Mealybug: Biology and control

Vaughn M. Walton
ARC Infruitec-Nietvoorbij, Stellenbosch

Mealybug (Planococcus ficus) is one of the key pests affecting vines in South Africa. The last two seasons favourable climatic conditions urged viticulturists to focus on this pest once more. To control the mealybug successfully, a thorough knowledge of the insect’s biology is required. In this article an attempt is made to shed light on the biology of mealybug and possible control strategies will be recommended.

Biology and Life cycle The rate of development of mealybug is directly dependent on environmental temperature. Eggs are laid in egg-sacs consisting of a mass of wax threads. Crawlers hatch after 7-10 days at an average temperature of 25°C (Fig.1).

Crawlers cast their skins to become 2nd and 3rd instar larvae (nymphs), while third instar larvae cast their skins to become adult females. After mating, the egg-sac develops and the female starts laying up to 750 eggs at a time. Damage is only caused by the female mealybug. She is more visible than the male, since the latter only lives between one and three days. The female feeds with sucking mouthparts and nutrients are extracted from the plant, while honeydew is excreted.

The male mealybug causes no damage as it has no feeding mouthparts. The only difference between the lifecycle of the two sexes is that in the case of the male, second instar larvae spin a cocoon in which the third instar larvae, pre-pupae and pupae develop. Adult winged males then appear from the cocoon. In summer mealybugs can complete their lifecycle in 3 to 4 weeks, causing populations to develop rapidly under favourable conditions.

Seasonal occurrence

In winter mealybugs hide beneath loose bark and in cracks on the trunks of vines (Whitehead, 1957). It was recently found that mealybug can also hibernate on vine roots (Fig. 2) and weeds (Fig. 5).

In winter they continue to feed on vine sap and lay eggs. Due to the low environmental temperature the lifecycle is very slow. As soon as temperatures start rising in spring and early summer, crawlers emerge from below the bark and cracks in search of the new growth with its higher nutrient concentrations. Crawlers settle on new leaf and shoot growth where they develop, mate and multiply. In January/February these populations are at their highest. Mealybugs then move to the bunches where the nutrients are increasing. Here they feed mainly on the bunch and berry stems. In late summer and autumn nutrients start moving to the roots from the leaves and stems (Conradie, 1985) and the downward migration of mealybugs takes place once again.

Symptoms and possible damage

Mealybugs excrete sticky honeydew on which sooty mildew (a fungus) grows. Affected bunches can be rejected in the case of table grapes and downgraded by wine cellars. When mealybug infestation is serious, browning and wilting of leaves may occur, as well as early shedding. The latter, as well as mealybugs feeding on vines, can result in loss of nutrients which may cause weakening of the vines (Fig. 3).

High infestations may result in desiccation of bunches, making them totally unmarketable (Fig. 4).

Repeated serious infestations can cause individual vines to die. Furthermore, while early shedding of leaves can cause sunburn damage to grapes, mealybug is also a vector of leafroll virus.

Integrated control

With integrated control, chemical, biological and cultivation methods are used in conjunction with each other. Each method contributes to the total control strategy. In an integrated programme various aspects should be considered to ensure an environmentally friendly, yet effective end result.

Integrated mealybug control may be implemented in the planning phase, even before planting. Factors that could play a role are cultivar differences and macro- as well as micro-climatic conditions.

Certain cultivars are more susceptible to mealybug than others and are indicated in Table 1 (Le Roux, 1996).

Before planting a new vineyard, the history of mealybug infestation in the area should be ascertained, especially if one of the more susceptible cultivars is being considered. If high average temperatures occur in a specific area, there is a bigger chance of mealybug problems. Favourable micro-climates, e.g. slopes that get excessive northerly sun, may also occur in certain blocks and encourage mealybug infestation.

a) Monitoring

It is of the utmost importance that a vineyard which is regularly plagued by mealybug should be monitored for the pest early in the season. By regular monitoring, the development of a problem may be predicted and possible control strategies implemented. An early indication of mealybug is the presence of ants. Certain weeds (as indicated below) have roots that may be checked for the presence of mealybug. These observations may serve as early warning signs. At least twenty vines, evenly distributed over a block of one hectare, must be monitored. Later in the season mealybug infested vines are more visible – they are covered in honeydew and sooty mildew and ants are usually present. In the late summer early shedding of leaves and desiccated bunches are further symptoms. In winter sooty mildew and sometimes even ants are already visible on infested vines. Infested vines should be marked throughout the season, in the course of pruning and vine preparation and also during harvesting. Vines that are marked make it easier to control mealybug accurately and effectively. Once monitoring has been done, a decision can be taken as to which control strategy should be followed.

b) Weed control
Before considering any other means of control, the first step is to control weeds. During the past season it was found that various weeds may serve as hosts for mealybug.

Mealybug occurs on the roots of the following weeds (For illustrations of weeds refer to Fourie et al., 1996) :

• Common blackjack (Bidens pilosa)
• Khaki weed (Tagetes minuta)
• Small mallow (Malva parviflora)
• Flax-leaf fleabane (Conyza bonariensis)
• Black nightshade (Solanum nigrum)
• Thornapple (Datura stramonium)
• Sowthistle (Sonchus oleraceus)
• Musk Herons Bill (Erodium moshantum)
• White goosefoot (Chenopodium album)

A correlation has been found between the occurrence of these weeds and mealybug problems in vines. The best method of control is the planting of cover crops as recommended by Fourie et al. (1997). Long flowering cover crops that do not host mealybug may reduce ant problems, may help to reduce the forming of dust, may serve as supplementary nutrition for natural enemies and may bind nitrogen. Weeds may also be controlled physically (shrub beaters) and chemically (herbicides) (Fourie et al., 1996). Weeds have to be controlled from early in the season, since they act as access routes to the vine for ants and do not contribute much to the quality of the soil. Ant control is impossible if weeds grow into the vines.

c) Ant control
Effective ant control is a prerequisite for mealybug control since ants protect mealybug against its natural enemies. Ueckermann (1998) found the most effective means of ant control to be circular spraying around the trunk. This method of control is also more environmentally friendly than chemical treatments on the soil. The purpose of this application method is to keep ants out of the vines, but still allow them on the soil surface. Here ants may act as predators of beetle, fruit fly and moth larvae and pupae. At the moment chlorpyrifos EC is the only registered pesticide against ants (Nel et al., 1999). Chlorpyrifos has also been tested at 41ml/L (not a registered concentration) with reasonably good results (Ueckermann, 1998). Trunk treatments for ants should be dependent on ant activity. Usually ants become more active from October onwards if mealybug is present.

d) Natural enemies and biological control
Natural enemies can only impact on mealybug control if prior practices have been implemented. There is worldwide resistance to the use of insecticides. South African wine farmers therefore have to limit the use of chemical products to the absolute minimum. In future the role of natural enemies will consequently increase. Research indicates that if integrated control is applied correctly, these natural enemies can control mealybug successfully. For this reason it is essential to recognise natural enemies in the vineyard and surroundings before considering the use of chemical products.

The most common natural enemies of mealybug include the parasitic wasp which plays an important role in most vineyards. The most important parasitic wasps are Anagyrus sp. (Fig. 6A), Leptomastix dactylopii (Fig. 6B), and Coccidoxenoides peregrinus (Fig.6C).

These insects eliminate mealybug populations by laying their eggs in the host. The parasite eggs develop in the body cavity of the mealybug. The host is eventually killed when the parasite engulfs the body cavity and hatches. Coccidoxenoides peregrinus is being bred on a large scale at Nietvoorbij and is then set free in trial blocks. Results obtained over the past two seasons are very promising and in future C. peregrinus can possibly be bred co-operatively and released to control mealybug. However, it is sometimes difficult to determine whether these insects are indeed present in vineyards. It is possible, nevertheless, to notice mummies of parasitised mealybug with the naked eye (Fig. 7).

Predatory beetles also occur in vineyards early in the season. The most dominant beetles include Nephus quadrivittatus (Fig. 6D) and Nephus bineavatus (Fig. 6E). These beetles eat various stages of mealybug.

d) Chemical control
This method of control should be seen as the last resort for mealybug control, since no insecticides are environmentally friendly. The substances mentioned below are organophosphates that are poisonous to people, cattle, birds, fish, natural enemies and bees. Since mealybug occurs in random spots in vineyards, an attempt should be made to give spot treatments in order to reduce the impact on the ecosystem.

Certain products are only applied in the growing season, while others are used both in winter and the growing season. Dosages (per 100l water) also differ, depending on the growth stage. Dormant treatments (before the new growth begins) should only be applied if more than 5% infestation occurs. This reduces hibernating mealybug populations to such an extent that natural enemies can control mealybug effectively the next season. Routine dormant spraying should be avoided. Try to spray only the vine that has been marked and the two vines on either side with hand-held spray guns and a high pressure pump. Atomiser sprays do not give sufficient coverage. The following chemical products and application methods are still being used at present (Nel et al., 1999; Vermeulen, 1999):

• 1.Chlorpyrifos EC (480g/l) at 200ml/100l is recommended twice, 14 days apart, prior to budding. After budding there are problems with phytotoxicity on young shoots and leaves. Chlorpyrifos at this dosage also suppresses the Argentine and Cocktail ant.
• 2.Profenofos EC (500g/l) at 100ml/100l is also recommended twice,14 days apart, prior to budding. Thorough drenching is required.
• 3.Protiofos EC (960 g/l) at 50ml/100l is recommended once before budswell. A safety period of 100 days must be maintained, however.

During the growing season the following chemical products and application methods are allowed (Nel et al., 1999; Vermeulen, 1999):

• 1.Chlorpyrifos EC (480g/l) is recommended at 75ml/100l from four weeks after budding up to 28 days before the harvest. Take note of the dosage, higher dosages will result in phytotoxicity on leaves.
• 2.Dichlorvos EC (1000g/l) is recommended at 75 ml/100l up to 7 days before harvest. This treatment is only supplementary to winter treatments.
• 3.Dimethoate EC (400g/l) is recommended at 125ml/100l up to 28 days before harvest.
• 4.Formothion EC (250g/l, 330g/l) is recommended at 150ml/100l up to 10 days before harvest.
• 5.Methidathion EC (420g/l) is recommended at 50ml/100l up to 8 days before harvest. This treatment is prescribed as a late corrective one-off treatment.
• 6.Mevinphos SL (500g/l) is recommended from 37,5-45 ml/100l up to 7 days before harvest.

e) Post-harvest spraying
Post-harvest spraying is not recommended, since this is the period when the populations of natural enemies, which are a lot more susceptible to insecticides than mealybug, are at their highest. Spraying at this stage interferes with biological control for the next season. If vines deteriorate, and early shedding of leaves occurs, the infested vines only can be sprayed.

f) Further guidelines

If a decision to use chemical control is taken, the following guidelines may be followed in order to interfere as little as possible with biological control of mealybug:

Apply the minimum amount of chemical products per season. If full cover has to be achieved with spraying, products that degrade rapidly (withdrawal period of 7-10 days) are recommended. In old vines loose bark around the main trunk and neck may be removed so that penetration of the insecticide will be more effective and more mealybugs killed. For ant and snoutbeetle control, synthetic pyretroids are recommended for trunk spraying instead of soil or full cover spraying. With monitoring and correct application no more than two trunk sprays per season are required. With snoutbeetle control, full cover spray with synthetic pyretroids (withdrawal periods of 28 days and longer) should be avoided. Rather apply fruit fly bait droplets than a full cover spray. Chemical control for budmite should only be applied if bud analysis at the time of pruning indicate this to be necessary.

Planting suitable cover crops has several advantages. It limits dust forming, which decreases natural enemies efficiently. It provides certain natural enemies with alternative nutrition and shelter. It benefits weed control and improves soil quality. Attempt to establish cover crops that flower early and for a long time during the season, thereby giving natural enemies an early advantage in the season.

In conclusion

Mealybug remains one of the key pests in the viticultural industry. To address this problem, five projects are currently registered with ARC Infruitec-Nietvoorbij, viz.:

• Mass breeding of natural enemies of mealybug
• Integrated control of mealybug in vineyards
• Bio-testing of insecticides on natural enemies of mealybug
• Determination of life-span tables of mealybug and an important natural enemy
• Determination of the economic threshold values of mealybug

With these projects an attempt is being made to develop effective and environmentally friendly control strategies against mealybug.

Further queries about mealybug or its natural enemies may be addressed to Vaughn Walton at Tel.: 021 809 3167, Fax: 021 809 3002, or e-mail: vaughn@nietvoor.agric.za.

Literature references

Annecke, D.P. & Moran, V.C. 1982. Insects and mites of cultivated plants in South Africa. Butterworths, Durban/Pretoria.

Conradie W.J. 1985. Nitrogen nutrition of the grapevine (Vitis vinifera spp.). Ph.D., University of Stellenbosch.

Fourie, J.C. 1996. Uitkenning en chemiese beheer van belangrike onkruide in Wingerde van Suid-Afrika. Nooitgedacht Pers, Kaapstad.

Fourie, J.C., Louw, P.J.E., & Agenbach, G.A. 1997. The effect of different cover crop species and cover crop management practices on the available N and N-status of young Sauvignon blanc vines on a sandy soil in Lutzville. Abstract, South African Society for Enology and Viticulture Congress, 27-28 November 1997, Cape Town.

Le Roux, P. 1996. Die chemiese beheer van wingerdwitluis, in Wingerdwitluis paneelbespreking, SAWWV – Kongres, Kaapstad, 8 November 1996.

Nel, A., Krause, M., Ramautar, N. & Van Zyl, K. 1999. A guide to the control of plant pests. Directorate: Agricultural Production Input, National Department of Agriculture, Republic of South Africa.

Prinsloo, G.L. 1984. An illustrated guide to the parasitic wasps associated with citrus pests in the Republic of South Africa. Department of Agriculture Science bulletin, No. 402.

Ueckermann, P. 1998. Ant control in vineyards. Wynboer Tegnies 105: 8-9.

Vermeulen, A.K. 1999. A Guide to the use of Registered Fungicides and Pesticides against Grapevine Diseases and Pests: Wine Grapes. ARC-Fruit Vine and Wine Research Institute, Private Bag X5026, Stellenbosch, 7599, South Africa.

Whitehead, V.B. 1957. A study of the predators and parasites of Planococcus citri (Risso) (Homoptera) on vines in the Western Cape Province, South Africa. M.Sc. thesis, Rhodes University, Grahamstown, South Africa.

Source: http://www.wynboer.co.za

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