Encarsia (Encarsia formosa) - whitefly parasitoid
Target pests: Greenhouse whitefly (Trialeurodes vaporariorum); silverleaf whitefly (Bemisia tabaci )
Encarsia formosa is a tiny wasp which parasitises whitefly larvae. It was one of the first biocontrol agents developed for greenhouse use and is still the most common parasitoid in commercial production for whitefly control worldwide. It works best under warm temperate conditions. The Encarsia population is at least 98% female, so all wasps can parasitise whiteflies.
Encarsia is supplied as parasitised greenhouse whitefly scales glued onto small cards. Each card should produce at least 100 wasps within 1-7 days. The cards are designed to be hung from leaf petioles. For pest hotspots, they may also be supplied as loose scales, which are best shaken into small boxes attached to plants, or sprinkled onto planting slabs.
Life history and biology
Encarsia formosa has a flying adult stage and a sedentary larval stage. The adult female is black with a yellow abdomen; males are very rare. The wasps are very small, <0.6 mm, and best seen with a hand lens. She feeds on host honeydew and also on the smallest whitefly scale stages, called host feeding. She lays eggs (one per scale) in older scale stages. She may lay 15 eggs per day for an average of 150 eggs per female. The egg hatches into a tiny grub, which pupates when mature to become a new wasp within the scale. This process turns the greenhouse whitefly scale black, while the silverleaf whitefly scale turns brown. The new adult escapes by biting a circular hole in the wall of the scale, visible if it is held up against the light. The complete life cycle takes about 28 days at 21°C, faster at higher temperatures. Encarsia will also host feed on juvenile whitefly scales killing the scales in the process. This is especially so for Bemesia where it will use all larval stages as a food source causing significant mortality.
Encarsia is effective against greenhouse whitefly in a broad range of greenhouse vegetable, ornamental and herb crops, including tomato, cucumber, capsicum, chrysanthemum, rose and gerbera. In summer and in warmer climates they may also be useful in outdoor nursery stock and gardens. It is less effective against silverleaf whitefly where rates need to be doubled unless there is a mixed population with greenhouse whitefly.
Encarsia is most effective at temperatures between 20-30°C, at a relative humidity of 50-70%, and at high light levels. The life cycle is slower than that of whiteflies at daytime temperatures less than 18°C; the wasps become less active and searching ability is limited. Some compensation for reduced winter activity can be obtained by increasing introduction rates and/or by raising daytime temperatures for a few hours each day to exceed 18°C while lowering night temperature, if this is compatible with crop development. Lengthy periods on a constant basis above 30°C is usually detrimental.
Storage and handling:
Encarsia cards are shipped in padded bags for immediate use. As a general principle they should be kept cool at 8-10°C until they go into the crop, and used within two days. Longer storage will reduce the emergence rate. Avoid handling the cards as much as possible and particularly the black scales in the centre of the card. Suspect predation by ants, mice or earwigs if black scales are absent from recently hung cards as rubbing-off is unlikely. Encarsia scales will remain on cards for weeks after release and will stay black even after wasps have emerged.
Encarsia cards should be carefully separated and hung low in the crop canopy in the shade. Wasps tend to fly upwards when they emerge. The cards should be well spaced through the crop to minimise flying distances for the tiny wasps. In case of heavy whitefly infestations, it is better to increase the number of cards in these hot spots rather than move all cards to these areas and risk leaving the rest of the crop unprotected.
Timing of application:
Encarsia should be introduced preventatively rather than curatively, particularly on susceptible crops such as cucumber, tomato, eggplant, strawberry and gerbera. The aim is to prevent whitefly scales from developing and to multiply Encarsia within the crop early in the crop cycle. Good control of an existing moderate or heavy whitefly infestation is very unlikely without chemical intervention. For new crops, the first Encarsia introduction should ideally be made within a week of planting, continuing weekly, while carefully monitoring the first appearance and build-up of whitefly adults through strategic placement of yellow sticky traps. Increase introduction rates if necessary. Uncontrolled, the next generation of adults will show up as a sharp increase 3-4 weeks after their first appearance. Adult whiteflies entering from outside the greenhouse in significant numbers will negatively impact on successful control with Encarsia. For existing or perennial crops, prior knock down of whitefly populations to very low levels is desirable, by judicious use of short residual, low toxicity pesticides such as insecticidal oils and soaps.
Release rates: Contact Biological Services for crop specific advice.
Greenhouse whitefly: general introduction rate is 1-3 Encarsia/m2 (10+/m2 for hotspots) weekly for 8 weeks or until 80% of the older whitefly scales are black. Use the lower rates for preventative control and higher rates for curative control and for crops with hairy leaves. If whiteflies are still being detected on yellow sticky cards, continue introductions at low rates, even if no black scales are found. This may indicate that host feeding is preventing whitefly population development, or that it is being kept below detectable levels. Encarsia wasps live for approximately two weeks, less at high temperatures, so it is essential to keep up introductions. Exceptions are when most whitefly scales are parasitised, many black scales are present in the crop, or the crop is within a month of removal.
Silverleaf whitefly: Double rates and monitor for brown rather than black scales, with the typical round exit hole once the wasp has emerged. Good control of silverleaf whitefly can be achieved in poinsettia and hibiscus with Encarsia. A recently introduced parasitoid of silverleaf whitefly, Eretmocerus hayati, has established widely in Queensland and has been found in New South Wales, but there is currently no commercial production of this parasitoid.
Monitoring control success
Whitefly eggs are primarily laid on the undersides of new foliage most near the top of the plant. Because of the relatively long life cycle, mature scales will be on leaves at least 3 weeks old. Whitefly infestations tend to be patchy between plants and between leaves. Check several leaves in areas where whitefly adults have been observed or have shown up on traps. Look for adult wasps engaged in egg laying and host feeding on upper and middle strata leaves with younger scales, and black/brown parasitised scales on lower leaves. At least 80% of these mature scales should be black/brown before reducing Encarsia releases. Count and change yellow sticky traps regularly to monitor whitefly population trends.
Tips for best results
Encarsia and most other parasitoid wasps are very sensitive to pesticide residues, including systemic pesticides applied to the root zone. The crop must be free of harmful pesticide residues before initiating biocontrol. Start your program by contacting your seedling supplier and ensuring that no harmful, residual pesticides will be applied. Start with a clean greenhouse free of pesticide residues, pests and weeds. Monitor flying pests with yellow sticky traps even before the crop goes in, and continue through the crop. An old maxim for cucumbers is that no more than one adult whitefly per hundred plants should be present at the start of a program. If whiteflies are moving into the greenhouse from outside in any numbers, installing screening over vents and openings is strongly advised. If whitefly honeydew builds up in hot spots, wash off with water or it will hinder Encarsia movement. Avoid excessive de-leafing of tomato and cucumber plants if large numbers of black scales with unemerged Encarsia are present. Growers with pipe and rail systems may allow these leaves to remain under the crop for 1-2 weeks without risk of disease. Cutting of herbs may also remove black scales. Consider banker plants at the ends of rows in this case. Treat hotspots with rates in excess of 10 parasites/m2 weekly until under control.
Each batch of Encarsia undergoes a quality control check at Biological Services. In case of problems during shipment, retain an undamaged card periodically and keep in a glass or plastic container at 20-25°C, out of direct sunlight. Examine after 7 days. There should be at least 100 tiny wasps within the container. Cards may also be checked 2 weeks after placement in the greenhouse, by holding the black scales against the light and checking with a 10X hand-lens for a small round hole in most of the scales. Note that excess scales are added to each card, and that some wasps emerge from the side of the scales.
Encarsia formosa is very sensitive to many pesticides, particularly pyrethroids, organophosphate and neonicotinoids. Residues on foliage and structural components may remain toxic for many weeks and negatively impact on their survival and ability to effect control. Check side-effects charts carefully and avoid using pesticides before and during Encarsia use unless they are known to be safe.
Encarsia technical sheet (160 kb)