there is a great diversity in dwelling places.

 

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Families of Homoptera

 

          Cicadidae. -- The cicadas are noted for their high-pitched call, which is made only by the male and is used to attract females.  The different species may be recognized by this call.  All cicadas have a rather broad head and prominent eyes with three glossy, beadlike ocelli between. 

 

 

          The nymphs are underground feeders.  They possess enlarged front legs for digging and may remain up to 17 years underground (= 17-year locust).  The last nymphal stage leaves the ground.  Adults are the largest of Homoptera and do serious damage to trees by boring into them to lay their eggs.

 

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          Membracidae. -- The head of treehoppers is situated below the pronotum and covers most of the wing. 

 

 

          Hook-like projections occur only in the larvae.  They feed on herbaceous plants and some species have different hosts in different parts of the life cycle.  For example, oak in springtime, goldenrod in summer and oak again in autumn.  These insects secrete very sugary, sweet honeydew, which attracts ants.  The Buffalo Treehopper is a notorious pest that causes damage to plants with its oviposition activity.  It feeds on cover crops in orchards and then lays eggs in trees.

 

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          Cercopidae. -- The immatures of spittle insects or froghoppers have a protective froth covering of spittle, which prevents desiccation.  Some species even construct a tube of spittle.  Mucus-like secretions derived from special glands cause the froth.  This is beat up and mixed with air by action of the hind legs.  It consists of water, anal fluid and air.

 

 

          These insects become free living as adults when they resemble some leafhoppers very closely, but can be distinguished by having spines on the apex of the hind tibia..  The adult is generally brownish or dark in color.  Some species show a pattern of colors.  Sometimes there are up to eight different forms in the same species.

 

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          Cicadellidae. -- Leafhoppers feed on plants in all stages and many are very economically important pests. These insects are characterized by long, slender wings, which are held roof-like above their body. 

 

 

          They are very active as jumpers due to their enlarged tibia, and the nymphs are agile and can run sideways. The hind legs have a double row of spines and not a single circular row as in spittlebugs.  Some species are brilliantly colored.

 

 

          Some leafhoppers are serious plant virus vectors, which have a virus incubation period that can last a year or more. The beet leafhopper migrates and transmits Curly Top Virus in North America They can also cause plant disease by their feeding secretions.  Potato yellow leaf or hopperburn is caused by the toxic effect due to the feeding of leafhoppers.  In alfalfa the disease is termed yellows.  The toxic effect is not only from their saliva but also due to their feeding on the vascular parts of plants (xylem and phloem).  They can cause white mottling on beans.  The potato leafhopper that causes all the symptoms noted above migrates to higher latitudes annually.

 

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          Aphidae. -- Aphids reproduce in large numbers.  They have complex life histories with winged and wingless forms and one or more hosts.  They prefer to feed mostly on tender shoots of rapidly growing plants, but will also feed at other sites even on the roots.  Besides causing direct injury to plants some species are also able to transmit plant viruses, which occur on their mouthparts as contaminants.

 

 

          Aphids possess cornicles that emit glandular secretions and can serve as a kind of defense.  The cornicles are useful in identification because of their many different shapes.

 

 

          Aphids secret honeydew from the anus.  When dropping onto plants various molds grow on it, which will damage plants.  Ants will care for aphids and extract honeydew from them by caressing them with their antennae.  Sometimes ants will carry aphids underground to overwinter.

 

          A typical life cycle is described as follows:  Winter is spent on eggs on the winter host plant.  In the spring the eggs hatch to produce the stem mother.  These mature without fertilization and reproduce.  Usually they are apterous and will produce apterous offspring parthenogenetically.  Winged forms are produced next, which fly to the alternate host plant.  If the winter host is a tree the alternate host will often be an herbaceous plant.  Wingless generations are produced.  If conditions become crowded on the alternate host, winged generations may be produced, which are all females.  These fly to another plant that may be the same or a different species of herbaceous plant.  In the autumn, there is a migration back to the winter host plant.  Also, some males may be produced on the summer host.  Also in autumn the female may produce several generations on the reinvaded winter host.  She may mate with the male and then go through an egg-laying phase that overwinters.

 

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          Coccidae. -- Scale insects are primarily tropical and subtropical insects that secrete a wax covering over their body, which protects them from external environmental conditions.  Three groups of scales are (1) hard scales, (2) soft scales and (3) mealybugs.

 

 

          Only a few males are produced and the life cycle is quite complicated.  Scale insects have great economic importance as pests, especially on tree crops.  However, the lack insect of India and the cochineal insect of Mexico have been favored commercially.  The Biblical "manna" is attributed to this family.

 

          A typical life history is as follows:  In springtime the eggs hatch and the crawlers, which look like aphids, spread over the plants.  The scale stage occurs primarily during summer.  In autumn and winter a mass of eggs forms in place of a degenerated female.  Overwintering occurs as eggs underneath the scale.  Most species are parthenogenetic, but if males are present they possess only one pair of hind wings.

 

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          Phylloxeridae. -- Members of this family almost ruined the wine industry in Europe in the late 17th Century.  They feed on the roots of plants.  Rootstock from America was grafted onto European grapes and stopped the seriousness of the attack.

 

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Virus-like Diseases of Plants

 

          The invertebrates that transmit virus diseases resembling viruses are primarily Homoptera, Acarina (excluding spider mites), Thysanoptera and some Orthoptera, Hemiptera and Coleoptera.  Insects are the most important vectors, but some viruses are also transmitted through vegetative propagation and seed.

 

          A plant usually does not recover once it contracts a virus.  One vector is sufficient to infest a plant so that the number of vectors is not important.  Viruses multiply within plant tissue, and symptoms are similar to genetic disorders.

 

          Two types of viruses are (1) Nonpersistant and (2) Persistant.  A nonpersistant virus lives in the insect only a short time.  Aphids are usually involved as well as herbaceous plants.  There is little or no latent period and mechanical transmission is very effective.  Persistant viruses may live in an insect vector for its entire lifetime.  Leafhoppers are usually involved as well as woody plants.  There is a definite latent period and mechanical transmission is rare.

 

          The number of viruses that can be transmitted by an insect may be large.  The green peach aphid is able to transmit over 50 species of virus all of which are of the nonpersistant type.  Some viruses show symptoms in only a few plants while still having a wide host range.  Certain woody plants show symptoms only after three years of initial inoculation (see <efl5.htm>.

 

          Producing resistant plant varieties may control plant viruses.  The removal of diseased plants through rogueing is effective, which eliminates the source of virus inoculum from the area.  Controlling the insect vector with pesticides is another approach, but it requires great thoroughness as only one individual insect can still transmit the virus.  Providing a host-free cycle in fields is sometimes effective as well as heat treatment of seed that may be infected.  Reducing weeds around a field can eliminate hideaways for vectors.

 

          Some examples of serious virus diseases are Tobacco Mosaic, Cucumber Mosaic, Potato Leaf Roll, Curly Top of Beets and Peach Mosaic.

 

          An example of complete recovery from a virus that is transmitted by leafhoppers is the Oleander Virus that was beginning to destroy plants in California in the latter 20th Century, especially those of with white blooms.  After about 10 years of rapid spread and infection, the plants began to show recovery, so that by 2010 most show no symptoms whatsoever over a wide area.

 

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Homoptera --Biological Control Projects (50.5% of total projects)

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Details of Insect Taxonomic Groups

 

          Examples of beneficial species occur in almost every insect order, and considerable information on morphology and habits has been assembled.  Therefore, the principal groups of insect parasitoids and predators provide details that refer to the entire class Insecta.  These details are available at <taxnames.htm>.

 

 

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References

 

                 Introduction                                                                                                                                                                                                                                                          Contents