The biological control of the invasive cactus, chain-fruit cholla (Cylindropuntia fulgida) by means of cochineal insects (the ‘cholla’ biotype of Dactylopius tomentosus)

H. Klein, ARC-PPRI, Private Bag X134, Queenswood 0121, Pretoria, South Africa [email protected]

Chain-fruit cholla In South Africa, this cactus species was initially thought to be Opuntia rosea (rosea cactus), but a few years ago scientists realised that it was in reality Opuntia fulgida, commonly known as chain-fruit cholla or jumping cholla. It originated in the USA and Mexico, from where it was imported as an ornamental plant. When humans or animals brush past it, segments of the cactus become detached and are carried to other areas. Wherever a segment falls, it roots to give rise to a new plant. In this way the cactus escapes from gardens and invades grazing land and natural environments in warm, dry areas. It injures humans and animals, kills small animals, and drastically reduces the productivity of farm land.

Herbicides can kill the cactus, but are expensive, and small plants are easily missed, so it is a never-ending process. The manual or mechanical removal of the cactus is also ineffective, because every little piece that remains behind, can regrow. In contrast, biological control costs very little, needs little labour, and once it becomes established in an Opuntia fulgida (left), with the area, it spreads by itself to surrounding cactus characteristic chain-fruits (right) plants and will find even the smallest plants. All biocontrol agents are intensively tested for their safety to the environment and crop plants, and only if they are found to be host- specific are they released.

Biological control agents against chain-fruit cholla The biocontrol agents used in South Africa to control chain-fruit cholla are a specific biotype of chochineal, Dactylopius tomen-tosus. Cochineal insects are a group of tiny A segment of chain-fruit cholla infested with cochineal. sap-sucking insects that feed and develop only Note the round “fluff-balls” representing females, the on cactus species. elongated, white male cocoons, and the tiny pink dots, representing nymphs. The females become fastened, through their feeding tube, to one spot on a cactus plant for the rest of their life. They produce large Biological control is the use of host- numbers of eggs that hatch into tiny, pink, specific insects or pathogens (disease- immature insects or nymphs, with very short causing organisms) to keep the numbers legs and long hairs on their backs. The nymphs of the target weed below the level

where the plant is a problem. crawl to the highest part of the cactus plant, The body fluid of a cochineal insect is a bright, where the wind usually blows them away. If dark red colour, which is visible when the they land on another plant of their host species insect is squashed. (chain-fruit cholla), they start feeding, moult several times, and eventually become mature Only the male cochineal develops wings and can cochineal insects. fly. Nymphs and females have no wings, and only very short legs. The nymphs can crawl male short distances, usually only to the highest tip of the plant, so that the wind can disperse them to fresh food plants. Mature females are permanently attached to the food plant, and cannot move around, but feed in one spot until they die.

nymph This means that the insects have to be distributed manually, on pieces of their host male plant, to any far-away cactus plants that need cocoon female to be controlled, because without human help they can only disperse as far as the wind blows them. Wind dispersal can take them to surrounding plants that grow within about 5 metres from the plant on which they hatch, if the surrounding vegetation isn’t too dense. In a large patch of cactus, they will disperse Microscopic view of the different developmental unaided from plant to plant, but when there stages of the cochineal, Dactylopius tomentosus are no more cactus plants within the range that they can reach unaided, the insects will starve A mature female cochineal insect looks like a and become extinct in that area, and there will fat, pink bag with a very long, thin, hair-like be no insects left that can control the cactus in feeding tube and very short legs. She is filled other areas. This is why it is very important to with eggs, and is completely covered with a collect plant material with cochineal before the layer of white, cottony wax threads. The insects have killed all the cactus in one area, to mature male cochineal does not resemble the ensure that the next cactus patch can be female in the least, but looks like a tiny, inoculated before the old one has died. white fly with two long tails. The male nymphs produce a white cocoon at the end of their Heavy rain can wash the cochineal insects off nymphal stage, from which a mature male later their host plants, and it might take a while for hatches. The males then fly off in search for the few surviving cochineal insects to build up females to fertilize, and die after a day or two, their numbers sufficiently to keep the cactus without feeding. under control. The cactus plants themselves grow better during the rainy season, and The nymphs and female cochineal insects kill recover more easily from the attack of the the cactus, probably by injecting a toxic cochineal. During the dry season, the insects do substance into the plant, while feeding. better, and the cactus plants cannot recover so Cochineal insects take about two months to fast, and as a result the insects achieve better complete their life cycle, from the time when control during the dry season. the nymph hatches from its egg until it becomes a mature male (which can fertilize a This particular biocontrol project is female) or female, which can start to produce exceptionally effective, and can kill large eggs. cactus plants in less than two years. Small cactus plants can be dead a few months after The white “fluff” that develops on a cactus cochineal has been introduced into the area. after it has been inoculated with cochineal is a Normally biocontrol takes much longer to shelter for the insects, which is secreted by the achieve such good results. cochineal insects themselves. Underneath each “fluff ball” is a feeding nymph or mature femal The cochineal insects have been tested very cochineal insect with her eggs and young thoroughly, and will certainly not attack any nymphs. plant other than a cactus. They can also distinguish between different cactus species, and although they might survive on a few cacti

2 that are closely related to chain-fruit cholla, in a radius of about 5 metres. Therefore, one they can only kill their target weed (chain-fruit infested segments every 5 metres is sufficient cholla). They are not harmful to animals or in dense cactus patches. If the cactus plants are humans. further than 5 metres apart, each large plant could be inoculated with one infested segment. If cochineal is already present in a cactus Harvesting and re-distribution of biocontrol patch, it is unnecessary to put any more agents (cochineal insects) infested material within 5 metres from the

closest cochineal.  Harvest the biocontrol agents (cochineal) by breaking segments with cochineal off cactus plants in the field in an area where the biocontrol agents are already

established. It might be necessary to first obtain permission to collect in a particular area.  Use tongs to grip one segment with live biocontrol agents on an infested cactus plant, and twist it off the plant without damaging the segment unnecessarily. If the stem segments are damaged, they dry out or start rotting easily, and will not survive long enough to allow the female cochineal insects, which cannot move to another plant, to complete their life cycle until they can produce eggs to infest the fresh plant.  Place the harvested, cochineal-infested segments into a collection container and cover it with a lid, to prevent the crawlers and mature male insects from escaping.  Harvest enough cactus segments with cochineal to distribute within a week of Chain-fruit cholla being killed by cochineal collection. a  Make sure that you do not remove all the cochineal from one plant, so that the plant Indications of the presence of cochineal will remain under biological control, and White wooly fluff will appear on the stem the agents can continue to breed. segments, around the spines, especially on the  The collection box containing biocontrol- lower side of the stems. If squashed, a dark-red infested material should always be stored in liquid will appear. a cool and dry area – do not leave inside or on the back of a vehicle in the sun. Damage caused by cochineal  Attach one infested segment to the plant The side branches droop, the stem segments you want to inoculate by pushing it into a drop off the plant, until dead segments become sheltered spot, protected from wind and heaped on the ground around the bare trunk of rain, attached by its spines. the cactus. A few new segmets may be  Re-visit the release the site after 2-3 produced, but if cochineal remains present on months to check on the progress. the plant, these will drop off the plant and die  It should be possible to harvest material in due course. Eventually the trunk also dies. after four or six months if the initial inoculation was large enough. If cochineal-infested stem segments are dragged around on animals’ coats, the insects How much material is necessary to establish could reach areas where the cactus plants are the cochineal? still uninfested, which will speed up the One segment, heavily infested with cochineal, progress of biological control. is enough to kill even the largest chain-fruit cholla plant, as well as all other cactus plants

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