Biological control of prickly pear has been one of Australia’s success stories when it comes to weed control. And, biological control continues to play a very significant role in controlling various prickly pear species in Australia, particularly in the drier, warmer areas.
The two most important biological control agents are COCHINEAL and CACTOBLASTIS. The two are entirely different! (Cochineal PDF print version available – 3 pages)
Cochineal (Dactylopius spp.) attaches to the outside of the plant and (a bit like the way a mosquito works on people) literally sucks the moisture out of the plant:
Comparison with Cactoblastis
Cactoblastis (Cactoblastis cactorum) works very differently to cochineal. The cactoblastis (black and yellow/orange striped) “grubs” tunnel into the plant and feed away on the contents until there is only an empty shell left…
Most people associate “cactoblastis” with “prickly pear”, because of the spectacular impact “cacto” had on major prickly pear infestations in Queensland and New South Wales during the 1920’s and 30’s. (Please see Prickly Pear History for more information on this story).
But, cochineal has also done incredible work in the control of various prickly pear species. It is still doing an exceptional job, particularly in the dry, warmer areas of eastern Australia, and does even better with some basic assistance from the property owners.
Cochineal has had a long history in Australia. The original cochineal insects, and the first prickly pear plants, were brought to Australia with Captain Arthur in the First Fleet, to start a cochineal (red) dye industry. Please see Prickly Pear History for more information on the origins of cochineal in Australia.
There are four main species of cochineal insects (and one mealy bug) of importance in the biological control of the main prickly pear (cactus) species found in Australia. The four species of cochineal insects look the same, but they’re not. And, very importantly, they will only survive on their own, specific host plant!
This table, below, (courtesy Dr JH Hosking, Tamworth NSW) shows which species of cochineal feeds on which species of prickly pear / cactus:
|Prickly pear species||Cochineal species|
|Common pest pear Opuntia stricta spp. Velvety tree pear O. tomentosa||Dactylopious opuntiae cochineal|
|Riverina pear O. paraguayensis (Riverina area) Smooth tree pear O. vulgaris (NSW coastal areas)||Dactylopious ceylonicus cochineal (NB D. opuntiae will “work” on smooth tree pear but it is not as effective as D. ceylonicus)|
|Rope pear Cylindropuntia imbricata||Dactylopious tomentosus cochineal|
|Tiger pear O. aurantiaca||Dactylopious austrinus cochineal|
|Harrisia cactus Harrisia martinii||Hypogeococcus ferterianus mealy bug|
DESCRIPTION OF COCHINEAL INSECTS The adult female cochineal is a soft, shapeless sac hidden beneath a white, silky covering. The female grows to about the size of a “match head”. Once the female attaches itself to the plant, it sheds its “legs” and cannot leave. The male is a small two-winged insect with mobility to visit the females (such is life?).
According to Dr John Hosking, entomologist with NSW DPI (Tamworth), one female can produce up to 2,000 offspring under ideal conditions. Rate of development is largely influenced by temperature. One generation may be completed in less that two months. A cochineal population can actually double its number every five days under right conditions (i.e. dry, and a consistent temperature around 26-28°C)!
A combination of wet and cold weather can wipe out cochineal insects in their thousands: rain washes away their (white) protective covering, exposing them to the cold. Ants sometimes take a special interest in cochineal, as they carry away their hapless victims. Another common, naturally-occurring predator is the devasting mealybug ladybird (cryptolaemus montrouzieri). The adult ladybirds fly from plant to plant to lay their eggs – both the adults and the larvae attack the cochineal insects.
Click on the photo, right, to see two of these (“many-legged”) insects amongst a colony of cochineal on velvet tree pear. The cochineal insects have no defence against the cryptolaemus – note the red areas where cochineal insects have been wiped out! Cochineal insects breed up in their thousands in their peak times, but the cryptolaemus can still have a serious impact on their numbers.
WHERE AND WHEN TO USE COCHINEAL? “Where” relates to how much pear there is. If you only have a small patch, if you want to keep your property really clean, or if you want to run special animals eg stud cattle or horses in the paddock, don’t rely on insects. On the other hand, if you have pear over a large and/or inaccessible area, biological control is an excellent, cost-effective and long-term option.
“When” to use cochineal relates to climatic conditions. In north-western New South Wales, cochineal insects are at their peak from November to May. But, they are delicate little critters and are easily wiped out by a combination of wet and cold weather.
MEANS OF DISTRIBUTION The eggs hatch into tiny “crawlers” within a few hours. The crawlers’ natural instinct is to move off to find a new food source. They have four (4) different means of travel:
1) They can crawl over smooth ground for up to 10 metres in search of new plants. But, they’re not so good on rough ground (eg they can disappear forever into a crack in black soil).
2) Cochineal insects can be blown long distances by the wind. The sketch, below, (courtesy VC Moran & BS Cobby, 1979, Rhodes University, Grahamstown, South Africa) shows the insects’ hairs which act as sails in the wind.
3) By being carried on the actual prickly pear plant material. Pieces of pear are spread by (a) floodwaters, (b) by being attached to a passing animal or (c) by being attached to tyres of passing motor vehicles or farm machinery.
4) By being manually transferred from an infected plant to a new, non-infected plant – using a bucket and a pair of tongs.
As you can imagine, distribution methods 1, 2 and 3 are very haphazard – especially where plants are very scattered. Manual distribution of cochineal insects (as in method 4) is the most reliable for new areas. And, one person can carry out a lot of insect distribution in a short time. Even an hour every now and then will boost the insect numbers.
Photo, right, shows simple method of distributing infected plant material into new areas. Bob Smith (NSW DPI Bingara) – using a bucket and a pair tongs. Easy, and environmentally friendly!
Tips to help the cochineal?
- The basic thing to remember is that cochineal insects are very delicate. They don’t like cold and wet weather (or extremes of heat). They don’t favour heavily shaded areas. When you put insects into a new plant (especially in the approach to winter) give them some protection from the elements. Cover the infected segments with parts of the plant, or branches, bark, cow pads etc.
- Covering newly-infected plants with cardboard, plywood or even a bit of old corrugated iron will offer the cochineal insects maximum protection from the weather. This photograph on the left demonstrates the point: cochineal thrived under the cardboard after what had been a long and cold winter. CAUTION: snakes may also take up residence under these protective coverings?
- Another trick is to breed the insects indoors, in readiness for release in early summer, the optimum time for release. Store infected plant material in cardboard boxes in a dry, warm area. Over a period of 6 to 8 weeks, generally, you’ll have good supplies to put out when the weather warms up.
USE THE RIGHT COCHINEAL… Tiger pear, common pear, tree pear and rope pear cochineal all look the same. But, they’re each specific to their host plants. Please take notice of the chart at the top of this page. The only way to be sure that you’re using the right insect is by collecting the correct host plant material which is already infected with cochineal.
Cactoblastis to learn about that other famous prickly pear insect. Also, information on individual prickly pear species is included in this website. Please return to Weeds List to access common pear, rope pear, velvety tree pear, prickly pear history etc.
Dr John R Hosking (NSW DPI, Tamworth) for providing the original technical information on the description and breeding of cochineal insects.
DISCLAIMER: The information contained in this web site is based on knowledge and understanding at the time of writing. However, because of advances in knowledge, users are reminded of the need to ensure that information upon which they rely is up to date and to check currency of the information with the appropriate officer of North West Weeds or the user’s independent adviser. LRT 31/8/2009, 9/6/2013, 6/5/2014, 21/72015, 02/10/2017