Mealybug insecticide resistance management strategy

The long-tailed mealybug Pseudococcus
longispinus is a minor pest of fruit crops
in New Zealand (Photo: David Cappaert,
www.insectimages.org).

J.G. Charles
Plant & Food Research, Private Bag 92169, Auckland 1142, New Zealand

(Revised October 2004)

Reason for strategy and update

Mealybugs are capable of becoming resistant to insecticides. Management strategies aimed at reducing or preventing resistance will help conserve existing products for ongoing use. This is an update of the earlier resistance management strategy (Charles 1996).

Background

Mealybugs (Hemiptera: Pseudococcidae) are pests of a wide variety of horticultural crops, ornamental plants, pasture and other ground covers, such as those used in bowling greens. Populations often do not increase to pest levels, but, when they do, they are generally effectively controlled by insecticides. Mealybugs most frequently cause concern by their presence on horticultural crops destined for export markets. However, relatively few insecticides can be used on fresh fruits without exceeding residues-tolerances set by those markets, and this situation is not likely to change in the near future. This restricted range of permitted insecticides increases the risk of resistance and the potential economic impact should it occur. Mealybugs may also be vectors of plant virus diseases, and are known to spread grapevine leafroll disease in New Zealand vineyards.

Mealybugs are difficult to kill with insecticides. They are covered with a powdery wax that repels water-based insecticide solutions. They also often live deep inside cracks and crevices in trees, or inside fruit or fruit bunches where they are protected from contact with insecticides. High volume applications of insecticides are essential for mealybug control and should be sprayed to "runoff". Mealybug control should not be attempted with low-volume application technology.

Overseas mealybug species also developed resistance to organophosphates, especially parathion, in the USA and South Africa (Flaherty et al. 1982; Myburgh & Siebert 1964). The potential for resistance to Applaud has been long recognised and it is generally recommended around the world that it is not used more than twice a year.

Mealybugs are usually secondary or quarantine pests of export horticultural crops in New Zealand. The species recorded as significant pests are listed in Table 1. All these species are widespread through the North and South Islands.

**Table 1:** Pest mealybug species in New Zealand.
Species	Common name
Pseudococcus longispinus	long-tailed mealybug
Pseudococcus calceolariae	citrophilus mealybug
Pseudococcus viburni (= affinis)	obscure mealybug
Planococcus mali	"blackcurrant" mealybug
Planococcus graminicola	"grass" mealybug
Balanococcus diminutus	"flax" mealybug
Balanococcus poae	"pasture" mealybug
Vryburgia lounsburyi	"bulb" mealybug
Rhizoecus spp.	"root" mealybugs

The common names of the Pseudococcus species are widely used around the world. The common names given for the other species describe the physical habitats of the mealybugs in New Zealand, but their use elsewhere may describe different species.

The three Pseudococcus species are cosmopolitan and typically inhabit the aerial parts of perennial plants. They have a worldwide distribution, and a very broad host range, and are the most important mealybug pests of fruit crops in New Zealand. They rarely become pests south of Marlborough. The other mealybug species are usually only minor pests, but may occasionally cause significant damage locally.

Planococcus mali is most frequently found as a pest in blackcurrants in the South Island.

Planococcus graminicola lives at the base of, or among the roots of grasses. It may move up into fruit trees, especially during droughts.

The two Balanococcus species are the only New Zealand native mealybug pests. B. diminutus is a pest of ornamentals, especially flax (Phormium spp.). Balanococcus poae is a pest of pasture which can affect ryegrass productivity. It may also affect turf in golf courses and playing fields.

Vryburgia lounsburyi appears to be restricted to bulbs, and can be a major pest of commercial nerines.

Rhizoecus species are root feeders on many plants, and can cause considerable damage to bowling greens of Leptinella (= Cotula) dioica.

Products with label claims for control of mealybugs in New Zealand

Products with label claims for control of mealybugs fall into five main groups. Table 2 shows products with label claims for application against mealybugs on the indicated crops, although it should be noted that some products might no longer be recommended for use on some crops. Products may also have label claims for use against pests on different crops.

The highly refined all purpose spraying oils have a narrow distillation range, and are useful materials for mealybug control, either alone or mixed with an insecticide. However, great care is needed with their application to ensure that they do not damage plants. Some cultivars are more susceptible to oil damage than others. Test spray a few plants of each cultivar first before spraying the whole crop. Read label notes carefully, and take heed of weather conditions, tank mixtures and target plants.

**Table 2:** Products with label claims for control of mealybugs in New Zealand (September 2002). Not all products listed for each pesticide may have a label claim for all crops indicated.
	Type of label claim for each crop¹
Pesticide category and IRAC chemical group Pesticide common and (product) names
Biological control
Cryptolaemus montrouzieri (Crypto)	Greenhouse and outdoor crops
Oils, oil/ insecticides
JMS stylet oil				X
Sunspray + insecticide		X		X		X	X
Carbamates 1A
carbaryl, (Carbaryl, Sevin)	X				X	X
methomyl (Lannate)				X
Organo-phosphates 1B
acephate (Lancer, Orthene)	X	X
chlorpyrifos (Chlorpyrifos, Lorsban, Pychlorex, Spectrum)				NT		X	X	X
diazinon (Basudin, Dew, Diazinon, Diazinyl, Gesapon)	X	X	X	X	X	X	X
dimethoate (Dimezyl, Rogor)		X				X	X
maldison (Malathion, Yates Maldison)		X		X		X	X	X
prothiofos (Tokuthion)				X		X
Organo-phosphates/pyrethroids 1B/3
permethrin/pirimiphos-methyl (Averte)	X				X
permethrin/pirimiphos-methyl (Attack)	X	X		X	X			X
Chloro-nicotinyl 4A
thiacloprid (Calypso)						A
Thiadiazine 16
buprofezin (Applaud)				X	X	X	G

¹A = apples only, G = golden queen peaches, NT = not table grapes. Ornamentals includes flowers.

Warning: Not all products listed may be suitable for crops being exported to certain markets. Check with your export agency before applying any pesticide on export crops. Observe withholding periods.

Current status of mealybug resistance in New Zealand

In the past, extensive and regular use of insecticides in New Zealand heightened the potential for mealybugs to develop resistance. The obscure mealybug in Hawke's Bay was reportedly resistant to DDT in 1959 (Congdon & Morrison 1959). Following anecdotal records of resistance to parathionmethyl in the 1970s, resistance to another organophosphate, chlorpyrifos, was demonstrated in the 1990s (Charles et al. 1993), with signs of cross-resistance to prothiofos (Charles et al. 1993). Cross-resistance to organochlorines was not examined. Resistance has not been recorded in any other New Zealand mealybugs.

With the advent of IFP and accompanying reduction in pesticides, the mealybug problem in pipfruit has diminished. The additional introduction and establishment of the parasitoid Pseudaphycus maculipennis, a biological control agent for obscure mealybug, should further assist in mealybug control in all fruit crops. Nevertheless, the cost of widespread mealybug resistance to New Zealand could be very significant, and this strategy is designed to reduce selection pressure for resistance as far as possible without jeopardising fruit quality.

Resistance management and prevention strategy

The general strategy is to reduce selection pressure for resistance by optimum spray timing, accurate delivery of insecticides and rotation of products with active ingredients from different chemical groups and used in a planned programme. This is combined with management practices for the crop and shelter trees that aim to reduce mealybug numbers and improve insecticide coverage. Details are provided below.

Note: Control failure does not always imply resistance

Crop management

Mark "spot" infestations of mealybugs during harvest.
During winter and spring remove protected sites that mealybugs hide in when fruits are not available. Prune away old 'snags', spurs or canes and mummified fruits, and remove loose bark. Prune cleanly to avoid splitting wood. Remove, mulch or burn old prunings.
Maintain tree and plant sizes and shapes suitable for good spray penetration and coverage.
Female mealybugs cannot fly, although crawlers are blown about by wind. Mealybugs may be transferred from orchard to orchard, or block to block, by machinery. Wash down tractors/harvesters/sprayers between jobs.
Keep ground cover low throughout the year to reduce mealybug populations.
Be aware of mealybug natural enemies and take actions to protect them. Insecticide use
Spray only when essential for control. Follow industry codes of conduct where appropriate.
Comply with label rates.
Use correct application procedures, observing correct tractor speeds and spraying conditions to obtain good insecticide coverage.
Calibrate sprayers at least once per season.
Follow spray programme recommendations.
Identify mealybugs present and learn their life cycle. Apply insecticides when the most vulnerable stage (first instar 'crawlers') is most common.
Confine sprays to the infested crop area: do not spray shelter or other areas around the orchard unless there is a clearly identified source of pest infestation.
Use a range of insecticides, especially if making more than one application per season.
Insecticide use must be designed to keep the mealybug population small enough to prevent significant infestation of fruit. For most horticultural crops, once mealybugs have established within the calyx of individuals fruits, or between fruits within bunches, they are virtually impossible to remove by insecticides. For most crops, the key infestation-period is from early January to harvest.

Fruit growers in general should aim to have mealybugs under control by early January.

Shelter management

Use non-host shelter species such as Casuarina and Bamboo. Avoid using shelter species that are favoured hosts, e.g. willows.
Remove wild fruit trees and other host plants in the immediate vicinity to reduce reservoirs of mealybug populations close to the crop.

Implementation and recommendations

Mealybug control insecticide labels should carry the following statement:

IMPORTANT - RESISTANCE MANAGEMENT
Resistance to this pesticide could develop from excessive use. To minimise this risk use strictly in accordance with label instructions. Avoid using this pesticide exclusively all season, and avoid unnecessary spraying. Maintain good cultural control practices.

This resistance management strategy should be included:

in industry-wide spray programmes for local and export horticultural crops.
on product labels for all insecticides used for mealybug control.

References

Congdon NB, Morison LG 1959. Mealy bug resurgence in Hawke's Bay orchards. New Zealand Journal of Agriculture 99: 481-487.

Charles JG 1996. Mealybug resistance management strategy. In: Bourdot GW, Suckling DM ed. Pesticide Resistance: Prevention & Management. New Zealand Plant Protection Society, Lincoln, New Zealand. Pp 172-176.

Charles JG, Walker JTS, White V 1993. Resistance to chlorpyriphos in the mealybug Pseudococcus affinis and P. longispinus in Hawke's Bay and Waikato pipfruit orchards. Proceedings of the 46th New Zealand Plant Protection Conference: 120-125.

Flaherty DL, Peacock WL, Bettiga L, Leavitt GM 1982. Chemicals losing effect against grape mealybug. California Agriculture 36: 15-16.

Myburgh AC, Siebert MW 1964. Experiments on parathion-resistant mealybugs. Deciduous Fruit Grower 14: 190-193.