Obscure mealybug - Natural enemies and diseases

Obscure mealybug is attacked by a range of natural enemies, including both parasitoid wasps and predators. The most effective mealybug natural enemies are usually considered to be small wasps of the family Encyrtidae, and seven species parasitise the major mealybug pests of pipfruit in New Zealand. However, none of these are parasitoids of obscure mealybug (see below). All of these encyrtids arrived accidentally. The only species of parasitoid wasp which was deliberately introduced was Coccophagus gurneyi (Aphelinidae) in 1933. This does attack the obscure mealybug but it is a less preferred host than the other pest mealybug species. The predator complex includes a variety of ladybird species, lacewings, a hover fly, and a predatory midge (Cecidomyiidae). The only species of mealybug predator deliberately introduced to New Zealand, the ladybird Cryptolaemus montrouzieri, is restricted to the north of the North Island. A survey of mealybug natural enemies from 1990-92 confirmed that obscure mealybug was not attacked by the encyrtid wasps and had fewer natural enemies than the longtailed and citrophilus mealybugs. This has prompted proposals for further introductions of natural enemies for the biological control of obscure mealybug, such as the encyrtid wasp Pseudaphycus maculipennis, which is an effective parasitoid in the Mediterranean region.

Diseases of mealybugs have been little studied. The known species are all fungi and only about five species have been confirmed as pathogenic. None have been reported specifically infecting obscure mealybug.

The known and possible natural enemies in New Zealand are:

Brown or Tasmanian lacewing, Micromus tasmaniae (Walker). This Australian species eats a wide variety of insects and is common in pipfruit orchards practising organic or integrated fruit production. It has been reported feeding on longtailed mealybugs in Australia, and as early as 1945 feeding on mealybugs in New Zealand. However, more recent records of predation are lacking. This lacewing was first reported in New Zealand in 1869.

Coccophagus gurneyi Compére (aphelinid wasp) was introduced to New Zealand in 1933 and is now widespread. It parasitises a number of mealybug species (polyphagous) but is reported to prefer citrophilus mealybug > longtailed mealybug > obscure mealybug. It has been found attacking all three species in New Zealand on a range of crops, including pipfruit. In California, this parasitoid is reported to be more effective in winter than summer, and there is similar evidence in New Zealand. Ants may interfere with its effectiveness as a parasitoid during their 'farming' (collecting) of honeydew from the mealybugs.

Hover fly, Melanostoma fasciatum (Macquart). Larvae of this native species have been observed feeding on longtailed mealybugs and may also be a predator of the obscure mealybug.

Lacewing, Cryptoscenea australiensis (Enderlein). This Australian species arrived in New Zealand accidentally and was first recorded in 1927. The larvae and adults are commonly found feeding on mealybugs, including in apple trees in Nelson and Hawkes Bay. They feed on all the major pest species and appear to be widespread from Nelson northwards.

Ladybird, Cryptolaemus montrouzieri Mulsant. This species is from Australia and has been introduced to a number of countries for mealybug control, including New Zealand in 1897, and again in the 1920s. It established well in Auckland and Northland and was thought to be confined to those regions because of its requirement for warm conditions, including a mild winter. It has now also been recorded attacking mealybugs in Bay of Plenty and Gisborne. Both adults and larvae are predatory but it is not clear whether obscure mealybugs are part of their diet, particularly on pipfruit. The different distributions of this ladybird and the obscure mealybug appears to provide limited opportunity for significant predation in pipfruit crops. The larvae of C. montrouzieri are superficially similar in appearance to mealybugs. This predator can be easily reared and is a candidate species for mass production and inundative release for mealybug control.

Ladybird, Midus pygmaeus Blackburn was first reported in New Zealand in 1974, an accidental introduction. It has been recorded as an occasional predator of mealybugs on apples and grapes from Auckland to Hawkes Bay, but not feeding specifically on obscure mealybug. It is similar in appearance and size to S. loewi, and the larvae resemble mealybugs due to their white waxy secretions.

Ladybird, Rhyzobius sp. An unknown species of Rhyzobius ladybird was recorded feeding on mealybugs on citrus in Northland during a survey of mealybug natural enemies in 1990-92. This predator may be present in other regions and crops.

Ladybird, Scymnus loewi Mulsant, originates in North and Central America. It was accidentally introduced to New Zealand and was first recorded at Auckland in 1941. Adults and larvae have been found feeding on obscure mealybugs on apple trees in Hawkes Bay. The larvae appear very similar to mealybugs, and are covered with a dense white waxy secretion, similar to that of the larger C. montrouzieri.

Ophelosia charlesii Berry and Ophelosia keatsi Girault (pteromalid wasps from Australia). O. charlesi is mainly a predator within the egg sacs and also a primary parasitoid of mealybugs, but the feeding habits of O. keatsi are unknown.. There are commonly three or four Ophelosia larvae (up to six may develop) in each mealybug egg sac. O. charlesi adults were reared from obscure mealybugs in the North Island during a survey of the natural enemies of mealybugs in 1990-92, including from apples in Hawkes Bay. Wasps of this genus have also been recorded from mealybugs in Nelson. O. charlesi was first recorded in New Zealand in 1925, and O. keatsi in 1963. O. charlesi has been reared in the laboratory at ambient temperatures and took about 52 days to complete development (from pairing to offspring adult emergence). An adult female has been recorded to survive 56 days in these conditions.

Predatory midge, Diadiplosis koebelei (Koebele). The predatory larvae of this midge were found to be the most widely distributed of the mealybug predators during a survey of their natural enemies from 1990-92. They attacked all the major pest species on all the crops surveyed, including pipfruit. The pupae were found in spun silken cells alongside the mealybug cadavers. Early imports of this predator to New Zealand in 1927-28 were not released and it is thought to have arrived accidentally on mealybug-infested produce.

Tetracnemoidea brevicornis (Girault) (encyrtid wasp) is widespread from Auckland to Southland. It is not known when it became established in New Zealand but the first record was in 1924. During a survey of mealybug natural enemies from 1990-92, this wasp was usually reared from citrophilus mealybugs, and secondarily from longtailed malybugs. Early records of this parasitoid from Pseudococcus spp. are unlikely to be obscure mealybug because there is no confirmed more recent case of parasitism of this species.

Tetracnemoidea peregrina (CompËre) (encyrtid wasp). Although this species has been regularly recorded parasitising longtailed (and occasionally citrophilus) mealybugs in New Zealand, there are no confirmed cases of parasitism of obscure mealybug. Overseas research has suggested that obscure mealybug may be a host.