What is a host plant?
One question frequently asked is why are some plants attacked by a herbivore and other plants left untouched. Within this simplequestion are two linked questions,
- why are some plant species host plants?
- why are some plants within a host plant species infested, while other plants of the same species remain uninfested?
The Plant-SyNZ™ database is concerned with documenting the host plants of herbivores and this information may give clues as to why some plant species are hosts and others are not.
For the Plant-SyNZ™ database, a host plant is a plant 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 an explanation of the criteria used to decide if a plant is a host for a herbivore, go to the page explaining the use of a reliability index for the quality of this information. A more detailed discussion about criteria used for deciding if a plant species is a host of a herbivore can be found in the following paper: Ward LK. 1988. The validity and interpretation of insect food plant records. British Journal of Entomology and Natural History 1 (4): 153-162.
The 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 vulgar (groundsel) or S. skirrhodon.
Herbivore numbers can change during the year because of the breeding cycle of the herbivore, 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 eachof these factors.
- Plant genotype, i.e. plant resistance. 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 plants 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, e.g. 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 is not found in the wetter rainforest of the Waitakere Ranges.
- The weather can also affect the abundance of herbivores on their host plants, either creating a short term environmental change that 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.
- Natural enemies. Pathogens, predators and parasites can all attack and kill herbivores. They are one factor that can reduce herbivore populations. This is the assumption on which biological control research is based. When populations of herbivores are reduced by natural enemies, then fewer plants a likely to be infested, and the chance that a particular plant will be infested is less.
- Chance. Even when a group of plants are equally susceptible to a herbivore species, an individual herbivore still has to find a plant. There is always an element of chance about which plant is found and which is missed.
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