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Insect pests of crops, pasture and forestry

by  Alison Popay

Armies of insects can infest crops and pasture, causing millions of dollars’ worth of damage. Mites, weevils, grubs and beetles nibble the leaves or suck the sap of a range of crops and pasture grasses, and some carry diseases.


Endemic pests of pasture

Insect pests are a major problem for New Zealand’s primary industries, causing millions of dollars worth of lost production each year. A number of native insects have adapted to pasture plants and some horticultural crops to become significant pests.

Grass grub

In the 1930s ryegrass and white clover became the basis of New Zealand’s pastures. The root-feeding larvae of one endemic insect, the grass grub (Costelytra zealandica), adapted well to the new pasture from its original habitat in native tussock grasslands. It is now estimated to infest 9 million hectares of pasture land throughout New Zealand, and costs farmers in the dairy, sheep and beef industries between $41 million and $90 million annually in lost production.

Adult beetles are shiny golden to dark brown, and about 10 millimetres in length. They fly at dusk during October and November to feed on a range of trees. The beetles lay their eggs in soil, usually close to where they first emerged. Larvae feed on the roots of white clover and ryegrass, causing severe damage in autumn, particularly to pasture that is up to three or four years old.

Porina

Porina (Wiseana spp.) are also endemic insects that have adapted to pasture. Six species have been identified, the most common of which are Wiseana cervinata and W. umbraculata. Porina is a major pest in the central and southern North Island and throughout the South Island. Adult moths are light to dark brown, with a hairy body. They are short-lived, flying between spring and autumn depending on the species, with female moths releasing their eggs as they fly. They are strongly attracted to light. The grey-green caterpillars initially live on the soil surface, but after about six weeks tunnel underground. They live in these burrows, emerging at night to graze on foliage. After eight or nine months they reach their full size of up to 70 millimetres in length.

Mānuka beetle

An endemic scarab, the mānuka beetle (Pyronota spp.) inhabits hill country and farmland near bush margins. The small, bright-green adult beetles feed on mānuka and other native species, while the larvae resemble small grass grubs and eat roots of various pasture plants.

Pasture mealybug

The pasture mealybug (Balanococcus poae) is an endemic species that sucks the sap from the roots of grasses. It has been identified in several regions including Canterbury, Hawke’s Bay, Marlborough and the Manawatū. These small, pink, oval-shaped insects are largely sedentary and surround themselves with tufts of white wax, which help protect them in the soil.


Introduced pests of pasture foliage

Accidental introductions of pests from overseas have posed some major threats to New Zealand agriculture, especially in recent years. New Zealand has increased its efforts to detect potential pests before they become established.

Argentine stem weevil

The Argentine stem weevil (Listronotus bonariensis) arrived in New Zealand in 1927 and has become a major pest of ryegrass, estimated to cause annual production losses of $250 million. The weevil can be found throughout New Zealand and generally produces two or three generations a year. Adults are 3 millimetres long, light grey to dark brown, with a hard body. They feed on leaves, and lay their eggs in the stems of grass tillers (sideshoots). Larvae kill plants by burrowing into these tillers, eating out the central part.

Black field cricket

The black field cricket (Teleogryllus commodus) is a problem in some North Island areas. These insects shelter in cracks in the ground which have opened up in summer. They eat the foliage of various pasture plants, and grow to about 25 millimetres in length.

Tasmanian grass grub

Aphodius tasmaniae originates in Australia, where it is known as the black-headed cockchafer. In New Zealand it is called the Tasmanian grass grub. These small and shiny black beetles fly between January and March, and are attracted to fresh dung. The larvae dig tunnels in the soil, surfacing to graze on foliage, particularly clover.

Clover flea

Also damaging to clover is the clover flea (Sminthurus viridis) from Australia, which is a springtail that jumps readily when disturbed. Their numbers can rapidly increase in localised areas when the climate is favourable in spring and autumn. They cause serious damage to clover by grazing on soft leaf tissue.


Introduced pests of pasture roots and foliage

Clover root weevil

In 1996, established populations of clover root weevil (Sitona lepidus) were found at two sites in the North Island. Over the next few years this pest spread rapidly, and in 2006 was reported in the South Island. Adult weevils feed on white clover leaves, making notches around the edges. Early-stage larvae do considerable damage by eating the nodules on clover roots where rhizobacteria fix nitrogen. Older larvae feed on roots. The cost of this pest to the pastoral industry is estimated at $300 million per year.

African black beetle

The African black beetle (Heteronychus arator), another major pest of grasses, was first recorded in New Zealand in 1937, and is thought to have arrived from Australia rather than from its native South Africa. The species became established in the northern half of the North Island, then spread down the east coast into Hawke’s Bay and down the west coast as far as Taranaki. Adult beetles are 15 millimetres long and reddish-brown, changing to shiny black as they mature. They feed at the base of grass tillers, causing most damage to seedlings. The larvae do the most harm, by eating the roots of grasses in summer.

Root aphid

The root aphid (Aploneura lentisci) originates in the Mediterranean region and, like the pasture mealybug, sucks grass sap and produces a white wax. It is found throughout New Zealand. This insect has been given little attention until recently, but is now thought to contribute to failing ryegrass in pasture.

Lucerne pests

Attempts by farmers in the 1970s to use special pastures such as lucerne were hampered by pest problems, despite lucerne being resistant to grass grubs. Larvae of the white-fringed weevil (Graphognathus leucoloma), which was first recorded in 1944, and the sitona weevil (Sitona discoideus), first noted in 1974, feed on lucerne roots. Lucerne productivity and persistence were further affected by invasions of three aphid species, and lucerne is no longer widely farmed.

Parasitic nematodes

Parasitic nematodes are not insects, but they also damage the roots of clover and ryegrass. The clover cyst nematode Heterodera trifolii is found in most parts of New Zealand. The larvae penetrate the roots, where the hardening of a cuticle around female nematodes causes cysts to form.

Two species, Paratylenchus and Pratylenchus, feed on the root systems of grasses. Damaged roots lose the ability to absorb nutrients and water, resulting in diminished plant growth. The nematodes are invisible to the naked eye, so they often go undetected.


Control of pasture pests

DDT

DDT (Dichloro-Diphenyl-Trichloroethane) became available for agricultural pest control in the 1940s. It was a relatively cheap way to rid the soil of grass grubs, porina and other insects, and farmers used it liberally, often applying it indiscriminately by plane. By the late 1960s, grass grubs were showing resistance to DDT, and evidence that it accumulated in the soil and entered the food chain led to it being banned in 1970. In the next few years, farmers experienced a resurgence of grass grubs and other pests, probably as a result of the long-term suppression of their natural enemies caused by frequent application of DDT and its persistence in the environment.

DDT dangers

DDT was first manufactured in 1874 but was not used as an insecticide until 1939, when it was used to control mosquitoes spreading malaria and typhus. After the Second World War it was widely applied as an agricultural pesticide. Rachel Carson’s 1962 book Silent spring highlighted the dangers of DDT to the environment, and the chemical was banned in most countries by the 1980s. It is still used in low dosages to control malaria, although mosquitoes are becoming increasingly resistant.

After DDT

After the banning of DDT and all organochlorine insecticides, it became less economic to control pasture pests with insecticides, although they are still used occasionally.

Natural control of grass grubs

Research has focused on finding natural ways to control insect pests. Grass grub larvae are affected by a range of bacterial and protozoan diseases, which can reduce populations but do not prevent serious outbreaks of the grubs.

The bacteria Serratia entomophila has been used as a natural insecticide to reduce the build-up of grass grub populations.

Using parasites

Three parasites are used to control weevils:

  • Small braconid wasps (Microctonus spp.) lay their eggs in adult weevils and cause sterility. They are effective against weevils on lucerne, clover and ryegrass.
  • M. aethiopoides has helped control sitona weevils since it was introduced in 1982. Another strain was released in 2006 to help control clover root weevils.
  • M. hyperodae has been successful against Argentine stem weevils on ryegrass since releases began in 1991.

Plant resistance and tolerance

Some plants are tolerant to insect pests, or are avoided by them. Grass grub larvae do not eat lucerne, birdsfoot trefoil or phalaris, although these plants are not widely used by farmers. Tall fescue grasses can survive grass grub attack, unlike ryegrass and white clover.

Endophytes

In 1982 a fungus was discovered in ryegrass that protected it from Argentine stem weevils. This endophytic fungus lives entirely inside the ryegrass plant and is transmitted only via ryegrass seeds. It produces various alkaloids which may benefit the plant by being toxic or repellent to insects. However, the alkaloids in an endophyte strain brought into New Zealand in ryegrass seeds cause ryegrass staggers and other health problems in grazing livestock.

Introduced endophytes

A strain of endophyte, AR1, which occurs naturally in European ryegrass, was commercially released in New Zealand after inoculation into local cultivars. It protects plants from Argentine stem weevils and pasture mealybugs, but is not toxic to grazing animals.

The endophyte AR37, released in 2007, reduces ryegrass damage caused by several insects including Argentine stem weevils, African black beetles, pasture mealybugs, porina and root aphids, with minimal harm to livestock.


Arable and cereal crop pests

Cereals such as wheat, oats and barley are important grain crops in Canterbury. Arable crops are often grown as a feed supplement for pasture-fed stock and include maize, winter oats and a range of brassicas such as kale, rape and turnips. Brassicas are affected by the same pests that affect horticulture crops.

Maize pests

Maize is widely grown in the northern North Island, where pests that attack ryegrass, such as African black beetles and Argentine stem weevils, can also do considerable damage to young maize plants.

The greasy cutworm (Agrotis ipsilon) severs young maize shoots at their base. These pests are generally controlled by an insecticide coating around seeds to protect plants as they get established.

Cereals

Several aphid species are common pests of cereal crops. By sucking sap, aphids can stunt and distort plant growth. But they cause most damage as transmitters of barley yellow dwarf virus and cereal yellow dwarf virus. Associated crop losses have been estimated to cost Canterbury growers $3.7 million annually.

Wheat bug

The wheat bug (Nysius huttoni) is a common insect that infests a range of crops. In wheat, it pierces the grain and sucks out the nutrients. In the process it injects an enzyme, which causes dough made from the wheat to become sticky and of poor quality.


Apple and kiwifruit pests

Apples and kiwifruit are the mainstay of New Zealand’s horticulture industry, but they can be damaged by a number of insect pests, making the fruit unsuitable for export. Some insects are known as quarantine pests because affected fruit is quarantined by countries receiving it.

Leafrollers

Six species of leafroller are serious horticultural pests. Four are endemic and have adapted successfully to exotic fruit trees – they are the greenheaded leafrollers (Planotortrix octo and P. excessana) and the brownheaded leafrollers (Ctenopseustis obliquana and C. herana). The other two are the black-lyre leafroller (Cnephasia jactatana) and the light-brown apple moth (Epiphyas postvittana), originally from Australia. The apple moth also damages the leaves and fruit of berry plants and grapevines.

Leafroller moths are small, with a wingspan of 12–18 millimetres, and vary in colour from pale to dark brown. The larvae are green with a dark central stripe and two side stripes. Leafrollers are so called because they web together leaf edges or leaves and fruit to form a shelter to live in – often rolling the leaves into a tube. The caterpillars eat leaves and fruit. Leafroller damage is not tolerated on fruit for export.

Painted apples and Asian gypsies

Painted apple moths and Asian gypsy moths are not established in New Zealand, and strict biosecurity controls aim to keep them out. Either of these pests could do serious damage to private gardens and plantation forests. Parts of suburban Hamilton and Auckland were sprayed after single insects of these species were found.

Codling moth

Major pests of apples are caterpillars of the codling moth (Cydia pomonella), originally from Europe. Mature caterpillars are about 20 millimetres long, and pinkish-white with a brown head. Adult moths are a similar size to leafroller moths, but the forewings are grey and crossed with fine brown bands. Codling moth larvae scar the outside of fruit, but the damage caused by older larvae, which enter the fruit and feed on the core, is particularly severe.

Oriental fruit moth

Larvae of the oriental fruit moth (Grapholita molesta) damage stonefruit, in particular peaches, by penetrating the fruit.

Scale insects

Greedy scale (Hemiberlesia rapax), latania scale (H. latania) and oleander scale (Aspidiotus nerii) attack kiwifruit, while apples are affected by the San Jose scale (Quadraspidiotus perniciosis). Infestations are spread by the mobile, young scale nymphs or ‘crawlers’ – older nymphs and adults are sedentary.

To feed, scale insects insert a stylet into plants and suck the sap, producing sticky secretions which cause sooty mould and reduce plant vigour. Although damage is generally cosmetic, it is not tolerated on export fruit.

The species that affect horticultural crops are called armoured scale insects because of the protective shell covering older nymphs and adults. The shell makes them difficult to kill, but young crawlers are susceptible to insecticides, and older insects can be destroyed using mineral oil, either by itself or mixed with an insecticide. Encarsia citrina is an important parasite of armoured scale insects, and is used for biological control.

Other pests

The apple leafcurling midge (Dasineura mali) was first recorded in New Zealand in 1950 and has since spread throughout New Zealand. Larvae of this small fly feed on developing shoots, causing severe stunting. This pest is parasitised by the wasp Platygaster demades, which was introduced to New Zealand in 1925 to control a related insect, the pear leafcurling midge (D. pyri).

Woolly apple aphid

The woolly apple aphid (Eriosoma lanigerum) infests the roots, leaves and branches of trees. It sucks plant sap, and heavy infestations prevent plants from growing and thriving. This aphid is parasitised by Aphelinus mali. Previously, organophosphate insecticides suppressed woolly apple aphid as well as apple leafcurling midges, but both pests have become more numerous with the introduction of an integrated pest-management system of fruit production, which relies less on chemical sprays.


Other fruit and vegetable pests

White butterfly

Larvae of the white butterfly (Pieris rapae) eat the leaves of cruciferous crops such as cabbage and broccoli. Adult white butterflies are cream with black spots on the wings, and are a common sight in summer. The yellow, bullet-shaped eggs are attached to the underside of brassica leaves. Larvae are covered in fine hairs, and are pale yellow-green when young, darkening to dull green with a thin orange stripe as they mature. The parasite Cotesia rubecula was introduced to New Zealand in 1993 to help control them.

Diamondback moth

The diamondback moth (Plutella xylostella) is small and slender, with a row of yellow, diamond-shaped markings visible on the wings when they are folded along the body. Eggs are laid singly or in batches on the underside of brassica leaves, and hatch into greyish-green larvae which grow to about 10 millimetres in length. These caterpillars feed on brassica leaves, leaving many small holes.

Potato tuber moth

The potato tuber moth (Phthorimaea operculella) is a major pest of potatoes in some places. It has slender grey-brown wings which are heavily fringed with hairs. Eggs are laid on potato foliage or exposed tubers. The pink or greenish larvae grow to approximately 10 millimetres long and bore into the tubers, making them unsaleable.

Tomato fruitworm

The tomato fruitworm or corn earworm (Helicoverpa armigera) affects many plants, including tomatoes and maize. The damage done by caterpillars to flowers and fruit causes considerable financial losses. Adult moths have brown-to-orange forewings, while the caterpillars are variable in colour, ranging from pink to green to dark brown. The caterpillars move in a looping motion, devour leaves, and eat into tomatoes and corn cobs. Two parasites, Cotesia kazak and Microplites croceipes, have been introduced to control tomato fruitworms.

Thrips

Thrips are mobile insects that rasp the surface of fruit and leaves, puncturing plant cells to suck the sap. Affected leaves take on a silvered appearance. Thrips also transmit diseases.

The endemic New Zealand flower thrips (Thrips obscuratus) infests several stonefruit, including nectarines, apricots, plums and cherries. Their feeding causes russetting on fruit. Greenhouse thrips (Heliothrips haemorrhoidalis), first found in New Zealand in the 1930s, has a wide host range that includes avocados and citrus fruits. Onion thrips (Thrips tabaci) attacks vegetable crops including onions, lettuces and tomatoes.

Mites

Mites are not insects, but belong in the same taxonomic class as spiders and have eight legs. They are tiny and often too small to see.

Two species known as spider mites damage horticultural crops by feeding on plant juices, and their presence puts export crops in quarantine. The two-spotted spider mite (Tetranychus urticae), named for the spots on its back, is a ubiquitous pest that attacks orchard and greenhouse crops. The mites produce a large amount of webbing on the foliage they eat, and heavy infestations make leaves look bronzed. The European red mite (Panonychus ulmi) is predominantly an orchard pest and causes leaf discolouration.

The predatory mites Typhlodromus pyri and Galendromus occidentalis can reduce populations of these pests.

Aphids and mealybugs

Various species of aphid and mealybug suck plant sap, and heavy infestations may inhibit and distort new plant growth. They produce sticky secretions on which sooty mould can grow and diseases spread. Three species of mealybug (Pseudococcus longispinus, P. calceolariae and P. viburni) infest grapevines and spread leafroll virus, a significant disease in vineyards.


Control of horticultural insect pests

Chemical sprays

Until the 1990s, orchard and vegetable pests were controlled by regular spraying of broad-spectrum organophosphate insecticides, and acaricides were used for mites. This regime killed most pests and allowed export fruit to meet stringent quarantine requirements. However, the pests started to become resistant, and there was growing awareness that chemical residues were harmful to the environment and human health.

Broad-spectrum organophosphates

Organophosphates are manufactured from phosphoric acid and are the world’s most widely used insecticides. They were developed in Germany in the Second World War as a by-product of nerve gas, and act by disrupting the central nervous system in insects. They are also dangerous to humans, especially children, who, depending on the degree of exposure, may suffer mild flu-like symptoms or respiratory failure.

Integrated pest management

Many growers now use integrated systems, in which they carefully monitor pests and aim to control them using natural predators. If pesticides are used, they are designed to target certain pests with the least possible harm to the environment or people.

Sex pheromones

Pheromones are usually produced by female moths to attract males of the same species. Pheromones of several moth species have been isolated and synthesised, and are widely used in traps to check for the presence of pests, or to confuse male moths and disrupt mating.


Forestry insect pests

In general, insect pests are less of a problem to the forestry industry than they are for agriculture and horticulture.

Conifer pests

Looper caterpillar

Conifers such as Pinus radiata can be attacked by a native looper caterpillar, Pseudocoremia suavisi, which also feeds on native trees. Epidemics occurred in pine forests in Canterbury in the 1950s and 1960s, and in Douglas fir in the 1970s. Caterpillar populations have been reduced by a naturally occurring virus and improvements in silviculture.

Moths and butterflies

Tomato fruitworms and leafrollers occasionally defoliate pine trees. They are most problematic in tree nurseries, where weeds provide an extra food source.

Beetles

Adult beetles such as the mānuka beetle (Pyronota spp.), grass grub (Costelytra zealandica) and Odontria species feed on a range of trees including pine during their flight season. The root-feeding larval stage of these insects can be pests of nursery trees, as are larvae of the black vine weevil (Otiorhyncus sulcatus).

Wood-boring wasp

The wood-boring wasp (Sirex noctilo), introduced from Europe, was once considered to be a major pest in pine plantations. By removing weak trees susceptible to attack and introducing parasitic wasps for biological control, the problems caused by this wasp have been eliminated.

Monterey pine aphid

In 1998 the Monterey pine aphid (Essigella californica) became established in New Zealand, and by 2000 it was widespread. Its main host in its native California is Pinus radiata, and it has damaged drought-stressed plants in Australia. In 2008, after 10 years presence in New Zealand, this insect has had no visible effect on tree health.

Log-boring insects

Wood- and bark-boring insects can be a problem in logs after felling. Adults of Hylastes ater (black pine beetle) and Hylurgus ligniperda (golden-haired bark beetle) tunnel and lay their eggs in fallen logs, and the larvae feed on phloem (plant tissue that transports food around the plant). When adult H. ater emerge from the logs, they eat the roots and collars of seedling pines and kill them. This beetle is also a carrier of sapstain fungi, which causes wood discolouration and is a significant economic problem in New Zealand.

Eucalyptus pests

Eucalyptus trees are mainly colonised by Australian insects, with the number of species found having increased over the last century. In 2008, 26 specialist eucalyptus feeders had been reported, although relatively few had become pests.

Eucalyptus tortoise beetle

The eucalyptus tortoise beetle (Paropsis charybdis) was first recorded in New Zealand in 1916. The beetles and their larvae eat eucalyptus leaves and can severely defoliate new growth. Some eucalypt species are less susceptible to attack by this insect, and biological control has had some impact on populations.

Gum tree weevil

Other eucalypt defoliators are the gum-tree weevil (Gonipterus scutellatus) and the larvae of the gum emperor moth (Antheraea eucalypti). Sap-sucking pests such as a psyllid, Ctenarytaina eucalypti, and the scale insect Eriococcus coriaceus can stunt and distort tree growth.

Native species

Two native species – the caterpillars of Anetus virescens and the pinhole borer Platypus apicalis – drill into living trees, and can damage the timber.

Parasites and predators have been introduced to control these insects. Another pest, the leaf blister sawfly (Phylacteophaga roggatti), was first recorded in New Zealand in 1985. Four years later the mass rearing and release of the parasite Bracon phylacteophagus reduced numbers of the leaf blister sawfly.


Hononga, rauemi nō waho

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How to cite this page: Alison Popay, 'Insect pests of crops, pasture and forestry', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/mi/insect-pests-of-crops-pasture-and-forestry/print (accessed 14 August 2020)

He kōrero nā Alison Popay, i tāngia i te 24 Nov 2008