Often misunderstood and unjustly maligned, these ancient insects have a rich evolutionary history spanning hundreds of millions of years. Some scientists suggest that as early humans migrated out of Africa to explore and settle in different parts of the world, these resilient insects likely accompanied them, thriving in new environments alongside human populations. This long history of coexistence has enabled pest cockroaches to become exceptionally efficient at exploiting human habitats for food and shelter. From the resilient pests that scuttle through our homes to the vibrant and diverse native species thriving in Australia’s unique landscapes, cockroaches exhibit incredible adaptability and survival skills.

Cockroaches

Cockroaches are an ancient group of insects that have remained relatively unchanged for hundreds of millions of years. They have a flattened body, long antennae, and bristly legs. Equipped with specialized sensors called ‘cerci’ (rear-mounted antennae-like structures), they are highly attuned to their surroundings. Any hint of danger sends them scurrying away. They are also remarkably versatile feeders, capable of eating almost anything, which gives them an edge in competing for food.

There are over 4,600 species of cockroaches known, with approximately 450 of them belonging to the Blattidae family. These species are typically large, ranging in length from 20 to 100 mm, and are brown or black in colour. They have two pairs of wings, with the front pair toughened and leathery and the back pair membranous.

Cockroach Lifecycle and Reproduction

Most cockroaches lay their eggs in a protective casing known as an ootheca. This ootheca is often carried by the female for some time before being placed on the ground or buried in the soil for further development.

However, cockroaches in the families Blaberidae and Blattellidae exhibit some unique reproductive strategies. In some species within these families, the female expels the ootheca but then retracts it into a uterus or brood sac where the eggs continue to develop. Other Blaberidae species bypass the ootheca stage entirely, allowing the eggs to move directly from the ovipositor into the uterus for development. Additionally, the species Pycnoscelus surinamensis is known for its parthenogenetic reproduction, where females can produce offspring without mating with males, although males are occasionally born.

Some female cockroaches can produce over 20,000 offspring in a year. Their sheer numbers and indiscriminate feeding habits mean they can potentially spread disease-causing organisms, although there is little concrete evidence to support this. However, they do cause strong allergic reactions in some people.

Cockroaches undergo incomplete metamorphosis, comprising three life stages: egg, nymph, and adult. The lifecycle begins with the female laying egg cases called oothecae, which contain multiple eggs. These egg cases are often hidden in protected environments. Upon hatching, nymphs emerge as smaller versions of adults without wings. As they grow, they molt several times, gradually developing wings and reproductive capabilities. The number of molts can vary among species. Cockroaches are prolific breeders; some species can produce several generations in a single year, contributing to their rapid population growth and persistence in various environments.

Australian Cockroaches

Australia can boast native cockroaches, most of which are rarely seen by those who do not venture far from urban areas. Many cockroach species are wingless in one or both sexes, and some have reduced wings. Those with wings often take flight when disturbed. The size of Australian cockroaches varies significantly, ranging from 3 mm to 70 mm in length.

While many species are predominantly dark brown and flattened from top to bottom, particularly the pest species, others are brightly colored with hues of orange, red, yellow, and even blue. These vivid species are often diurnal, found basking on leaves and bushes in sunlight. Notably, one Australian species, Polyzosteria mitchelli, features transparent windows in its upper skin, although the purpose of this unique trait remains unknown.

1. Australian Cockroach (Periplaneta australasiae)
This species is often mistaken for the American cockroach but can be distinguished by the yellow markings on its thorax and wings. The Australian cockroach is typically found outdoors, favoring warm and humid environments such as greenhouses and gardens. It is known for its ability to fly and its rapid movement.

2. Bush Cockroach (Ellipsidion humerale)
The Bush cockroach is a native species commonly found in leaf litter and under bark in forested areas. Unlike pest species, it does not invade homes. It has a distinctive light brown to tan coloration with a pale, almost translucent appearance.

3. Giant Burrowing Cockroach (Macropanesthia rhinoceros)
Also known as the rhinoceros cockroach, this species is one of the heaviest cockroaches in the world, weighing up to 35 grams. It is a wingless, burrowing species found in Queensland. It feeds on dry leaves and can live up to 10 years.

4. Smokybrown Cockroach (Periplaneta fuliginosa)
This species is dark brown to black and often found in warm, moist environments such as tree hollows and compost heaps. The Smokybrown cockroach is a strong flier and is attracted to lights at night. It is more commonly found in outdoor settings.

5. Mitchell’s diurnal cockroach (Polyzosteria mitchelli)
Known for its unique transparent windows in its upper skin, Polyzosteria mitchelli is a day-active species often found on leaves and bushes. It displays striking colors and is a good example of the vibrant diversity among Australian cockroaches.

Several species are brightly colored, featuring hues of orange, red, yellow, and blue, which is unusual compared to the typically dull-colored pest species found globally. A significant number of Australian cockroach species are wingless, which differentiates them from many other global species that rely on flight for escape and dispersal.

Australian cockroach species are highly adaptable, capable of thriving in diverse environments from rainforests to arid regions. The large yellow roach that comes flying around the room to the light in Sydney is Periplaneta americana, an introduction from America that has almost driven the smaller indigenous Periplaneta Australasiae out of our houses; whereas in some southern and eastern states of America, our Australian roach has been introduced and has become the common domestic pest. The German Roach, also known as “The Croten Bug of America”, Phyllodromia qermanica, is occasionally found near Sydney wharves. Many of these bush and house roaches have glands at the tip of their abdomen from which they can emit a foetid odour as a form of defence when disturbed. Most of our common native forms are wingless and live under rotten logs or stones; some of the largest species can be found in the dry interior.

Some species, like the Giant Burrowing Cockroach, have adapted to life underground, using their strong legs to burrow and create extensive tunnel systems. Unlike their pest counterparts, many native Australian cockroaches prefer natural habitats such as forests and grasslands and are not typically found in human dwellings.

Cockroach oothecae are often parasitized by various wasp species. Some of these wasps produce a single adult per ootheca, while others produce multiple offspring from a single ootheca. Adult cockroaches face predation from a range of invertebrates, birds, lizards, frogs, and mammals, and they can also be parasitized by roundworms and wasps. To defend themselves, some cockroach species secrete defensive chemicals to deter attackers, while others can roll into a ball for protection.

One native cockroach species is listed as endangered: the Lord Howe Island wood-eating cockroach (Panesthia sp). This wingless, burrowing species may have been driven to near extinction following the introduction of rats to Lord Howe Island.