INTERESTING INSECTS

By N A Martin (2010, revised 2015)

Small hoverfly

Plesia fasciata Macquart, 1850

Melanostoma apertum Hutton, 1901

This endemic hoverfly is present throughout New Zealand, where it occurs in grassland, vegetable crops, field crops, cereals, gardens and other habitats with low growing vegetation. The adults feed in flowers while the larvae feed on insects.

Conservation status: The small hoverfly is widespread and not threatened. It is a useful biological control agent in many commercial and domestic crops.

The adults are typical flies with one pair of wings. The wings are usually held over the body when not used for flying. The head has large compound eyes and the thorax (the middle part of the body) is dark and shiny on top. The abdomen, usually hidden by the wings, has a pair of yellow patches on three segments. Like most insects, this hoverfly has three pairs of legs. The female small hoverfly lays small white eggs either singly or more commonly in small groups on leaves, usually near infestations of prey. A pale larva hatches from each egg. It has no legs and moves with a motion similar to that of a seal on land. The mouth is used to catch and hold prey. At the rear end of the larva is a projection that at its tip has a pair of openings of the trachea, breathing tubes. As the larva grows, it moults (changes its skin). There are several larval instars (stages). As the larvae grow, they become darker in colour. When the larva is fully grown, it attaches itself to a sheltered place on a plant and moults into a pupa. The pupa has a smooth teardrop shape. Adults hatch from pupae and mate. The length of time of each life stage depends on temperature, being shorter at higher temperatures.

Annual cycle

It is not known which stages of the small hoverfly are present in winter. Some species of hoverfly overwinter as adult females. Small hoverfly adults and larvae are seen in spring, summer and autumn. There are several generations per year.

Walking and flying

Larvae of the small hoverfly are legless. They move over the surface of plants with a motion that resembles that of seals on land.

Adults have both wings and three pairs of legs. They are proficient and agile fliers and as their name suggests, they can hover in one place. They are also able to walk nimbly over flowers.

Feeding

Adults feed on nectar and pollen from flowers. They have a short proboscis and can only feed on nectar in flowers where it is exposed, such as flowers of the carrot family. They eat pollen directly from the anthers. Beverly Holloway, in her study of pollen feeding by New Zealand hoverflies, found that plantain (Plantago sp.) pollen was favoured by the small hoverfly. They also fed on pollen of grasses and dandelion types of flowers. After feeding on a flower, adults may brush their bodies with their legs to collect any pollen attached.

Small hoverfly larvae feed on aphids, caterpillars and other small insects, including their own species. When a larva is close to its prey, it thrusts its head forward and grabs hold of the insect with its mouth. It then sucks the prey dry.

Top, dorsal, side of adult small hoverfly, Melanostoma fasciatum (Diptera: Syrphidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 15MW]

Underside of adult small hoverfly, Melanostoma fasciatum (Diptera: Syrphidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 15MV]

Top, dorsal, side of adult small hoverfly, Melanostoma fasciatum (Diptera: Syrphidae) cleaning its eye with its foreleg. Creator: Tim Holmes. © Plant & Food Research. [Image: 15MU]

Eggs of the small hoverfly, Melanostoma fasciatum (Diptera: Syrphidae), on a lettuce leaf. Creator: Plant & Food Research Photographer. © Plant & Food Research. [Image: 3F8]

A young larva of the small hoverfly, Melanostoma fasciatum (Diptera: Syrphidae), on a lettuce leaf. Creator: Plant & Food Research Photographer. © Plant & Food Research. [Image: 3F9]

Larva of the small hoverfly, Melanostoma fasciatum (Diptera: Syrphidae), feeding on aphids on barley. Creator: Plant & Food Research Photographer. © Plant & Food Research. [Image: 3F7]

Two pupae of the small hoverfly, Melanostoma fasciatum (Diptera: Syrphidae) on a lettuce leaf. Creator: Plant & Food Research Photographer. © Plant & Food Research. [Image: 3FB]

A pupa of the small hoverfly, Melanostoma fasciatum (Diptera: Syrphidae), showing the eyes of the developing adult. Creator: Plant & Food Research Photographer. © Plant & Food Research. [Image: 3FC]

The small hoverfly, Melanostoma fasciatum is very similar in appearance to the large hoverfly, Melangyna novaezealandiae (Macquart, 1855), but is smaller and has larger yellow patches on the upper (dorsal) side of the abdomen. The larvae of both species are similar in appearance except for the size difference of the mature larvae. The small hoverfly is found throughout New Zealand, while the large hoverfly is much less common in the northern North Island.

Pinned specimen of adult small hoverfly, Melanostoma fasciatum (Diptera: Syrphidae) collected by J Armstrong in 1960 from Taupo, note the large yellow patches on the abdomen are obvious even though the colours have faded. Creator: Birgit E Rhode. © Landcare Research. [Image: 15MX]

Top, dorsal, side of adult small hoverfly, Melanostoma fasciatum (Diptera: Syrphidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 15MW]

Top, dorsal, side of adult large hoverfly, Melangyna novaezealandiae (Diptera: Syrphidae) showing small yellow areas on abdomen. Creator: Tim Holmes. © Plant & Food Research. [Image: 1J33]

Top, dorsal, side of adult large hoverfly, Melangyna novaezealandiae (Diptera: Syrphidae) showing small yellow areas on abdomen. Creator: Tim Holmes. © Plant & Food Research. [Image: 1J32]

No pathogens of the small hoverfly are known.

Parasitoids

Hoverfly larvae may be parasitized by Diplazon laetatorius (Fabricius, 1781) (Hymenoptera: Ichneumonidae). The adult female lays eggs on the hoverfly larva, which completes development and pupates. The parasite pupates inside the hoverfly pupa. When the adult parasitoid hatches from its pupa, it chews an exit hole in the syrphid pupa.

Predators

Birds and predatory insects have been recorded feeding on the hoverfly. Spiders are also likely to catch and feed on them. Larvae of the small hoverfly are known to feed on each other.

Adults feed on nectar and pollen from flowers. They have a short proboscis and can only feed on nectar in flowers where it is exposed, such as flowers of the carrot family. They eat pollen directly from the anthers. Beverly Holloway, in her study of pollen feeding by New Zealand hoverflies, found that plantain (Plantago sp.) pollen was favoured by the small hoverfly. They also fed on pollen of grasses and dandelion types of flowers. After feeding on a flower, adults may brush their bodies with their legs to collect any pollen attached.

Small hoverfly larvae have been recorded feeding on several aphid species that live on vegetable crops, cereals, grassland plants and native plants. Although usually regarded as an aphid predator, they have also been recorded feeding on caterpillars, mealybugs, psyllids and their own species. The hoverfly probably has a wide prey range.

When a larva is close to its prey, it thrusts its head forward and grabs hold of the insect with its mouth. It then sucks the prey dry.

Biological control of pests

Biological control of aphids and other herbivorous pests can reduce the impact of the pests and the need to use insecticides. The small hoverfly is an important predator in grassland, various vegetable crops, in gardens and in native ecosystems. Their activity reduces the need for other means to control many pests. If pesticides are needed to control some pests, it is advisable to use chemicals that will have minimal harmful effects on the hoverflies or to use them at a time when hoverflies are not present.

Research projects

Several important aspects of the biology and annual cycle of the small hoverfly are unknown:

1. which stages are present in the winter,

2. is there a winter diapause (resting stage), and

3. how many larval instars are there?

This last problem could be investigated by collecting larvae and determining the length and shape of the mandibles and posterior stigmata (opening of breathing tubes) to see if these form distinct groups representing each instar.

Early J 1984. Parasites and predators. In: Scott RR ed. New Zealand Pest and Beneficial Insects. Canterbury, Lincoln University College of Agriculture. Pp. 271-308.

Hickman JM, Lovei GL, Wratten SD 1995. Pollen feeding by adults of the hoverfly Melanostoma fasciatum (Diptera: Syrphidae). New Zealand Journal of Zoology 22: 387-392.

Holloway BA 1976. Pollen-feeding hover-flies (Diptera: Syrphidae). New Zealand Journal of Zoology 3(4): 339-350.

Plant-SyNZ: Invertebrate herbivore-host plant association database. plant-synz.landcareresearch.co.nz/.

Valentine EW 1967. A list of the hosts of entomophagous insects of New Zealand. New Zealand Journal of Science 10(4): 1100-1209.

Graham Walker and Peter Workman for information on the biology of the hoverfly and helpful comments.

Birgit E Rhode for the photograph of the pinned fly.

The New Zealand Institute for Plant & Food Research Limited (Plant & Food Research) for permission to use photographs.

Landcare Research New Zealand Limited (Landcare Research) for permission to use photographs.

5 June 2015. NA Martin. Names: synonym added. Annual Cycle and recognition: new photographs of adult flies. Prey table added. Natural enemy table added.