What is a host plant?
Why do insect herbivores attack some plants and not others? This simple question is linked to two others: Why are some plant species host plants and why are some plants within a host plant species infested while others of the same species remain uninfested (see below)?
Clues as to why some plant species are hosts and others are not may be found in information held within the Plant-SyNZ database, which documents the host plants of herbivores. For the purposes of that database, a host plant is defined as a species on which at least one life stage of a herbivore feeds without being harmed and can pass on to the next life stage or lay fertile eggs. For more explanation on how to decide if a plant is a host for a herbivore, read about the use of a reliability index for determining the quality of this information. For a more detailed discussion about criteria used for deciding if a plant species is a host plant, see: Ward LK 1988. The validity and interpretation of insect food plant records. British Journal of Entomology and Natural History 1 (4): 153-162.
The Plant-SyNZ database only shows whether a plant can be a host for a herbivore; it does not provide information on the relative abundance of a herbivore on different host plant species. All host plants listed for a herbivore are not equally good hosts, e.g. leaf mines of Chromatomyia syngenesiae (Diptera: Agromyzidae) are commonly seen on Sonchus species, Senecio bipinnatisectus and S. jacobaea (ragwort), but are rarely seen on Senecio vulgaris (groundsel) or S. skirrhodon. Herbivore numbers can change during the year according to its breeding cycle, but infestation rates can also vary between plants at a similar stage of the herbivore's annual cycle.
There are four main reasons for variable infestation of plants by herbivores: plant resistance, environment, natural enemies, and chance. On a local scale, without careful experimentation it can be very difficult to determine the relative importance of each of these factors.
Plant resistance, i.e. plant genotype: The genetic variation of plants in a species may make some plants more or less susceptible to the herbivore. Those on which the herbivore cannot live or breed are called resistant. Resistance may be due to the plant being toxic (poisonous) to the herbivore or if the herbivore feeds on a particular growth stage, e.g. flowers, they may not be available at the normal time for the plant species and therefore miss attack by the herbivore. Plant breeders exploit naturally occurring genetic resistance in plants to breed crops that are resistant to pests and diseases.
Environment: The environment in which a plant grows, or even part of a plant, can affect its suitability as a host for a herbivore. The climate in part of the plant's geographic range may not be suitable for the herbivore. This may be due to temperature or humidity preferences, though often the precise reason for restricted distribution is not known. For example. Rastrococcus namartini (Hemiptera: Pseudococcidae), a mealybug that lives on leaves of the tree Myrsine australis, is found on trees in the drier central and eastern areas of Auckland Region, but not in the wetter rainforest of the Waitakere Ranges. The weather can also affect the abundance of herbivores on their host plants, creating a short-term environmental change that either favours or suppresses herbivore populations. Fluctuations in populations from year to year may be due directly or indirectly to the weather and may be mediated through the plants or by the activities of natural enemies. There are few studies of the long-term natural changes in abundance of New Zealand indigenous herbivores. The growing media, soil and rock substrate on which a plant is growing may affect access to water and nutrients and as a result the quality of plant tissues, which in turn may make a plant more or less suitable as a host plant for a herbivore. These effects may be very localised. Another factor, local environment, can also affect the suitability of plants as host for herbivores. For example exposure to wind or frost may make plants less suitable than those in protected sites. On the other hand, plants on the edge of forest appear to more susceptible to some scale insects, e.g. Icerya purchasi (Hemiptera: Margarodidae). It also appears that location of leaves or twigs on a tree can affect susceptibility to a herbivore. For example, tree or shrub leaves in shady positions seem more likely to be infested by some herbivores than leaves in full sun.
Biostatus, an explanation of the terms used
The plants and animals found in New Zealand today have very different origins. They may have evolved in New Zealand; arrived naturally since New Zealand separated from other land masses and still exist in the country of origin (or may have become extinct in their country of origin); or they may have arrived since the arrival of man as a deliberate or accidental introduction.
Biostatus information for each species indicates their origins and kinds of places they occur in New Zealand. Various terms are used for both origins and occurrence. They have been standardised in the factsheets and are described below together with the alternatives in use. The terms used differ slightly for plants and invertebrates.
In the insect factsheets only one term is used for each taxon and it emphasises origins though in some cases it also covers occurrence. The Landcare Research NZFlora database uses separate terms for occurrence in New Zealand and the origin of a taxon.
Endemic. These species occur naturally only in New Zealand. They are indigenous to this region.
Non-endemic. These species occur naturally in New Zealand and other regions of the world. The term native is also used for these species, which are also indigenous to this region.
Naturalised. These are non-indigenous species that grow wild in New Zealand. They may also be called exotic, introduced, alien or adventive species. They may have arrived accidentally or have been deliberately introduced to the country. This group includes plants that have 'escaped' from cultivation and have established breeding populations in the wild.
Cultivated. These are plants that exist only in cultivation in New Zealand. Some plants that are mainly present in cultivation also have wild populations and are therefore classed as naturalised, exotic or exotic (casual).
Overseas. This is for plants that do not grow in New Zealand, but for which there is information about New Zealand invertebrate herbivores that are associated with them.
Unknown. This is for species where the biostatus is unknown. This mainly refers to plants where the genus is known and the species is not known, and where species in the genus have variable origins, i.e. some are endemic or native in New Zealand and some come from other countries.
Endemic. These species occur naturally only in New Zealand. These species are indigenous to this region.
Native. These species occur naturally in New Zealand and other regions of the world. The term non-endemic is also used for these species, which are also indigenous to this region.
Adventive. These are non-indigenous species that are living in New Zealand. They may also be called exotic, introduced, alien or naturalised species. They may have arrived accidentally or have been deliberately introduced to the country. If they have been deliberately introduced, for example for biological control of a weed, they may be termed established.
Captive. These are non-indigenous invertebrate species that are or have been held in captivity in New Zealand and do not exist in the 'wild'. The term may include species that have been released into New Zealand, but have not established.
Overseas. This is for invertebrates that do not occur in New Zealand, but for which there is information on their association with indigenous New Zealand plants.
Unknown. This is for species where the biostatus is unknown. This mainly refers to invertebrates where the genus is known and the species is not known, and where different species in the genus have variable origins, i.e. some are endemic or native in New Zealand and some come from other countries.
Information used in the Insect Factsheets comes from both published and unpublished sources. 'Citations' are used to show the source of information. Citations may be shown in the text of the factsheet and the tag names for undescribed species. However, in general the source(s) of information used is shown in the section 'Sources of information'.
Citation conventions: text of factsheet
Where the information is based on unpublished information, this is acknowledged by giving the person's name, including initials, and the year when the information was provided, e.g. NA Martin 1999.
Where the information is published, scientific-journal-type formats are used with the author(s) name and the year of publication, e.g. Other 2003 or Martin & Other 2004. Note that the author initials are not used in citations of published information. Full details of where an item was published can be found in the section 'Information used'.
Names used for undescribed species are based on published and unpublished sources.
Published sources give the name of the author(s) and the year of publication, e.g. Liriomyza sp. 'Hydrocotyle' (Spencer 1976) for a species of leaf mining fly found in Hydrocotyle.
Unpublished sources of names are similar but insert 'of' after the tag name and the author and year are not in brackets, e.g. Phytomyza sp. 'Ranunculus reflexus' of Martin 2001 for a leaf- and stem-mining fly in Ranunculus reflexus.
A plant or animal can have several names: a scientific name, a common or vernacular name, or a Māori name. Where there is no scientific name, they may have an informal 'tag' name.
Scientific names are given to plants and animals that have been formally described in scientific literature.
The scientific names used in the Insect Factsheets are those in the Landcare Research NZBugs (invertebrates) and NZFlora databases. In addition, the factsheets contain some scientific names of plants that are not in the Flora database. These either are plant species that do not occur in New Zealand or some species of cultivated plants. The factsheets also includes names of insects and mites that are not in the NZBugs database. These include species that are not in New Zealand and names for undescribed taxa.
Synonyms. Sometimes the scientific name of a species changes as a result of new information. The redundant name is called a synonym.
See tag names for names used for species that have not been formally described.
Sometimes a specimen can only be identified to a genus. In the database it is named as Genus sp., e.g. Pittosporum sp. or Liriomyza sp.
For more information about scientific names see below.
Common or vernacular names
Common or vernacular names are used in the Insect Factsheets. Where possible, existing common names are used, otherwise an appropriate name has been created. Sometimes an existing common name can refer to more than one species of organism. And conversely an organism can have more than one common name, in which case they are listed as synonyms under 'other names'. Common names for plants are listed in NZFlora.
Māori names are given either as the common name or as a synonym. Sometimes a Maori name can refer to more than one species of organism. Sometimes an organism can have more than one Māori name, in which case they are listed as synonyms under 'Other names'. Māori names for plants are listed in NZFlora.
Some plants and animals that are recognised as belonging to distinct species have not received a formal description. Tag names are used so that information about the species can be referred to in publications and databases. Two forms of tag names are used in the Plant-SyNZ database and may be used in the Insect Factsheets.
Genus known, species undescribed. Where the genus is known, the format is Genus sp. 'tag name' (Author year), i.e. for the leaf-mining fly reared from Hydrocotyle species, Liriomyza sp. 'hydrocotyle' (Spencer 1976) and for the leaf-mining fly reared from Melicytus alpina, Liriomyza sp. 'Melicytus alpina' of Martin 2000. The first species was first mentioned by Spencer in a 1976 publication, while the second species has not been recorded in a publication.
Genus unknown, species undescribed. Where the genus is unknown and the species undescribed two conventions have been followed. The first is as follows, Gen. nov. 'tag name' sp. 'tag name' of author year, e.g. Gen. nov. 'Greenthrips' sp. 'Hunua' of Martin 2009.
The second convention is used mainly for species of gall-forming insects and mites where a longer description of gall type is useful and biologically meaningful. The following examples illustrate the concept. Coprosma small bud gall sp. 'areolata' of Martin 1999, Coprosma shoot tip gall sp. 'areolata' of Martin 2001, Olearia small leaf blister gall sp. 'albida' of Martin 2005.
Scientific names, an explanation
The scientific name for a species consists of two parts, a generic name and a specific epithet. The generic name always starts with a capital letter while the specific epithet is never capitalised even if it is based on the name of a place or person. The scientific name is always italicised or underlined, e.g. Thrips obscuratus or Thrips obscuratus.
The scientific name may be followed by the name of the person(s) who originally described the species. The persons (authors) name is never italicised or underlined. If, after the original description, the species is moved to a different genus, the original author's name is enclosed by brackets, e.g. Thrips obscuratus (Crawford). For plants the convention is that when a species is moved to a different genus, the name of the person who makes the change is also added, e.g. Ranunculus membranifolius (Kirk) Garnock-Jones. Animal names can also include the year when a species was originally described, e.g. Thrips obscuratus (Crawford, 1941).
In addition to the basic scientific name, there are various additional subdivisions. For example a genus may be divided into two or more subgenera. The subgenus name is inserted between the generic name and the specific epithet. Species themselves can also be subdivided. These subdivisions can include, subspecies (spp.), varieties (var.), forms (f.), and cultivars (cv.).
A species is a human concept for biological entities. Over time ideas about the definition of a species and its relationship to other species may change, hence the movement of species to other genera or, if two 'species' are later believed to be the same, the joining (synonymising) of the two names. When this happens the oldest name is retained. In order to provide a reference for each species, type specimens are designated. The most important are the holotype which is a single specimen selected and designated at the time of the original description, and the paratypes, which are any specimens from the original series of specimens from which a description was prepared.
The simple answer is an animal – vertebrate or invertebrate – that feeds on plants.
However, as in most aspects of biology, there are some grey areas. These include deciding what is or is not a plant, and what are the boundaries between herbivores that mostly feed on live plants and saprovores that feed on dead plants and dead animals and are all 'pollinators' herbivores.
The place of a species within the environment can be defined several ways: by its feeding type, by its 'ecological function' or by the concept of 'ecosystem services'. These concepts are discussed below.
Herbivores are often regarded as pests, because they damage plants. When a herbivore may be regarded as a pests is discussed below.
In addition there are terms for the host range (number of species) that an organism lives on.
The following explanations of different 'life styles' are based on information in Gordh G., Headrick D. 2001. A Dictionary of Entomology. CABI Publishing, Wallingford, UK. 1032 p.
Fungivores are animals that feed on fungi. Some insects that feed on dead parts of plants (saprovores) may be primarily consuming the fungi in the dead tissues of the plant.
Herbivores are animals that feed on plants. Although fungi belong to the plant kingdom insects that feed on them are not usually regarded as herbivores, but are called fungivores. Algal- and lichen-feeding invertebrates as well as fungi should strictly speaking be regarded as herbivores. Some insects that feed on dead parts of plants (saprovores) may be primarily consuming the fungi in the dead tissues of the plant. However, it may be difficult to categorise them as either herbivores or saprovores, because some wood-boring insects may start feeding on live stems or trees, but through their feeding the whole plant or part of it dies and the insect completes its life feeding on dead plant tissue. Wood borers have been included in herbivores.
Parasites are organisms that live on other organisms, but do not usually kill their host.
Parasitoids are insects that have characteristics of both parasites and predators. Usually a single larva feeds on and kills its host. It usually lives on or inside one host. Sometimes several parasitoid larvae will live on or inside one host insect.
Pathogens are micro-organisms that live on/in other living organisms. They include fungi, bacteria, and viruses. Sometimes the fungal pathogens are more visible than the insect host and can then be used to identify the presence of the insect, especially scale insects.
Pollinators are organisms that assist in the transfer of pollen between flowers. Many insect pollinators also feed on the pollen and nectar produced by the plant and are technically herbivores. At present, insects that are primarily pollinators are not included in the Plant-SyNZ database. Not all insects visiting flowers are pollinators. It can be difficult to know whether insects visiting flowers are effective pollinators.
Predators are animals that catch, kill and consume other organisms. One predator usually catches and consumes several or many prey in its lifetime. The definition of a predator is usually restricted to animals that catch other animals. However, the term is sometimes used for herbivores that feed on plants. For example seed-eating insects may be called seed predators. Leaf-feeding insects may also be called predators.
Saprovores are organisms that feed on dead and decomposing organisms. Saprovores may be primarily feeding on the micro-organisms in the dead organism. Those that feed on dead plant tissue are important in the process of recycling and making the nutrients in dead plants and animals available to living plants for their growth.
Ecological function and ecosystem services
The ecological function and ecosystem services of pollinators are obvious from the name of this group of organisms. Pathogens, parasites, parasitoids and predators can be thought to regulate the numbers of other organisms, such as herbivores, and prevent extreme fluctuations in numbers and severe damage to their plant hosts most of the time.
Herbivores can be regarded as converting plant tissue into animal protein that is more suitable as food of other organisms. The beneficiaries of this include insect-eating birds and lizards. It is noteworthy that some mainly herbivorous birds switch to insect eating when they are feeding their young. A second benefit from insects and mites feeding on plants is that they contribute to an increase in the productivity of the ecosystem by increasing the rate of nutrient recycling through making plant nutrients available through their excreta and dead bodies.
Three terms are commonly used to describe the host range of animals (including herbivores): monophagous, oligophagous and polyphagous.
Monophagous. Feeds on only one species (of plant).
Oligophagous. Feeds on several species (of plant). Oligophagous herbivores may be divided into two groups, those that feed on species in one genus and those feeding on species in several genera and which can be in up to two families. The first group is indicated as oligophagous (genus).
Polyphagous. Feeds on many species (of plant). A herbivore is sometimes defined as polyphagous if it feeds on species from three or more families of plants.
There is a common saying that a weed is a plant in the wrong place. Similarly, a herbivore is only a pest when it is in the wrong place.
New Zealand native herbivores are part of New Zealand's biodiversity and are not pests in native ecosystems and like native birds should, in general, be encouraged on their native host plants outside native ecosystems. However, some species cause unacceptable damage to valued non-native plants, such as lemon tree borer on citrus trees and grass grub on pasture grasses. In these circumstances these species are regarded as pests.
The feeding of some native herbivores on native plants in gardens and parks can also cause unacceptable damage, especially while a plant is young – and at this stage the plant may need some protection. However, if people grow native plants they should expect that once the plant is established, it will host native invertebrates. By doing so they are contributing to increasing the native biodiversity of the area, and in some cases helping monophagous herbivores to survive, e.g. Aceria clianthi Lamb 1952 (Acari: Eriophyidae) on Clianthus puniceus (G. Don) Lindley (Fabaceae) (kakabeak).
Herbivores from overseas may also be either pests or beneficial. Many crop pests are overseas herbivores, though some herbivores have been deliberately introduced into New Zealand to help control weeds. An example of a weed biological control agent is the gorse mite, Tetranychus linearius Dufour (Acari: Tetranychidae) which forms webbing on young growth of gorse (Ulex europaeus L.).
Herbivores from overseas that feed on native herbivores are pests, especially where they occur in native ecosystems. Many do not cause serious harm to the native host plants, but heavily infested plants are sometimes found and this may be particularly serious where it occurs in native ecosystems.