A Research Summary of Mediterranean Fruit Fly

By Nick Sakovich

The possibility of the Mediterranean Fruit Fly coming across our borders and invading precious agricultural lands has been a real threat for many years. Each past invasion has been successfully turned back. (Although, there are theories about residual populations remaining) With the increase of worldwide travel and tighter budgets for border protection, along with an uneducated public, we will most likely continue with these invasions, and perhaps one day, Medfly will become established. Because this is a real threat, we need to be informed about this pest, and what we can do to control it. Fortunately, there are teams of scientist throughout the world who are presently conducting Medfly research. One of the finest teams is located in Hawaii (Hawaii already has an established Medfly population). This past year, I visited the USDA-ARS facilities in Hilo and on the island of Oahu, in addition to talking with researchers at the University of Hawaii. The Hilo office of the USDA-ARS is under the capable guidance of Dr. Eric Jang, who is conducting some fascinating research of his own.

I hope Medfly never gets established in California, and I advocate that we do not relax any regulations established to keep it out. But, I must say that for me, the fear of this pest has greatly diminished after learning about the wide range of exciting research presently being conducted. Presented here is a brief description of some of this excellent work.

Medfly control can be categorized according to the following treatments. 1. chemical 2. hot and cold treatments 3. sterile releases 4. biological control, and 5. attractants.

Chemical Treatment

One of the new and most exciting materials available for the control of medfly is Sure Dye. It is a red dye which has been used in the cosmetic industry for many years. Obviously, it has a very low mammalian toxicity. This dye, when mixed with a bait, ingested, and under the presence of light, undergoes a chemical change which basically explodes the stomach of the fly. The exact mechanism of toxicity is not actually known. It is thought that a multitude of cellular components within the insect becomes rapidly oxidized, resulting in death. The length of time the insect is exposed to light does have an effect on the dyes’ ability to kill, but flies will most likely die even if kept in the dark, but it will take longer. Flies that have ingested the dye can easily be distinguished by their tiny pink bellies. A corn-based product called maizopherm is a good bait that can be mixed with the dye. An adjuvant may also be added for stabilization. This mixture has been quite successful in Guatemala when applied as an aerial spray.

Care must be taken however, because for some plants the dye acts as a defoliant, and can put holes in leaves. The less waxy the leaf is, the worse the damage will be. Another drawback to this product is its use in an urban setting, i.e. aerial eradication sprays over LA. The dye will stain cars and buildings. It will also stain some fruits, but will wash off the fruit. It is uncertain at this time just when Sure Dye will be registered in California. There are also other dyes which are being investigated.

One research project is presently looking at the impact of Sure Dye on nontarget species. So far it has been found that the parasite Fopius arisanus will eat the dye, but fortunately, the dosage that kills the medfly is much smaller than that at which the parasite will die.

Another new pesticide, produced by Dow AgroSciences, is call Success (active ingredient-Spinosad). It is produced through the fermentation of a naturally occurring microorganism. It has been used in Guatemala and works well. It is currently registered in California on several crops including citrus, but it is not registered for Medfly. A section 18 has been approved for Florida, for Medfly, and hopefully approval will soon come to California.

Sterile releases

Sterile medfly releases may be either all male or all female, or even both. The steriles are produced by irradiating the pupal stage of the fly. California prefers male releases and is currently releasing between 500 and 600 million sterile medflies each week. Presently, 500 million are being released over 2,165 square miles in the LA Basin alone. Male only releases are being used in Chiles’ eradication program as well as in Argentina. Males seem to search and disperse more than the females. Some disadvantages to female releases include the fact that even though the female is sterile she will still sting the fruit, causing possible cosmetic injury or an entry point for disease. In addition, wild males can mate with several females, and one of them might not be sterile.

When separating out males and females, to produce the one gender steriles, photoelectric scanners are used. The male pupae are brown, while females are white. One strain however, called Temperature Sensitive Lethal (TSL), allows for temperature --34 degrees C for 12 hrs -- to kill only the females, thus producing all males. Guatemala is presently mass rearing this strain, and soon Hawaii will also. It is harder to rear, and therefore more costly, but it is thought that mating frequency will increase.

When releasing sterile flies, a very high ratio of steriles to wilds is required. A good guess is that a given population would need 70-80% mating with the steriles in order to turn the population around. In Guatemala, a ratio of 100:1 in favor of the steriles was needed. In Kauai only 30:1 was needed, but the land was flat and the host was coffee (a favored host), planted in rows. Under this cultivated environment, it is easier for the sterile males to find the females. In the wild, a higher ratio would be needed for males to effectively search and find females. It is thought that with a ratio of somewhere between 50 and 100:1 the sterile fly program can work.

There is however a behavioral resistance encountered when releasing steriles. Females prefer the wild males (to steriles) by 50%. Therefore wild females will mate with 75% wild males and only 25% sterile males. Thus, the population needs to be flooded with the steriles. There is also a definite drop in male quality with the irradiated males. Perhaps a slower speed in wing beating (part of the mating ritual), or the production of incomplete chemical phermones. It is thought that the periodic injection of wild genes into the lab strain will help invigorate the stock. However, it is unfortunate, but the lab strain has adapted so well to laboratory conditions, that most likely the wilds would die out rather quickly. The common strain today, Hilab, is 42 years old. There is definite resistance in this strain when mating with the wild flies. Maui-med is a new strain which mates well with the wilds.

A lek sight is a mating arena. Usually one male per leaf, on the upwind side of the tree. Sunlight must also be able to penetrate the leaf. Sights may move within the day, depending upon the light. A female will land on top of the leaf and walk to the underside, guided by the pheromone. The pheromone is released by the male on the underside of the leaf, fanning his wings to direct the pheromone. The males can see the silhouette of the female above. Wild males will go back to these lek sites generation after generation. The laboratory reared sterile males go all over. In order to have mating, he will have to be at a natural lek site. Females will not go to an artificial site. Being able to better understand the formation of lek sites and the flies intricate behavior toward these sites, will enable researchers to manipulate the environment in order to obtain a higher frequency of sterile mating, and thus tremendously improve the sterile release program.

Integrated Pest Management IPM

When Mediterranean fruitfly becomes established in an area, there is hope for control through IPM.

The potential steps would be as following: 1. Sterile releases; 2. Male annihilation with lures. This can work with the Oriental Fruitfly, but for Medfly, the lure that is presently used is not quite efficient enough; and 3. Establishment of parasites - Fopius arisanus (formally Biosteres) is a good parasitoid. Presently, the USDA in Hawaii raises 7 different parasites which have good potential for biological control.

Irradiation of the Fruit

There may be quality problems with irradiated fruit-- surface pitting with citrus. Officially, Japan says they will not buy anything irradiated. Pummelos and mandarins are the only tropical fruits that Japan has accepted, and they do maintain decent quality. One main hurdle with irradiated fruits and vegetables is whether Japan will officially ever accept them. The other, is the expense. Presently, it costs over two million dollars for a small irradiation unit.

Medfly Lures

Alpha-copamine is an excellent lure for attracting medfly, but it is expensive and breaks down quickly. Trimedlure is the standard female lure, although many times a 50/50 male-female ratio is found in the traps. It is less expensive but not nearly as good as the alpha-copamine. The USDA in Hawaii has identified a female attractant, produced by the male, which contains over 50 compounds. In field cages, it works well to attract the female, but when put out into the field, it does not work so well. Research needs to be conducted to isolate the most potent of these 50 compounds and find out in which combination they work best. This will be an immense task.

Fruit flies require certain food sources, such as flowers, to provide them with nutrients for pheromone production. The flies do not seem to be able to produce all the compounds that are required, the remaining need to be supplied by feeding on specific food sources. It has been found that substances like ginger root oil and angelica seed oil contain alpha-copamine. The mating rate was increased three times when flies were fed compliments of these foods. Present research needs to concentrate on specific diets for the reared sterile flies, to enable them to better compete in attracting and mating with wild females.

Hot and Cold Treatments

If all our control measures worked well enough to downgrade this pest to the same status as say, red scale or thrips, we would still have to contend with a major obstacle…. Japan. That is, we will be quarantined and either loose certain markets or be made to conform to their quarantine rules and regulations. This is where cold treatment would then allow us to ship to these destinations. Actual research on cold treatment was conducted in the 1920's. APHIS has established protocol for shipping citrus fruits infested with medfly. Clementines, avocados and other fruits are presently cold treated. The cold treatment can be done in cartons, and in transit. However, the cartons would have to be placed in small storage units before they are placed in the hold of a ship. This is because uniform cooling and monitoring cannot be achieved in large areas. Thus, each smaller unit would have its own cooling apparatus and electronic monitoring. This can be done, but with an obvious higher cost per carton. We also need to be aware of any seasonal changes of the fruit itself. That is, research shows that chilling injury to lemons is greater with early-season desert fruit than mid or late-season fruit.

Heat treatments, whether hot water dips, vapor heat, dry heat or forced hot air, all have their problems with fruit quality. With lemons, pitting could occur due to rupture of the oil glands. Forced hot air treatment, which is the most common heat treatment for fruits in general, can cause problems with smell, texture, flavor and/or odor. The navel fruit becomes bitter, valencias have an off- flavor and loose their aroma. Grapefruit have the best tolerance along with Dancys’. Treatment would be 4-7 hours at 47.2 degrees centigrade at the center.

Although hot water dips are used for fruits like lychee, it would not be good for citrus, leaving the fruit in poor quality. Vapor heat treatments were used in Florida in the 1930's with fair success. It is now used quite extensively for papayas. There are many fruit quality problems with dry heat, but lemons and grapefruit seem to do quite well. Five thousand pounds is the optimal size for this heat treatment, but the industry wants 1 million pounds a day treated. This treatment would be much more expensive than cold treatment.

What plants is a good host for Medfly?

Many papers have been published which document the host range of Mediterranean fruit fly. Santa Clara County prepared a preferred host list which listed sweet orange and lemons as an ‘excellent host’ and avocado as ‘good’. The Entomological Society of America publication, Host Plants of the Mediterranean Fruit Fly: An Annotated World Review, lists 50 pages of hosts including several comments about field infestations of lemon fruits (1914 to 1990). Here are two: (1) "50 ripe fruits (lemon) were examined and found to have 1,422 eggs, 29 live larvae, and 354 dead larvae; in host preference tests with mango and ball kamani, flies did not oviposit in lemons (1915)"; and (2) 15-30 fruits per lemon cultivar were selected and placed in emergence units in a screened house; fruits were inspected weekly and the removed pupae were held until adult emergence; 20 kg of green Eureka fruit produced 0.7 pupae/kg (1988).

We must be careful not to make absolute statements about plant susceptibility. A good host in one geographic location may not be as good a host in another, and visa versa. Coffee is a very good host of Medfly. However, is it an excellent host solely because of the chemistry of the plant, or is there other environmental factors involved? As an illustration, if we look at coffee plants in a very high rainfall area, will those berries absorb more water and swell, thus creating a poorer environment for the larvae to develop. In other words, as plants are grown in different locations, slight changes may occur which influence their desirability. In addition, what is the makeup of the plant species in a given area; what is the competing host range? A desirable host in one area may become less desirable in another, because a better host is now available. If I were going to plant an orange orchard in a medfly infested area, I wouldn’t mind putting it next to a coffee plantation. The point is, we can’t be too dogmatic about hosts. Simply to say, for a given area, one must sample to see what the best hosts are, and during various time of the year.

One day in Hawaii, in one particular orchard, researchers were collecting soil samples. They placed the topsoil in sealed plastic bags. As the bags lay on the ground, large numbers of male medflies were observed congregating on the bags. Even when double bagged, the males were still strongly attracted to the bags. In the laboratory, the soil was place on a checkerboard along with other soils. The flies were attracted only to this particular orchard soil and none of the others. It is safe to say, that the soil from this orchard contains a powerful attractant. The task now is to analyze and identify the various components in this soil which are creating this very potent lure.