Lesson 3 - Plant pests
Pests are a part of growing food. You just need to be aware of who might turn up to munch on your plants and be prepared. Some are simply pesky and can be deterred quite easily, some need a bit more work.
Plants are part of the food chain. They provide food for other living creatures (including us). In nature we can probably assume that every plant can be eaten by one animal or another, and that every plant part is potentially food for one or another organism. In many cases, animals that eat plants can, in turn, be eaten by others. When we choose to grow plants for our own purposes, almost inevitably we will end up competing with other herbivorous animals for that crop. In this context, we call the animals we are competing with pests.
In the specific case of Australia, some of the pests that we encounter have been introduced along with the crop plant (for example, cabbage white butterfly, onion aphid, harlequin bug), some were introduced either deliberately or accidentally and have become pests of crop plants (rabbit, garden snail, blackbird), and some are native animals that have adapted to feed on introduced plants (light brown apple moth, grapevine moth, Queensland fruit fly, cockatoo).
Arthropod pests
The Arthropoda contains animals that have external skeletons and jointed legs. In terms of pest species, the largest group of arthropods that are plant pests are the insects (Insecta – 6 legs), followed by Arachnida (mites (8 legs), and millipedes.
Life cycle differences
Most arthropods have a life cycle that goes: egg, larva, adult, egg. In some groups, the larval forms a small versions of the adult and moulting allows the larva to increase in size until adulthood is reached. In some pest insects (butterlies, moths, flies, wasps, beetles), the larval form is quite different to the adult (often a grub or caterpillar) and undergoes a dormant pupal stage with the adult emerging from the pupa after metamorphosis to the adult form. In some pests all forms of the emerged pest are capable of causing damage. In others only the larval form does this. The amount of damage caused by a pest tends to depend on the size of the animal, the number of animals, and the size of the host (smaller plants are more susceptible).
The table below sets out a list of arthropod pest groups, the type of damage they cause, and summarizes approaches to their management. Because there are so many potential pests, and so many hosts, detailed descriptions of all are not feasible but two case studies are given at the end of this lesson.
| Anthropod Pest Damage | ||
Pest type | Damage | |
Myriapoda | ||
Millipedes | All life stages can eat plant material, especially young seedlings. | Baited traps |
Arachnida | ||
Mites | Very small animals, often not visible to the naked eye as individuals. All life stages feed by piercing plant cells and sucking sap. Heavy damage interferes with leaf function and can be visually unappealing. Two spotted mite is a common pest, especially in greenhouse culture. | Oil sprays, miticide sprays, use of predatory mites, some of which can be purchased from suppliers. |
Insecta | ||
True flies Order Diptera | The larval stage can cause damage in pest species such as Queensland fruit fly. | Exclusion netting, baits and lure traps, hygienic removal and destruction of damaged fruit. |
Wasps Order Hymenoptera | There are relatively few pest species in this order but sawfly larvae (e.g. pear and cherry slug) eat foliage, and adult European wasps can cause severe loss in some fruit crops, as well as being dangerous during harvest. The citrus gall wasp is a serious pest of that group of trees. The damage they cause results from eggs being laid under the bark of the citrus tree and as the larvae grow inside the stem a swelling results. | Bait traps, netting, insectide sprays, predatory insects. |
Beetles Order Coleoptera | Both the larval stage (e.g. curl grubs, wireworms) and the adult forms (e.g. pumpkin beetle, vine weevil) of beetles can be pests. | Physical removal and destruction, insecticide sprays. |
True bugs Order Hemiptera | All life stages feed on plants. They have tubular mouthparts that allow them to feed by tapping into the sap stream of their host plants. Severe infestations lead to loss of vigour, damage to fruit, and distorted growth. Some species transmit plant viruses. Examples include scale insects, aphids, plant bugs, leaf hoppers. In some species, excess sugar is excreted as honey dew which causes sooty mould (not a disease, but a symptom of the problem. Ants also feed on honey dew and some ant species “farm” true bugs for their honey dew. | Oil sprays, insecticide sprays, fostering of predatory insect populations with diverse plantings, physical removal (fingers, high pressure water) in small infestations. |
Butterflies and moths Order Lepidoptera | Only the larval stage of life is a plant pest. The larvae (caterpillars) eat plant tissue (predominantly foliage, but some species eat or damage fruit (e.g. codlin moth, light brown apple moth, tomato budworm), and some species tunnel under the bark of trees | Netting, fostering predatory insects with diverse plantings, physical removal (fingers), pheromone reproductive disruptors, insecticides sprays. |
Grasshoppers/locusts Order Orthoptera | All life stages of these are plant feeders, eating green vegetation. During plague infestations serious damage can occur but this is relatively uncommon. | Netting, encouraging predatory insects and birds, insecticides. |
Molluscs
Snails and slugs are the major pest issues in this group. They feed on plant material and can causes significant losses, especially with emerging seedlings and small transplants.
Control of damage by these pests usually involves a multi-pronged approach. This can include removing hiding places adjacent to crop areas and destruction of any snails or slugs found hiding, hand collection at night when the pests are moving around, use of bait traps, spreading of barriers to mollusc movement (a wide band of diatomaceous earth has been shown to deter molluscs, but it must be kept dry), and the use of toxic baits to poison the pests. Some baits must be used cautiously as they are toxic to pets and humans. Mollusc baits are usually based on metaldehyde, or iron EDTA.
Mammals
This group includes rats and mice, rabbits and hares, possums, deer, goats, flying foxes, kangaroos and wallabies. All these potential pests eat plant material, the scale of the damage being related to how large the animal is. Rats and mice will eat emerging seedlings and some fruit. Larger animals can defoliate larger plants and even destroy whole plants.
Control of mammalian pests is difficult. Rats and mice can be trapped and killed. Baits can also be used for rats and mice but be aware that animals poisoned by baits can then kill birds that feed on carrion. For most pest animals exclusion (netting, fencing of various types and height) is probably the only successful long term control. The type of protection required will depend on the animal.
Nematodes
Nematodes are microscopic, soil-living unsegmented worms. They are completely unrelated to earthworms. There are thousands of species, many of which feed on decaying plant material or fungi or are predatory on soil animals. Some however feed on plant roots. Presence of large numbers of nematodes can reduce plant vigour and productivity. They are most likely to build up in large numbers in sandy soils, and in cropping situations where limited crop rotation is used (that is, the same crop is grown repeatedly in the same patch of ground). The root knot nematodes (Meloidogyne spp.) are common pests of a wide range of species of fruit and vegetables.
Management of nematode problems
Crop rotation ensures that there are breaks between crops of susceptible species, preventing the development of large populations of nematodes.
Solarization uses sunlight to heat the soil to temperatures high enough to kill nematodes. Moist soil is tightly covered with a clear polyethylene sheet. The treatment requires high summer temperatures and takes about one month. Solarization can also be used for some insect, fungal disease, and weed problems).
Biofumigation uses cover crops of plants that release volatile biocidal chemicals as they decompose after incorporation into the soil. Sulphur compounds (isothiocyantes) from members of the Family Brassicaceae (e.g. mustards) are a good example of these. Typically the cover crop is grown and incorporated into moist soil at flowering. The soil is then covered with polyethylene sheet for four weeks to retain the volatiles. Biofumigation can also be used for some fungal diseases.
Maintaining good soil health through increase and maintenance of soil organic matter fosters populations of antagonistic microorganisms and predators that can help suppress nematode numbers.
In some species of crops, nematode tolerant varieties exist, or tolerant rootstocks can be used.
Birds
Many bird species can be pest problems in productive gardens. While small birds like sparrows and blackbirds can eat young plants, the most serious damage is usually the eating of fruit. Serious fruit-eating pest birds include blackbird, starling, Indian mynah, white eye, currawong, raven, and parrots and cockatoos. In areas with serious bird pest problems, complete crop loss is not uncommon. Various methods, such as bird scares, have been used to try and manage this issue but realistically, in most situations netting is required. Netting can be a temporary protection for new seedlings, or for berry crops, or even for dwarf fruit trees. The netting is removed after the crop is harvested. For larger trees, permanent netting covered frames or trellises may be required.
Diggers tip
Be aware that some states and territories, there are requirements that netting be of certain dimensions to minimize the chance of birds being trapped in the net. In Victoria and the ACT, legislation requires that tree netting used in domestic environments has a maximum mesh gap size of 5 mm at full stretch. White netting is preferred for visibility. No other state or territory has this legislation although gardeners in those states and territories are encouraged to use netting that is visible to birds and bats and has small mesh gaps. While shooting and trapping of birds is feasible, native bird species are virtually always protected, and these methods cannot be used on them.
Queensland fruit fly
Queensland fruit fly (Bactrocera tryoni) is a native insect that is found in NT, QLD, NSW, and VIC. It is a serious pest and causes major crop losses in susceptible fruit species. This list includes many fruit trees, and some vegetables such as tomatoes and peppers. The insect overwinters in soil as a pupa, emerging in Spring. The females lay eggs under the skin of developing fruit and the larva (small white maggots with brown heads) which hatch out and destroy the fruit from the inside.
Management of the pest is difficult and involves a number of approaches. For small growers, exclusion netting with insect proof fine nets is probably the most effective method as it prevents the eggs being laid if the host is covered early enough. Other approaches include protein traps for females, pheromone traps for males, and hygiene through the removal and destruction of damaged fruit and removal of unwanted or unused host plants. Insecticides can be used to provide protective layers over the fruit. The organic approved insecticide spinosad is one treatment but remember that it will affect other insects as well. In some places fruit fly pressure may cause growers to not attempt some crops.
Cabbage white butterfly
The cabbage white butterfly (Pieris rapae) is a serious pest of members of the Brassicaceae (broccoli, cabbage, cauliflower, kale, Brussels sprouts). The white butterflies hatch out of their pupal stage in Spring and are active over the summer, laying eggs on host plants. The eggs hatch into small green caterpillars, that increase in size, eating plant foliage. They can be hard to see when small because of their colour, but leaf damage is obvious, and fecal pellets in leaf axils are also a clue. A severe infestation can result in complete destruction of a crop, and late summer is especially difficult. (Figure 10)
Pest management involves monitoring for the presence of the butterflies, early identification of caterpillar hatchings, hand removal of caterpillars in small crops, exclusion netting using fine mesh netting (very effective control if egg free seedlings are planted and butterflies can’t get under the nets), and the use of insecticides to kill caterpillars. Bacillus thuringiensis (Dipel®) is probably the ideal material to use as it only affects caterpillars. It is sprayed on foliage early in the season to kills caterpillars as they hatch and start feeding. Other “soft” chemicals include pyrethrum, and the soil bacterium-derived spinetoram (both have broader spectrum effects and can affect other insects, including bees).