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Natural enemies of locusts
Egg parasites
Flies (Diptera)
Mites
Nematodes
Protozoans
Fungi
Predators
Further reading
Egg parasites
A number of species of tiny wasps, belonging to the genus Scelio, parasitise the eggs of locusts and grasshoppers. In some seasons they can cause very high egg mortality. Scelio fulgidus is the most important species and is found throughout the locust infestation area. It is a black wasp 3-5 mm long.
The female burrows through the froth plug on top of the locust egg pod and using her retractable long needle-like ovipositor she lays one egg in each locust egg. On hatching, the wasp grub feeds on the locust egg killing the developing embryo. Along with its host, S. fulgidus enters diapause in autumn. S. fulgidus remains in diapause longer than locusts do so the adult wasp emerges sometime after locust nymphs. Some of the S. fulgidus adults can remain dormant in the locust egg for a month or more before emerging. These two adaptations ensure that some Scelio adults are present when plague locusts reach the adult stage.
Flies (Diptera)
Various species of fly parasitise locust nymphs and adults. They are usually seen in higher rainfall areas and can kill or reduce the fertility of locusts. Blaesoxipha spp. (parasitic blowfly) is a common parasite but because of its rapid larval development (4-6 days), it is rarely found in more than 10% of locusts adults at a given time. The female fly deposits larvae on the locust; these minute maggot-like larvae then bore into it. The fully grown larva emerges through the locust body wall and pupates in the ground. The parasite has several generations a year. It usually does not kill the adult, but a parasitised female may lay fewer eggs. Nymphs generally die following emergence of the parasite.
Trichopsidea oestracea (bee-fly) - females of this fly scatter thousands of eggs on the ground or on standing dead timber such as tree stumps or fence posts. The resulting larvae move around freely until they find a locust or grasshopper which they enter via the spiracle (breathing hole). Only one larva develops inside each host; after emerging it burrows into the ground. Those which emerge in autumn (April) become adults late in the nymphal development period (October). There is a second generation in the summer. Very high levels of parasitism (60-80%) have been recorded. The parasitised locusts do not mature eggs and they usually die when the fly larva emerges.
Ceracia sp. (tachinid maggot) is a rare parasite that strongly resembles Blaesoxipha sp. The female fly glues her eggs directly onto the body of locusts. On hatching a small larva burrows through the base of the egg and enters the host where it feeds on non-essential tissues. When fully-fed (10-14 days after hatching) the larva emerges from the locust by pushing through the segments in the body. Pupation takes place in the soil, the adult fly emerging 12-15 days later.
Mites
Tarsonemid mites (tracheal mites) are frequently overlooked as they live on the inside of locust tracheae (breathing organs). Their eggs can be seen on the dorsal surface of the locust when its wings are expanded.
Leptus sp. (body mites) is a red mite larvae that is frequently found attached to the wings and bodies particularly of old locusts. The host is not affected, except perhaps when heavily parasitised.
Nematodes
Nematodes (round worms) are important parasites of locusts and grasshoppers in high rainfall areas but they are rarely found in the drier western areas of Australia.
Amphimermis is an important parasite of wingless grasshoppers (Phaulacridium vittatum). The eggs are laid in the soil and hatching is stimulated by rain. On hatching the worm larvae wriggle to the soil surface where they locate and penetrate a grasshopper or locust. They remain in the host for 4-5 weeks, retarding development and hindering sexual maturation. The host is killed when the nematode emerges to return to the soil.
Photo: M. Hill (From G. L. Baker, with permission from NSW Agriculture)
Mermis lay their eggs on the foodplant of locusts. When the locusts feed on the vegetation they also inadvertently consume the nematode eggs which hatch in the gut. The resulting nematode larva penetrates through the gut wall and feeds in the body cavity, eventually killing the locust on emergence.
Protozoans
Little is known about the protozoan parasites of locusts and they are frequently overlooked. Protozoans live as cysts attached to the lining of the mid-gut of locusts. The gut of a heavily parasitised host is yellow; a large number of parasites will interfere with digestion, reduce female fecundity or even kill the locust.
Fungi
Locusts infected by fungi are rarely seen in the field and this may be due to the general aridity of the environment where outbreaks occur. However, a strain of the fungus Metarhizium anisopliae isolated from spur-throated locusts has been successfully cultured in the laboratory and now forms the basis of biological control research program conducted jointly by CSIRO and the APLC (see Research page).
Predators
A variety of birds, mammals and insects predate on locusts but generally they are non-specific feeders and have little impact on locust numbers during an outbreak. Typically, their consumption capacity is limited and their appetite is quickly satisfied. Vertebrate predators, unlike many parasites, are unable to increase rapidly in numbers to take advantage of a locust outbreak.
Further reading
Baker, G. L. 1983. Parasites of locusts and grasshoppers. Agfact AE2, Dept. Agric. NSW, 16 pp.
CSIRO (ed.) 1991. The Insects of Australia. A Textbook for Students and Research Workers. Melbourne University Press. 2nd ed., 1137 pp.
Dangerfield, P. C., Austin, A. D. Andrew and Baker, G. L. 2001. Biology, ecology and systematics of Australian scelio: wasp parasitoids of locust and grasshopper eggs. CSIRO Publishing, Collingwood, Victoria, Australia, 254 pp.