Hosts or Prey
California red scale and certain other armored scales
Identification
Obvious evidence of the activity this tiny wasp or its relatives is that after maturing and emerging as an adult the wasp leaves a rounded hole in scale covers. Beneath the scale cover Aphytis leaves behind a flat and dehydrated scale body and its cast skin. Black fecal pellets (meconium) of the wasp may be observed outside of the scale body, but under the scale cover. Note that a portion of the adult Aphytis parasitoids emerge by pushing out between the scale cover and plant substrate; the cover of these parasitized scales commonly falls off the plant, removing evidence of parasitoid presence.
The A. melinus adult has a yellow abdomen and thorax and is about 1/16 inch (1.5 mm) long. The head has short, knobby antennae and 3 tiny red eye spots arranged in a triangle on top. Large, true eyes bulge from the side of its broad, rounded head.
The eggs, larvae, and pupae of A. melinus occur on the host scale's body but hidden under the scale cover. Aphytis eggs are 1/125 inch (0.2 mm) long and oval with a distinct stalk. The pale larvae are oval or saclike with distinct segments. At maturity the last (third) instar is about 1/32 inch (0.8 mm) long. The pupa is oblong with distinct segments. As it ages distinct appendages develop folded against the body and the wasp's head with bulging eyes becomes apparent.
Lookalikes
Adult male California red scales resemble adult A. melinus. They occur together, are about the same size, and have yellow bodies. However, Aphytis has two pairs of wings and short antennae that are not obvious. The male scale has one pair of wings and relatively long antennae. The male scale has a dark reddish ring around the thorax near the base of the wings; adult Aphytis lack a red ring. The male red scale has two, small, red eyes and a triangular-shaped head that is relatively narrow in front. The head of Aphytis is rounded with two, large, bulging, pale greenish to yellow eyes.
California red scale is also commonly attacked by two other parasitoids, each of which prefers to attack certain scale life stages. Comperiella bifasciata is a common internal parasitoid of California red scale. In comparison with Aphytis, C. bifasciata leaves a larger, more irregularly shaped exit hole in the scale cover when it emerges. Aphytis melinus prefers to parasitize scales on fruit and leaves while C. bifasciata prefers scales on stems. Aphytis melinus prefers to parasitize third instar females while C. bifasciata prefers first and second instar scales.
Encarsia perniciosi and several other Aphytis species including A. aonidiae also parasitize California red scale. The adults and immatures of the other Aphytis generally resemble those of A. melinus. But unlike Aphytis the E. perniciosi adult is black to dark brown and its immatures feed inside host scales, not attached to their outside as with Aphytis species.
Life Cycle
Aphytis melinus develops four life stages: egg, larva, pupa, and adult. After hatching from an egg, the larva develops though three increasingly larger instars, pupates, then emerges as an adult.
The Aphytis female inserts her ovipositor through the cover of armored scales and injects the body with venom that prevents the scale from developing further. She then lays 1 to several eggs under the cover and on the outside of the scale body. Each Aphytis female can lay about 100 eggs during her lifespan of about 2 weeks.
Scale first instars and the molt stages (when the body and cover are tightly connected) are not acceptable for egg laying by A. melinus. The wasp prefers to lay eggs on third-instar, virgin female scales because they are large and provide all of the food needed for its progeny to survive. It also parasitizes male and late second-instar female scales if the third-instar female scales are not available.
The armored scale is killed as the parasitoid larvae feed on it. The Aphytis larvae accumulate food in their gut until they mature into prepupae and then excrete the blackish to dark brown fecal pellets. The mature larvae (prepupae) then molt into pupae, which molt into adult wasps and emerge.
The adult Aphytis female also feeds on hosts to obtain protein to mature her eggs. During host feeding the female parasitoid punctures scale bodies with her ovipositor and consumes the exuding body contents. Aphytis also mutilates scales by puncturing and killing them without using those scales for egg laying or host feeding. At times, host feeding and mutilation kill as many scales as parasitization.
Aphytis develops from an egg to an adult in about 2 weeks when temperatures are warm. There can be 2 to 3 Aphytis generations per generation of scales. Aphytis melinus can have up to about 1 dozen generations per year.
Habitat
Aphytis melinus is the most important natural enemy of California red scale. Other hosts of A. melinus include latania scale, oleander scale, and San Jose scale.
In the San Joaquin Valley Aphytis must be periodically released to ensure effective biological control of California red scale. The hot, interior valley weather causes scale life stages suitable for parasitization to sometimes be absent and as a result the parasitoid is unable to continually maintain high enough populations to always control the scale.
The parasitoid performs best against scales when all their life stages are present. Because of milder weather, multiple parasitoid-susceptible host stages are more prevalent year round in growing areas in southern interior California and along the coast, increasing the naturally occurring biological control.
Commercial Availability
Where resident natural enemies are insufficient, releasing purchased A. melinus can control California red scale. To improve the effectives of resident natural enemies and any that are released
- Control ants because they attack natural enemies of scale insects and other pests.
- Grow flowering insectary plants to provide nectar to nourish adult parasitoids and predators.
- Reduce dustiness that disrupts the activities of natural enemies (e.g., periodically hose off shrubs).
- Avoid the application of broad-spectrum, persistent insecticides for all pests because they are toxic to natural enemies.
- Rely on pesticides compatible with natural enemies and prune tree canopies to improve parasitoid access to scales.
How many and how often to release A. melinus varies by location and grove history. Consult the Pest Management Guidelines: Citrus for specific release recommendations in commercial citrus. See The Association of Natural Biocontrol Producers, Biological Control Organisms for Insects and Mites (PDF), Natural Enemy Releases for Biological Control of Crop Pests, and Protecting Natural Enemies and Pollinators for more information.
Species
At least 23 Aphytis species occur in California. They each parasitize multiple species of armored scales. For example A. melinus parasitizes at least 24 species of armored scales. California red scale in California is parasitized by at least 16 Aphytis species of which A. melinus is the most common and important.
More Information
- California Red Scale Aonidiella aurantii (Maskell) Diaspididae, UC Riverside
- Competition, Resource Partitioning and Coexistence of an Endoparasitoid Encarsia perniciosi and an Ectoparasitoid Aphytis melinus of the California Red Scale, UC Riverside, Ecological Entomology
- Competitive Displacement: Extinction of the Yellow Scale, Aonidiella citrina (Coq.) (Homoptera: Diaspididae), by its Ecological Homologue, the California Red Scale, Aonidiella aurantii (Mask.) in Southern California, Hilgardia
- Integrated Pest Management for Citrus, UC Integrated Pest Management Program
- Life Stages of California Red Scale and Its Parasitoids, UC Agriculture and Natural Resources
- Pest Management Guidelines: Citrus, UC Integrated Pest Management Program
- Universal Chalcidoidea Database, Natural History Museum of London
Scientific classification:
- Phylum: Arthropoda
- Class: Insecta
- Order: Hymenoptera
- Family: Aphelinidae
