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?
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 miners 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 their breeding cycle, but infestation rates can also vary between plants at a similar stage of the herbivore's annual cycle.
Why are some plants of a host plant species infested and others not?
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 a herbivore. Those on which a 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. 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, grows can affect its suitability as a host for a herbivore. For ecample, the climiate in an area of the plant's geographic range may not be suitable for the herbivore. This may be due to temperature or humidity preferences, 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 Waitākere 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 on the long-term natural changes in abundance of New Zealand indigenous herbivores. The growing medium, 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 can 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 be more susceptible to some scale insects (e.g. Icerya purchasi (Hemiptera: Margarodidae)). It also appears that the location of leaves or twigs on a tree can affect their 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.
What is biostatus?
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 arrived since human occupation as a deliberate or accidental introduction.
Biostatus information for each species indicates their origins and the kinds of places they occur in New Zealand. Various terms are used for both origins and occurrence. 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 Manaaki Whenua – Landcare Research BiotaNZ database uses separate terms for occurrence in New Zealand and the origin of a taxon. View the glossary for the definitions of these terms.
When are herbivores pests?
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 our biodiversity and are not pests in native ecosystems. Like native birds they 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, so 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 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 the 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.
Why do some images not load?
Some pages contain a large number of images. Occasionally this may result in some images not loading before they time out. If this occurs you can try refreshing the entire page or, alternatively, right click on the small icon that indicates an image should be shown but did not load, then select Load image.