Papaya Mealybug and bio safety

ANURA WIJESEKARA

In spite of the tireless efforts of the Department of Agriculture (DOA) personnel, public awareness on the threatening pest species, Papaya Mealybug (PM), remained at a low level until some political parties used ‘Piti Makuna’ in the Western Provincial Council election speeches. The Papaya Mealybug is an alien invasive insect species, scientifically known as Paracoccus marginatus.

This type of an alien pest can accompany any live material like fruits, flowers, seeds, floriculture/foliage plant parts that the public usually bring in their luggage from foreign countries. Therefore, any one could have inadvertently brought it to Sri Lanka. This is another very good example for the need to respect the quarantine regulations of the country which are blatantly violated by some due to ignorance, carelessness or power.

Papaya plants

This polyphagous (feed on many plant species) pest was first detected in mid 2008 and it has already spread throughout Colombo and Gampaha Districts and now threatening its spread throughout the country. It has already killed many papaya plants and capable of surviving on and damaging more than 80 different plant species. Due to the nature of the pest and the variety of host plants it attacks insecticidal control is not practicable.

Therefore, to control this pest the DOA has decided to introduce a tiny (less than 0.5 mm in length) parasitic insect that has already proven to be successful in controlling PM, elsewhere in the world. Recently I told one of my friends that the DOA has taken necessary actions for managing this pest by way of introducing a natural enemy.

His response was that the public is aware of the proposed biocontrol attempt but doubtful of the appropriateness of introducing another alien insect to control the pest. What my friend told me was exemplified by the huge title in one of the daily newspapers which read; ‘Many problems due to importing American insects to control Mealybug’. The newspaper article stated that a minister has raised the issue of effects on non target organisms due to introduction of alien parasitoids.

This doubtfulness, I consider as a very positive indicator of the increased environmental concern and awareness of our society.

The objective of this article is to discuss the proposed PM management procedure by the DOA and the public concern mentioned above.

Deliberate introduction of natural enemies into a new country to control an exotic pest (insect, disease or weed) is known as Classical Biological Control. Historically people have realized when an organism invades a new country with suitable habitats the invading organisms thrive very well mainly due to lack of its natural enemies. This understanding led to exploring of the pests’ original habitat for its natural enemies and introducing the natural enemies to newly invaded areas by the pest.

Termed Classical biological control, this practice is considered an economical, ecologically and socially sound pest control strategy in contrast to use of pesticides.

Papaya Mealybug is an exotic insect known to be native to Central America (Mexico), which has no natural enemies in Sri Lanka. Since its introduction to the country in 2008 no local predator or parasitoid has managed to keep Papaya Mealybug population under control irrespective of many known local parasitoids and predators of 37 native mealybug species already present in Sri Lanka.

This and impracticability of using insecticides for control make PM a typical candidate for classical biological control attempt.

Survey of scientific literature shows that the PM has spread from Mexico where it is not a pest due to the presence of natural enemies, to Caribbean region in 1994, Florida in 1998 and to the Pacific Islands in 2000. More recently it has established in Malaysia, Indonesia, Thailand and India.

Control programs

The Mealybug has become a very serious pest of papaya in Florida, the Carribian and Pacific regions. Entomologist have surveyed Mexico and found three parasitoids from the original habitats of the Mealybug. These parasitoids have been multiplied and introduced into Florida, the Caribbean and Pacific regions to control the papaya Mealybugs. One year after introduction the pest population has decreased by 97-99 percent making those classical biological control programs an astounding success.

All three insects Anagyrus loecki, Pseudleptomastix maxicana and Acerophagus papayae, belonging to the family Encyrtidae (Hymanoptera) are tiny parasitoids (less than 0.5 mm length) of mealybugs. Detail studies of the impact of these parasitoids on PM have shown that most of the control has been done by Acerophagus papayae after introduction. Acerophagus papayae is known to be a specific parasitoid i.e. feed only on Papaya Mealybug while Anagyrus loecki feed on more than one mealybug species.

Based on this knowledge the DOA scientists have decided to import and release Acerophagus papayae to control Papaya Mealybug.

The parasitoids will be imported from Puerto Rico where PM parasitoids are being cultured by Puerto Rico Department of Agriculture in collaboration with Animal and Plant Health Inspection Service (APHIS) of the United States Department of Agriculture (USDA). Release of the parasitoids and monitoring of its establishment will be carried out in selected areas where the pest has already spread.

Decision to import and release Acerophagus papayae give rise to two major questions. Will this parasitoid be able to control the pest? and will there be any impact of parasitoid on non target organisms?

Although a certain answer cannot be given to any of the questions, available scientific evidence and our experience suggest that release of A. papayae will bring down the papaya Mealybug population to a negligible/manageable level and there will be no impact on non target organisms. One might wonder what will happen to the parasitoid after it control the PM.

Two things can happen. Either parasitoid will co exist with PM in a low population equilibrium or the parasitoid will go extinct if it manage to eradiate the PM.

This is not the first time Sri Lanka has faced such threatening exotic pests. In 1970 the coconut leaf miner (Promecotheca cumingi) devastated our coconut cultivation and we imported a parasitod (Dimmokia javanica) which completely eradicated the exotic pest. In 1998 beet leaf minor (Liriomyza huidobrensis) threatened the vegetable cultivation in the hill country.

The DOA imported a parasitoid (Diglyphus isaea) which is still keeping the pest under control. In 1990 Guava white fly (Aleurodicus dispurses) killed many guava trees and infested almost every home garden tree. This pest population was brought down to a negligible level by a parasitoid, Encarsia transvena.

The above-mentioned are not the only biocontrol programs that DOA has implemented. In addition to classical biocontrol programs many neoclassical (non classical) biocontrol programs have been implemented in Sri Lanka. No adverse impacts on non target organisms have so far been recorded from any of these biocontrol agent introductions (both alien insects and pathogens).

However, it should be mentioned that no research has been conducted to find out any such impacts either, mainly due to lack of support for basic research.

Target species

Popular press stories of horrors due to Biological Control attempts, such as ‘cane toad introduction in Australia’ have made some people doubt the value of classical biological control as an acceptable pest control method.

It is true that there is some scientific evidence on the harmful impacts of introduced natural enemies on non target organisms. These few cases need to be individually analyzed to understand what has gone wrong.

Most of such introductions with unwanted results were done in the past using general predators before biological control advance into a modern scientific discipline.

Predators (e.g. lady bird beetles) are usually generalists. That means they feed on more than one species. Since the capability is already there in predators to eat many species it is easy for them to shift their host and attack non target species when introduced to a new area.

But the relationship of parasitoids with their hosts is very specific. Most parasitoids have co evolved for millions of years to acquire the capability of developing within the body of their hosts while the host itself continues to grow and moult (Koinobionts). Such relationships are highly specific and host shifts can expect to be evolutionary events not ecological events.

The proposed parasitoid for release, Acerophagus papayae, is a Koinobiont and therefore no impact is expected on any non target species. Today there are international standard guidelines and protocols to follow when introducing natural enemies to new areas. Under the International Plant Protection Convention there are guidelines for safe importation and release of biological control agents (ISPM No. 3 of 2005). The DOA will adhere to these guidelines.

Piti Makuna pest

Accordingly first imported consignment of parasitoids will be kept under quarantine and host specificity tests and pest risk analysis will be carried out before field release of the imported parasitoids.

Once released field monitoring of the parasitoid establishment will be carried out until pest population reduce to an economically insignificant level. Such a population reduction could be expected within 6-12 months after releasing the parasitoids if they successfully establish in Sri Lanka. With this effort I am confident that Piti Makuna will not be a significant pest in Sri Lanka by year 2010.

The writer is Research Officer Horticulture Research and Development Institute Gannoruwa