CONTROL OF INSECT PESTS
The main methods of controlling insect pests are:
By Chemicals: Since 1945, when DDT was introduced commercially to New Zealand, there have been many types of organic insecticides produced, the number growing with each year. DDT is the most common, about 1,200 tons of technical material having been used for control of grass grub, Costelytra zealandica (Wh.), in 1960–61. DDT is one of the chlorinated hydrocarbons; others of this class are DDD, lindane, dieldrin, and aldrin. The chlorinated hydrocarbons are nerve poisons, the first symptoms in poisoned insects being tremulousness of the entire body and its appendages, a condition known as “DDT jitters”. The second large class of organic insecticides contains the organophosphates, such as parathion, malathion, and gusathion. This group also includes insecticides that are systemic in their action, that is, they are taken up in the sap stream of the plant to which they may be applied and may kill insects (aphids, for example) through their feeding on the sap of the plant. Systox and metasystox are examples of such insecticides. Organophosphates are also nerve poisons, but they have a specific action which is not possessed by the chlorinated hydrocarbons. They inhibit the formation of cholinesterase, which is an enzyme necessary to the normal functioning of nerve tissue. Other organic insecticides that have appeared in recent years are the carbamates, such as sevin, and crotonates, such as karathane (acaricide).
Lead arsenate, once a very important insecticide, is now much restricted in use as is lime sulphur. Petroleum oils are still used, but there is now a tendency to rely on lighter fractions at or soon after bud movement instead of the older winter oils. Insecticides of plant origin, such as nicotine, derris, and pyrethrins, are still considerably used. An insecticide of vegetable origin new to New Zealand is ryania, which is just now being tried experimentally against codling moth, Cydia pomonella (L.). There are also fumigants, such as hydrocyanic-acid gas and methyl bromide, which are used to treat, for example, infested grain and similar materials.
A phenomenon that has appeared throughout the world since the advent of DDT is insect resistance to insecticides. Through repeated applications of chemical, certain insects, especially those with short life cycles, produce strains upon which the chemical is no longer effective. In New Zealand instances of this have appeared in the European red mite, Panonychus ulmi (Koch); the leaf roller, Epiphyas postvittana (Walk.); and the leaf hopper, Edwardsiana australis (Frogg.).
By Biological Means: This is commonly called “biological control”. Insect pests are controlled by parasites or predators, usually purposely introduced from overseas. New Zealand appears to be particularly suited for this kind of work as many of our pests are introduced species that have entered the country without their natural enemies. Several examples of this kind of control will be given: (1) The ladybird, Rodolia cardinalis (Muls.), on cottony cushion scale of citrus; (2) Aphelinus mali (Hald.) on woolly apple aphid, Eriosoma lanigerum (Hausm); (3) The parasites Pteromalus puparum (L.) and Apanteles glomeratus (L.) on white butterfly, Pieris rapae (L.); (4) The parasites Angitia cerophaga (Grav.) and Diadromus collaris (Grav.) on diamond-back moth, Plutella maculipennis (Curt.); (5) The parasite Prosactogaster demades (Wlk.) on pear midge, Dasyneura pyri (Bouche); (6) The parasite Habrolepis dalmanni (Westw.) on the golden oak scale, Asterolecanium variolosum (Ratz.); (7) The latest successful case is the control of the oak leaf miner, Lithocolletis messaniella (Zell.), by the parasites Apanteles circumscriptus (Nees), and Enaysma splendens (de L.).
In addition to insects, there are other agents of biological control, such as protozoa, bacteria, nematodes, virus, and fungi. The viruses are especially interesting because there are native species which are quite potent. In particular, there is one on white butterfly, Pieris rapae (L.), that, in the North Island at least, helps largely to control this pest on field crops of crucifers. At present it comes rather later in the season than is desirable and attempts are being made to introduce it earlier into the field. Then there is a bacillus, namely, Bacillus thuringiensis, (Berl.), produced commercially, which has some promise of controlling certain pests; it might, for example, be a satisfactory control of white butterfly and diamond-back moth on crucifers.
By Breeding of Resistant Varieties of Host Plants: The outstanding example of this is the resistance shown by Calder variety of swede to the cabbage aphid, Brevicoryne brassicae (L.). There is also a variety of rape into which this resistance to the aphid has been bred from the swede. And there are indications that some varieties of carrot are more resistant than others to the rust fly, Psila rosae (F.), and the aphid Cavariella aegopodii (Scop.).
By Sterilisation in Cold Storage: This method is applied to sweet oranges imported from South Africa. In the latter country there are the two species of fruit flies, the Mediterranean species, Ceratitis capitata (Weid.), and the South African species, C. rosa (Karsch.). It has been found that under certain conditions of transport at 31°F in the cold chambers of ships none of this fruit-fly material survives the journey from South Africa.
By Arranging Suitable Planting Dates: Seedling crucifers, especially in the South Island, if present when the adult beetles of the grass grub, Costelytra zealandica (Wh.), are about, are liable to be severely damaged. In this instance the pest is known as the “turnip fly”. The remedy is to delay planting until the main flights of the beetle are over, this safe period being usually about Christmas or the end of December.
Hygiene: The proper disposal of vegetable rubbish in which house flies could breed is of the utmost importance. Other control methods are mechanical measures by using heat, cultural practices or management, and by legislation (quarantine).
The chemical method is probably the commonest and main means of insect control, especially in such crops as fruit, vegetables, and tomatoes, but it has some disadvantages. First, the materials used are usually poisonous to human beings and leave residues on food; secondly, they cause resistance to chemicals to appear in certain plant pests, thus making controls more difficult; and thirdly, they are usually harmful to insect agents of biological control. It appears to be advisable, therefore, to make the most of the biological as well as the chemical method. It is important today that they should be “integrated”; that is, the chemical method should not be used so that it eliminates the biological one; rather, the aim should be to make control the sum of both methods.
by William Cottier D.SC., Late Director, Entomology Division, Department of Scientific and Industrial Research, Nelson.
- Plant Protection in New Zealand, Atkinson, J. D., et. al. (1956)
- The Diseases of Farm Animals in New Zealand, Reid, H. A. (1923)
- D.S.I.R. Bulletin No. 120 (1956), “Bibliography of New Zealand Entomology”, Miller, D.
- N.Z. Dept. of Agriculture Bulletin, No. 244 (1951), “Grass Grub and Grass Caterpillar Control”, Kelsey, J. M.
- Ent. Res. Sta. Publication No. 14 (1955), “Control of Grass Grub and Subterranean Caterpillar”, Kelsey, J. M.