There are many ‘good bugs’ that naturally occur within horticultural systems and supress pest species, and many other commercially mass reared that can be released for specific pest problems. The Australasian Biological Control Association lists all the suppliers of beneficial arthropods for use in horticulture, agriculture and turf in both Australia and New Zealand. It also provides tables listing all the commonly used chemicals to a range of beneficial species.
The ‘good’ arthropods that naturally occur within orchard systems to supress pest species in the trees can be categorised as predators or parasitoids. Predatory arthropods can belong to several different groups including beetles, bugs, spiders, ants, lacewings and flies. Parasitoids are most commonly from the wasp group but includes some flies as well. Parasitoids lay eggs on or within the body of another arthropod and their developing larvae feed on the prey, eventually killing it. Parasitoids are often limited/targeted to a handful of host species, while predators usually have a wider host range.
Common Predatory Arthropods in Orchard Environments
Spiders are a large group of predators with widely varied hunting strategies that are often highly abundant in orchard ecosystems with reduced pesticide use. Spiders are generalist predators, meaning they are not highly selective in the prey that they target, but instead consume a range of prey from different arthropod taxa. This makes spiders particularly useful as biological control agents because they can wait out period of low ‘pest’ abundance by eating alternative prey.
Spiders can be grouped in several different ways, but for our purposes we can group them as either active hunters or web-builders. Within these groups is a diverse range of behavioural strategies that results in some degree of prey-specialisation. For example, some web-builders will spin webs at night while some spin webs in the day, thereby catching different guilds of prey (i.e. moths at night, vs. flies and bugs during the day). Active-hunting spiders have their specialised niches too, with some occupying territory on the tree trunks, some on the ground, and others specialising in ambush among the leaves and flowers.
In orchard environments, web-building spiders may be important predators of fly-in heteropteran pests such as fruit spotting bug, whilst active-hunting spiders such as Salticidae and Thomisidae may be important predators of leaf-hoppers, aphids, lace bugs, thrips and mites.
The most commonly found predatory beetle found within tree canopies are the ‘ladybird beetles’ (family Coccinellidae), but there are other predatory beetles that hunt at ground-level, including ground beetles (family Carabidae) and rove beetles (family Staphylinidae).
In Australia, ladybird beetles, or ladybugs, are typically small, round to oval and dome-shaped with distinctive colourful markings. Many are orange and black, or yellow and black, but several are mostly black or black and red. Both adult and larval stages of ladybird beetles are voracious feeders of aphids, and are highly effective predators where aphid populations are abundant.
True bugs are a large group of insects which includes many plant pests. There are, however, many true bugs that are important predators. These range in size from the minute pirate bug to the larger Assassin bug. All true bugs undergo a gradual metamorphosis, usually through five instar stages in which they shed their skins until they develop wings and become an adult.
Minute pirate bugs, Orius tantillus (Hemiptera: Anthocoridae), feed on thrips, during both the adult and immature life stages. If thrip numbers are low, they can survive on alternate prey such as aphids, mite, whitefly and moth eggs. Adults grow to approx. 2.5 mm long and are predominantly black with light wing patches.
Big eyed bugs, Geocoris and Germalus species (Hemiptera: Geocoridae), feed on a variety of soft bodied insects and mites. As adults they are approximately 3 mm long and are distinguished by their very large eyes.
Damsel bugs (Hemiptera: Nabidae) are medium sized (adults are approx. 8 mm long) and feed on the immature and adults stages of other hemiptera, as well as caterpillars and other small arthropods. Damsel bugs are fast moving and hunt within the canopy of the crop. Many damsel bugs catch and hold prey with their forelegs, similar to mantids.
Assassin bugs (Hemiptera: Reduviidae) are large (adults are 25-30 mm long) and typically hunt in the plant canopy. They attack any soft bodied arthropod, including other bugs, caterpillars, bees, wasps and spiders. Assassin bugs use their rostrum to catch and pierce their prey. They ‘suck’ out their prey from the inside, leaving a dry, empty skin/shell.
Black mirids, Tytthus spp. (Hemiptera: Miridae), feed on the eggs of various planthoppers. Depending on the species, adults range from millimetre-long species up to 4 mm in length. They are hard to spot because they are so small, but adults have dark heads and the eyes have a yellow dot on the inside edge.
Predatory shield bugs (Hemiptera: Pentatomidae) in Australia include several species, the most common of which are the spined predatory shield bug, Oechalia schellenbergii, and the glossy shield bug, Cermatulus nasalis nasalis. Both species feed on a range of important plant pests, primarily caterpillars but also weevils and chrysomelid beetles. Adults range from approximately 10-20 mm in length.
Minute pirate bug (Hemiptera: Anthocoridae)
Big eyed bug (Hemiptera: Geocoridae)
Damsel bug adult (Hemiptera: Nabidae)
Assassin bug (Hemiptera: Reduviidae)
Predatory shield bugs (Hemiptera: Pentatomidae)
Ants are a well-known group of insects that are prevalent in crop environments. In terms of their feeding they can be foragers, scavengers and predators, and most are omnivores (eating both plant and animal). There are many ant species that are exclusively ground-dwelling but there are many whose workers forage mostly on plants as predators as well as gathering exudates or attending Hemipterans such as aphids for sugary-secretions.
In tropical/subtropical orchard environments, ants dominate the tree canopy and are often key predators of moth eggs and larvae. In temperate regions, ants are mostly absent from trees. Hunting strategies can be either solitary or cooperative. Groups of ants can hunt cooperatively in a swarm-type situation, enabling them to overwhelm large prey item.
Green lacewings (Chrysopidae) and brown lacewings (Hemerobiidae) are highly beneficial within agricultural crops because they feed on a wide variety of plant pests such as lace bug, thrips, aphids, scale, mites, mealybugs, moth eggs and caterpillars. Lacewings persist in cool temperatures, thereby lowering pest populations during winter and reducing the lag in biological pest control during early spring.
It may be surprising to find out that some flies are predatory! Robber flies are opportunistic predators, with most of their diet comprising of other flies, beetles, wasps, bees, true bugs and moths/caterpillars. Robber flies seize their prey in flight and inject their victims with saliva containing neurotoxic enzymes. There are many species of robber flies with a range range in size, but on average they are between 9 and 15 mm in length.
Hover flies are a wonderful insect to have in your crop because the adults commonly act as pollinators (and look very much like small bees, without the sting), whilst the larvae commonly feed on aphids, scale, thrips and caterpillars. The larvae are flattened, legless and maggot-like, mostly green or brown in colour, however they are often mistaken for pest caterpillars. They can be easily distinguished from caterpillars because they lack a defined head capsule.
Parasitoids Often Found in Crops and Gardens
Locally occurring parasitoids can be highly significant in suppressing pest populations. Their activity – especially that of egg parasitoids – is likely to go un-noticed, unless you have been shown what to look for.
Trichogramma wasps are found in most crops where there are moth pests and where spraying is minimised.
Telenomus species are another group of egg parasitoids- larger than trichogramma they lay just one egg into a heliothis moth egg and are important early season parasitoids.
Microplitis are larval parasitoids with one parasitoid emerging from one heliothis caterpillar.
Cotesia are parasitoids of caterpillar larva and produce characteristic bundles of white or yellow cocoons.
Orgilus lepidus is a major parasitoid of potato moth larva.
Copidosoma is also a potato moth larva parasitoid.
Diadegma is an important parasitoid of cabbage moth larva.
Tachinid fly species are many and their maggots have numerous hosts, from bugs to caterpillars.
There are many species of aphid parasitoids other than Aphidius colemani which are very active in crops.
Aphytis species are mass reared for red scale in citrus and Metaphycus for soft scales but there are many other species active in crops.
Leptomastix dactylopii has proved to be a very effective parasitoid for citrus mealybug and is commonly found in orchards. It used to be mass reared but an unidentified pathogen in mass culture has caused production to cease.
Tachinidae is a family of flies who larvae are parasitic on many different arthropod groups, primarily Lepidoptera (moths and butterflies), Coleoptera (beetles), Hemiptera (true bugs) and Orthoptera (grasshoppers, locusts and crickets). Photos below show a Trichopoda fly (family Tachinidae) whose eggs are laid on Green Vegetable Bugs (see white eggs). The hatched larvae will parasitise late instar and adult Green Vegetable Bugs.
This Green Vegetable Bug egg parasitoid was introduced from Egypt in the 1940’s. It has been important in suppressing Green Vegetable Bug populations in field crops, especially soy beans.