In agriculture we have come to view chemical control as the standard procedure for dealing with crop pests. It started as far back as 2500BC when the Sumerians used sulphur type compounds to control insects and mites. After World War II chemical use became an integral part of modern agricultural practices but as early as the 1950s it was already becoming clear that the widespread use of chemicals could induce an ecological backlash with resurgence of pests, replacement of pest complexes with others, and resistance to pesticides.
Male mealybugs are small winged insects that pose no threat since they are short-lived and do not feed. Females are wingless crawlers that can be spread to other host plants by workers, machines and, whilst young, through wind. Females lay up to 600 eggs in a dense, fluffy secretion that serves as an egg sac. Within a few days new nymphs hatch and begin to squirm out of this ovisac. Light infestations can be easily overlooked because mealybugs shelter in plant crevices. As numbers increase, mealybugs of all sizes can be seen crawling and feeding on all exposed plant surfaces.
This phytosanitary pest is well-known amongst citrus and grape farmers all over the world. A high infestation can lead to fruit drop, fruit deformation (high shoulders), and the development of discoloured welts on the rind. Mealybugs feed by sucking on plant juices and sap, and secrete copious quantities of honeydew which in turn is a growth medium for the sooty mould fungus. This fungus may stain fruit, thereby lowering grade, as well as cause a delay in fruit colouration. Photosynthetic potential, especially of young plants, may be negatively affected if infestations are severe and can lead to the death of the plant, low yields, or poor performance. Premature leaf drop can also result because of the mealybug’s toxic saliva.
Ants and mealybugs go hand in hand, as the ants feed on the honeydew that mealybugs excrete. Ants protect mealybugs in order to safeguard this food source and will defend mealybugs against other insects, therefore ant control should be done thoroughly where mealybugs are a problem.
Mealybugs spend the winter in the soil, on roots, or on the plant. They hide in fruit calyxes, crevices in the bark and other protected areas such as curled leaves. Populations begin to increase from September, peak in late December/early January, and begin to decline around March/April, depending on temperature, predators and parasites. Due to the mealybug’s waxy covering and its habit of feeding in sheltered areas, they are difficult to control with pesticides or chemicals, especially late in the season.
Mealybugs rapidly develop resistance to insecticides. The continuous application of broad-spectrum pesticides such as organophosphates and pyrethroids can be partially responsible for mealybug outbreaks on crops.
Integrated pest management of mealybug
Du Roi IPM, a South African company that has been mass rearing beneficial parasites and predators successfully since 1999, has focused strongly on developing solutions for the mealybug problem. Beneficial insects being reared for mealybug control are parasitic wasps Coccidoxenoides perminutus (Planopar/Timberlake), Cryptolaemus montrouzieri (Mulsant), and Anagyrus pseudococci (Girault).
C. perminutus (often referred to as Cocci’s), are small wasps which originate from Hawaii and now occur widely throughout the world. They are extremely effective parasites of citrus and vine mealybug and provide control of both low-density and high-density infestations. C. perminutus are black in colour with noticeable translucent wings, have relatively long antennae, and are approximately 3 mm in length. Females parasitise the first three instars of mealybug and are able to lay 60 to 90 eggs each. The eggs develop into pupae within the mealybug insect, slowly feeding off the intestines of the host. Parasitised mealybugs continue to develop for about six days but are eventually killed by the growing pupa. About 16 days after parasitism adult C. perminutus wasps emerge from pupae and are immediately ready to mate, further parasitise and continue the cycle. The tempo of the lifecycle is dependent on temperature and humidity. Generally C. perminutus adults are active for about seven days and are most effective at temperatures between 20°C and 30°C and humidity between 50% and 90%.
Releasing C. perminutus into vineyards
C. perminutus are best released in spring and early summer, before mealybug populations build up to unacceptable levels. It is important to determine the type of mealybug present before considering the use of C. perminutus since the parasite is not very effective against species other than citrus and vine mealybug. Winter releases are not recommended due to low temperature and the consequent reduction in mealybug populations.
C. perminutus are sold in pupal form and supplied in small degradable boxes with a small hook for attaching them to plants. Each box contains 1 000+ pupae, which will begin to hatch on the hatching date (approximate), indicated on the box. Once the wasps begin to hatch they will be attracted to the exit hole by sunlight and emerge. Two to three weeks after release an inspection should be done to check that C. perminutus have become established. The insects are difficult to detect but a thorough search for the tiny adults, as well as for parasitised mealybug pupil cases will reveal their presence.
Conditions of high humidity and little wind and dust will assist with the establishment of C. perminutus. Windbreaks are helpful in this regard. Ants should be controlled beforehand by selective spraying, treatment of ant nests, or the application of ant barriers. These should be carefully selected to ensure low impact on the introduced beneficial insect population.
Cryptolaemus are ladybird beetles that prey on mealybug and other insects such as Cochineal and some soft scales. These beetles are high-density feeders with a voracious appetite for mealybug and are often referred to as the ‘mealybug destroyer’. Adult beetles are about 4 mm long, oval in shape, black in colour with a light brown head and posterior. The larvae grow up to 13 mm long and are covered with long, white, waxy filaments. Their appearance is very similar to that of mealybug and they are often incorrectly identified as mealybug by the inexperienced eye.
Cryptolaemus adults and larvae feed on all stages of mealybug (a single larva may consume up to 250 small mealybugs) but they have a preference for mealybug eggs. Adults live for about 2 months with a mature female laying up to 550 eggs during her lifetime. Eggs are laid in the cottony egg sacs of female adult mealybugs and hatch about five to six days later. Approximately 15 days after hatching larvae prepare to pupate by moving to protected areas (undersides of leaves and crevices in the bark). Pupation takes about 8 days.
At temperatures of 27°C the full lifecycle is accomplished in about 30 days. Cryptolaemus prefer temperatures of between 20°C to 30°C and cannot tolerate cold winters. Cryptolaemus beetles are supplied in paper bags with 50 beetles in each bag.
Releasing C. montrouzieri into vineyards
Recommended release rates vary from 500 to 2 000 beetles per ha, released in hot spots. After release, the beetles rapidly disperse over the treated area and begin feeding. Eventually egg-laying will commence. It will take about three to four weeks before Cryptolaemus larvae are observed feeding on the target pest.
Adult beetles may be difficult to find after release but this should not be of concern. To ensure that Cryptolaemus have established, it is recommended that an experienced scout do regular monitoring after their release.
A. pseudococci (Hymenoptera: Encyritidae) is a small parasitic wasp that is effective at parasitising a variety of mealybug species. These host-specific wasps are especially robust predators of vine mealybug (Planococcus ficus) and can provide up to 90% parasitism. They are active late in the grape growing season and can reduce vine mealybug populations before the pest moves to the lower part of the trunk. Combined with C. montrouzieri and C. perminutus, the use of A. pseudococci results in excellent control because each of these targets different instars of the mealybug lifecycle.
Each female can lay up to 45 offspring, one per mealybug nymph, mainly from the third instar to adult female mealybug. Males (40% – 50%) are much smaller than the females (0.8 mm) and are entirely black. Adult females are 1.5 to 2 mm in size and are light brown in colour with distinctive white and black antennae. Females can live up to about 40 days and males about 30 days.
Releasing A. pseudococci into vineyards
A. pseudococci are supplied in small plastic containers with a ready ant-barrier. Recommended release rates are 1 000 to 1 500 wasps per ha. Two or three weeks after release the first mummies will be clearly visible in the field. Through regular scouting a decline in mealybug population should be observed.
The beneficial insects bred by Du Roi IPM are host-specific and the chances of these becoming pests are very unlikely. Being host specific means that they will only attack insects that they were designed to predate or parasitise by nature. If the target pest insect is not present the beneficial insects will simply move to where the host is present, or die off.
Although not indigenous to South Africa, these parasites and predators have been present for many decades.
Please note: Beneficial insects should not be released too soon after applying toxic chemicals and harsh chemical applications should be avoided after the insects have been released.
Some of the benefits of biological control:
- Completely natural.
- Decreases dependence on pesticides.
- Less dependent on weather than spraying.
- No pest immunity issues.
- Saves money on the long run by reducing pesticide use.
- Eliminates much of pesticide administration and reporting.
- Environmentally friendly – safe for animals, humans and other species.
- No contamination of ground water.
- Little direct effect on plants.
- Does not require mechanical means – no machines and fuel used.
- Greater market acceptance – green!
Du Roi IPM,
For further information contact Brahm Jonker at 082 077 5940 or email@example.com.