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Abstract

Biological Control of the Prickly-pear Pest.

Abstract

A few years ago it was estimated that over 22, 000, 000 acres in eastern Australia were infested with prickly pear, or one-third more than the total cultivated area in the continent.
Attempts have been made to control this pest by legislation, by mechanical methods, by chemical means, and, to a slight extent, by biological agencies. The most effective, so far, has been the chemical method, but its cost is frequently prohlbitive, and it does not afford protection against fresh invasions of the plant.
The biological method has not yet been given a fair trial. As a result of the work of the Travelling Commission appointed by the Queensland Government, two natural enemies, Dactylopius (Coccus) confusus indicus and D. (C.) confusus capensis were introduced [R.A.E., A, iii, 125] and became established. It is unfortunate that they attack Opuntia monacantha only, to which the former is so injurious as to effect eradication in all districts where it has access to this cactus. It is posslble that D. (C.) confusus indicus may be a native of South America, and that the species descrlbed from Argentina, Coccus argentinus, may be synonymous. It is also posslble that D. confusus indicus may not normally attack O. monacantha in South America, but that it may infest some other species of cactus in a less injurious manner.
The above-recorded success led the Commonwealth Institute of Science and Industry to invite the co-operation of the Governments of the Commonwealth, New South Wales and Queensland, and a scheme, which came into operation officially in June 1920, was devised to provide for the collection and shipment of desired material from North and South America.
The author brought some natural enemies from South America, the chief being two fungi, Sclerotinia cactaceantm and Montagnella opuntiarum, and a Syrphid fly. The fungi are being experimented with; the flies that were bred from the imported Syrphid larvae have failed to reproduce themselves.
A collection of cactus insects, fungi and bacteria that has been recently received from southern Florida and Texas includes the moth-borer, Melitara prodenialis; the weevils, Gerstaeckeria hubbardi, G. nobilis, G. porosa, G. clathrata, and G. basalis; the bugs, Chelinidea vittigera and another species of this genus; one or two kinds of wild cochineal insects; three or four species of scavenging flies which breed in injured plants- Volucella esuriens, V. fasciata, Copestylum marginatum and a large dark-coloured species of Hermetia; four fungi, Gloeosponum lunatum, Hendersonia opuntiae, Phoma sp. and Perisporium wrighti; and a bacterial rot.
A consignment of material from Argentina, which should reach Brisbane shortly, contains the Argentine cochineal insect, as well as the fungi, Sclerotinia and Montagnella, mentioned above, and the moth-borer, Cactoblastis (Zophodia) cactorum, the attempted introduction of which by the Queensland Travelling Commission had been unsuccessful.
The laboratory has also received some wild cochineal, Coccus tomentosus[Dactylopius opuntiae], from California. It is apparently the same as the Texan form recently received and has attacked three Queensland prickly pears: the common prickly pear, the spiny pear of the Burnett and Rockhampton districts, and the tree pear (O. tomentosa).Up to the present its effects have been negliglble, as also are those produced by some of the cochineal insects from recent consignments.
Apart from the bacterial disease now being carefully studied, the most important enemy received seems to be Melitara prodenialis. The South American Zophodia should have a similar effect. The larvae of these moths bore into the joints and feed there. The Queensland prickly pears are readily attacked by these borers, which-like all the other insects mentioned here-restrict themselves to cactaceous plants. Until the hlbernating larvae of Melitara breed, it will not be posslble to know whether the importation has been successful.
The bugs (Chelinidea), weevils (Gerstaeckeria)and cochineal insects all appear to be breeding satisfactorily. Though scavenging flies emerged in large numbers from the Florida and Texan material, nearly all have died, and none have bred. Extreme care was taken in Texas and Florida to eliminate all parasitic or predatory insects detrimental to the organisms that have been imported into Queensland. The South American fungi have not responded well to the cultural methods; in Argentina they are very effective.
In this attack on the prickly-pear pest the author is endeavouring to utilize organisms acting in various ways: -(1) Insects that actually eat the plant (e.g., moth-borers, weevils, etc.); (2) insects that suck the juices, either weakening or poisoning the plant (cactus bugs, cochineal insects); (3) insects that attack the fruit (e.g., certain midges, Cecidomyia (Itonida), Asphondylia, not yet imported); (4) insects, such as scavenging flies, that continue destructive work such as is done by those of the first group and passively assist the introduction of saprophytic fungi and bacteria; (5) actual disease-producing agents, such as fungi and bacteria.
Such of the organisms established in the laboratory as appear unlikely to threaten economic plants will be studied in the field in prickly-pear centres and then distrlbuted if satisfactory.
It is perhaps unwise to expect that the new arrivals will be so destructive to the Queensland prickly pear as the Indian cochineal insect has been to O. monacantha, but it is hoped that they will be sufficiently effective to leave only a remnant of the pest that can be dealt with without difficultv.