Laboratory-Rearing Techniques for Tephritid fruit flies
in Pacific Islands Countries and Territories

Fruit collecting and colony establishment 

Fruit collecting and colony establishment

Vigorous fruit fly colonies of the economic species are best established by collecting as many fruit fly individuals from as many different hosts and habitats as possible because the evolutionary potential of a laboratory population is essentially determined when the breeding stock is isolated from the field. This can be achieved by collecting large quantities of ripe host fruits and vegetables and setting up as for collection of pupae with drainage for wet fruit to prevent larvae from drowning. Collections at different times of the year are also recommended. Resultant pupae should be placed in a small emergence cage containing water and sugar. Emerging adults are identified and transferred to a separate culturing cage. This process eliminates other insects that may emerge from the fruit, including other fruit fly species and parasitoids. Every effort must be made to minimise mortality during this process.

The major bottleneck in laboratory rearing usually occurs within three to five generations of the colonies being established, particularly the first generation. To preserve the genetic variability of the colony, every effort must be made to minimize this mortality. High mortality can be avoided by not overcrowding the flies, which causes stress-related mortality, by providing excess food, and by avoiding temperatures above 30°C. In the initial stages of establishment, it is better to use small cages (about 20 x 35 x 25 cm) and combine as many adults from different hosts or habitats together, after identification of the fruit fly species, to maximize mating, yet avoiding overcrowding. Adults of similar age (within 2-3 weeks) should be combined in the same cage and as the numbers are increased into the hundreds, all the adults should be transferred to larger cages (50 x 50 x 50 cm). Colonies should be increased to 5-6 large cages for each species required for research purposes, with each cage containing 5000-10000 flies.

Whole fruit rearing

If a good diet has not yet been developed for the species being reared, or adult numbers collected from the field are low, whole fruit may be used to establish the colony. This alternative requires collecting preferred host fruit before it is infested (e.g. pawpaw before colour break) or protecting fruit by bagging to ensure it is not infested before use. The whole-fruit egging system requires spiking the fruit with a number of small 1 mm holes (number depends on the number of females and size of fruit) and ensuring sufficient fruit for each larva (2 g per larva). One female may oviposit 10-30 eggs in 24 hours so enough weight of fruit must be provided to sustain the possible number of larvae. Fruit should be set up with drainage to ensure larvae do not drown in liquid (see host fruit surveying).

Maintenance

Maintaining viable fruit fly colonies is a matter of careful rearing, diligent monitoring of quality control, and periodic strain restoration or replacement. Colonies evolve rapidly and then remain relatively stable if rearing techniques are not changed. Replenishment of a colony is preferable to total replacement, and the success of strain maintenance strategies (i.e. the frequency and degree of restoration or replacement) depends on the quality of the mass production system.

There is severe selection pressure during laboratory mass-rearing with consequent rapid adaptation of fruit flies to artificial rearing regimes. To ensure the colony remains biologically similar to the wild populations, the flies are regularly rejuvenated by crossing wild males exclusively with laboratory-reared females and at the same time crossing the wild females with laboratory-reared males every 12 months. This is a labour intensive task, but it is an integral part of rearing maintenance. Field collected host fruit are set up as for collecting and adult males aspirated out daily and placed in a cage containing only laboratory females. Adult females from host fruits are combined with laboratory reared males. Adult density is maximized to promote mating (without overcrowding). All new cages are set up with the progeny of these two cages. Flies from older cages are discarded once a healthy, 'new' colony is established (at about three cages, each with 5000 adult flies).

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 Page updated on: 30 June, 2004