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

Rearing larvae

Use of whole fruit

Whole fruit are used when the available artificial diets are not suitable for the particular species or when an artificial rearing system is still under development. Whole host fruit may also be used when, and if, adult fly numbers are very low.

Artificial diets

It is essential to develop inexpensive diets that are nutritionally suitable for larval development, contain ingredients that are continuously available, are of known quality, and free of any pollutants. Virtually all artificial larval diets used world-wide have common characteristics. They normally include water, microbial inhibitors, sources of protein, carbohydrate and lipid, plus vitamins, salts, minerals and sterols. The two other ingredients usually added to fruit fly larval diets are a bulking agent and an agent for adjusting pH. In Hawaii, wheat millfeed standard diets have been used for Bactrocera dorsalis (Oriental fruit fly), Ceratitis capitata (Medfly) and Bactrocera cucurbitae (melon fly) and an improved diet based on bran has been developed for rearing Bactrocera latifrons.  Note that cucurbit-infesting species requires special techniques that differ from the standard methodology described here, this information will be available subsequently.

Initial developments of the diets for the Pacific Island Countries and Territories were carried out in Cook Islands and in Fiji Islands. The initial bulking agent used in Cook Islands was cassava and then sugarcane bagasse in Fiji Islands. Pawpaw is now the main ingredient used because it is generally available all year and, if picked at colour break, is not infested by fruit flies. Another advantage of using fresh fruit in the diet is that the pH requires no adjustment. The pH is normally held at 4-5.5 to deter bacterial and fungal contamination. The other ingredients are Torula yeast and Nipagin, which are standard ingredients in insect diets. Torula yeast is a standard source of protein in larval diets although Brewer's yeast (Saccharomyces spp.) may also be used. Nipagin (methyl p-hydroxy-benzoate), or sodium benzoate at 0.1% of the diet, are added to stem both bacterial and fungal development, deleterious to fruit fly larvae.

Variations in the quality of the bulking agents, cassava and sugarcane bagasse used in Cook Islands and Fiji Islands led to unreliable data in larval developmental studies, and together with difficulties in extraction of larvae from these diets led to further development work on the diet. Researchers working in Cook Islands discovered that storing the other ingredients (pawpaw, Torula yeast and Nipagin) after mixing without the bulking agent at 4ºC for at least 24 hours led to gelling of the ingredients. Excess water produced in this gelling process could be decanted off, producing a diet from which larvae could be easily extracted using a sieve under running water. This, now standard diet has been further developed by substituting pawpaw with other available fruits, particularly breadfruit (used in Fiji Islands, Solomon Islands and Vanuatu) and banana in New Caledonia.

All range of diets used in the ten different Pacific Island Countries and Territories for various fruit fly species are given in the table below. The standard diet, either with pawpaw, breadfruit or banana as the main ingredient, is used for all species in all countries except B. umbrosa in New Caledonia, which is reared on dried potato/carrot diet. This species is reared on breadfruit diet in the Solomon Islands and Vanuatu. The standard diet is simple to make and stores well in the refrigerator but loses some quality if frozen. The two main problems with these diets are the threat that the fruit may be contaminated by pesticides, or that they may contain wild fruit fly eggs or larvae. It is essential to ensure that fruit are not infested by picking them at the appropriate stage or protecting them by bagging. Pawpaw is picked at colour break and breadfruit and banana are picked at the mature green stage, about one week before use. All of the fruits are then carefully stored to prevent insect infestations and allow natural ripening.

Table: Artificial fruit fly diets used in the Pacific Island Countries and Territories (ingredient amounts in grams).

Other standard diets may be used. For example, in New Caledonia, researchers were having difficulty mass-rearing B. umbrosa. However, a suitable diet based on potato and carrot was developed by Clare. The use of carrot for fruit fly larval diets is well known, with dried carrot diet the most commonly used diet in small-scale rearing around the world. There is evidence that carotene (from carrot) is an important feeding stimulant which promotes growth, particularly during early larval development. Water is also very important, both for minimising the effects of metabolic heat build-up during the final stages of larval development and in affording greater access to nutrients. There are indications that B. cucurbitae needs a diet of higher moisture content than some other species. The standard diets used in the PICTs that are based on natural fruit pulp have a naturally high water content, but the addition of a little water is sometimes necessary for the right consistency when using fruit that are not fully ripe.

General preparation of fruit diets

All ingredients must be thoroughly mixed, particularly the Nipagin. This is achieved by mixing the Nipagin, dissolved in a little warm water if necessary, and yeast to small quantities of fruit. Use a blender if available. The diet is stored at 4°C for at least 24 hours which allows the mixture to gel prior to decanting off excess water before use.

Larval trays and storage containers

Larvae may be reared in various shallow trays or dishes containing the diet. Diet thickness is an important factor as the greater the surface area to volume ratio the greater the likelihood of metabolic heat dissipation. However, thin diet is prone to drying. Diet (500-800 g) is spread onto a shallow tray at a thickness of 3-5 cm. Diet trays with eggs are placed in plastic containers with tight-fitting lids sealed with tape and labeled. The lid must have a large ventilation hole covered with fine insect screen to keep Drosophila (vinegar flies) out. Ventilation is important for efficient gas and heat exchange. However, during the first 3-4 days the vent should be sealed and covered to ensure high humidity for good egg hatch and prevent the entry of light which may promote excess yeast growth on the diet surface.

Eggs deposited on diet (left), larvae in diet tray placed over moist sawdust (centre) and larvae in diet (right) (Photos: L. Leblanc)

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 Page updated on: 17 October, 2002