Know Your Friends

Metaphycus helvolus, Parasitoid of Soft Scales

Metaphycus helvolus is a small encyrtid wasp native to South Africa. It was introduced into the United States in 1937 as part of a major biological control campaign against black scale in California. Black scale was the most serious pest of citrus in southern California until M. helvolus became established, and has since become a secondary pest of that crop. M. helvolus is a highly effective parasite in semi-tropical areas--or in indoor plantings in the Midwest--but does not survive in areas with colder winters. It is a polyphagous species with a host range including many different soft scales. M. helvolus readily attacks black and hemispherical scales, as well as brown soft scale, nigra scale, citricola scale, and European fruit lecanium.

This tiny wasp is only 1 mm long. The female is orange-yellow and the male is dark brown. Each female lays up to 5 eggs per day in the bodies of late second and early third instar scales. Larger scales are not parasitized because it takes too long for the female to drill through the thick scale covering. Well-fed females live 2-3 months and lay an average of 400 eggs over their relatively long lives. Wasp larvae hatch in 2 days and develop singly inside the scale bodies. After about two weeks the adult wasp emerges by cutting a small hole in the scale. Males emerge from small scales and females from large scales. Several generations may be produced each year as long as appropriate sized scales are available. Optimum conditions are 50% humidity and 24-30C, but the wasps will be active at cooler temperatures.

In addition to parasitizing scales, the females also kill many more scales by host feeding. The wasp makes a hole in the scale with her ovipositor, then feeds on body fluids exuding from the wound. They host feed primarily on scales smaller than those used for oviposition. Host feeding allows the wasps to produce more eggs and doubles their longevity, but the wasps must also have a carbohydrate source (honeydew or nectar) to feed on.

In greenhouses or interiorscapes M. helvolus works well against hemispherical scale and has provided control of black scale, but may not provide satisfactory control of brown soft scale. Many populations of brown soft scale in interiorscapes encapsulate and kill the developing wasp larvae, so biological control is less than satisfactory. But intensive host feeding, which kills ten scales for each one parasitized, may be sufficient. Other parasitoids (such as M. alberti, which is not currently commercially available) may provide better control of brown soft scale.

M. helvolus is commercially available from many suppliers produced primarily for augmentative control of black scale in California citrus and olives. For indoor plantings, release M. helvolus adults two or three times at 2- to 3-week intervals at a rate of 5-10 per infested plant, 10/m2 or up to 1,000 parasitoids per acre. Scatter the wasps on plants evenly throughout the infested area. The best time to release the wasps for control of hemispherical scale is just before the scale reaches the third instar--when a prominent H shaped ridge is present on the scale surface. Wasp emergence holes should be visible after 2 weeks and control should be achieved in 2-3 months. Periodic additional releases may be necessary to maintain control.

Insecticides should not be used on the plantings within several months before releasing wasps. Toxic residues persist much longer in greenhouse and interiorscapes than in other situations. Heavy scale populations should be reduced (use insecticidal soap or manually remove mature adult scales) 2-3 weeks before making releases. Dust can be almost as deadly as insecticides to these tiny wasps, so occasionally washing the plants with water to remove dust may conserve their populations.

- Susan Mahr, University of Wisconsin - Madison

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