How whiteflies threaten vegetable production

In Arusha region for example, the vegetables which comprise of living cells containing all the essential compounds and nutrients necessary to human health and life have badly been affected by the whiteflies.

The small scale vegetable growers of green bell peppers, beans, cassava, potatoes, flowers and tomatoes are increasingly being frustrated by the whiteflies and often resorting to heavy use of pesticides which put their lives and that of customers at risk.

Speaking during a field tour of scientists and researchers at several farms of small scale farmers recently, they said that application of regular pesticides have never helped them to wipe off the increasing number of the whiteflies.

The scientists’ tour to the small holder farmers was part of the activities during the second international whitefly symposium held in Arusha recently.
Husna Abdullah (33) is one of them. She is a small holder farmer growing green peppers in a green house in Arusha. Her curiosity and zeal to continue growing peppers is slowly fading away.

She is currently struggling to stamp out the whiteflies which have infested her quarter-Acre green pepper plot but in vain.
Husna says she has been growing potatoes for years, but this year she decided to expand her farming plot by venturing into green peppers to increase her income but the whitefly might stop her from investing her energy and money into vegetables.

As the fresh, crisp green bell pepper started sprouting; Husna started experiencing whitish tiny, sap-sucking insects, excreting honey dew which in turn serves as a media for sooty mold fungi growth, which darkens leaves and fruits.

Husna, who appeared frustrated for a great loss, she has employed all agronomic practices, but the methods could not pack a punch against the massive whiteflies currently on the increase in several farms.

She said that since she planted the crop in December 2015, she has incurred more than 175,000/-shillings for purchasing pesticides alone, leave alone other costs.
She said “If the government will not make an intervention on this problem, many farmers will abandon growing these vegetables because of high costs involved to protect the vegetables from the whiteflies”.

Another small holder farmer, Paulina Kiule who grows tomatoes, said that many farmers are getting difficult to open large farms due to the wide spread of the whiteflies.

Paulina, a resident of Arumeru who is one of the members of Mtanzamo women group cited another challenge as lack of strong pesticides to kill the whiteflies and skyrocketing of prices of inputs for the farmers.

“The problem here is whitefly; I am forced to spray pesticides every week due to these infections. Otherwise, I do harvest 5 crates every week and every crate we sell at local price between 40,000/-50,000/shillings” she said.

Explaining about global dynamics of whitefly-driven plant viruses epidemics, Dr. James Legg, from International Institute of Tropical Agriculture (IITA) in Dar es asalaam, Tanzania, said that the whitefly of the Bemisia tabaci species complex have been recognized as important crop pests for more than 120 years.

He said that they are now considered to be one of the most economically damaging pests worldwide. “This group of insects is responsible for transmitting more than 200 virus species from five plant virus families, the numbers which continue to increase” he said.

He said that three of the most severely affected production systems are tomatoes-the world’s most important vegetable, cotton-the premier fibre crop, and cassava-a vital food staple throughout the tropics which has a critical food security value in Africa.

For his part, Dr. Peter Sseruwagi, Assistant project coordinator at MARI, (Technical and research) said that in Africa the crops most affected by whiteflies and whitefly transmitted viruses include cassava, tomato and tobacco.

Presenting a paper on whitefly research perspectives in Africa: history, past, future challenges and pathways to solutions, he said “Until the 1990’s the whitefly were less known as a serious problem by large section of the community.

When the pandemic of severe cassava mosaic disease (CMD) and recently the cassava brown streak diseases (CBSD) were associated with super abundant whitefly populations on cassava, whiteflies were less appreciated as a major problem in African agriculture.

Not only were small holder farmers unaware of the whiteflies and associated whitefly transmitted viruses but also there were a general lack of knowledge among African scientists and governments about the extent of the problems.

“…….however, management of whiteflies is still a challenge and efforts to develop durable management strategies for whiteflies are hindered by lack of human and research infrastructure in national agriculture research institutes” he said.

He said that until recently, efforts in whitefly research have mainly focused on understanding species diversity and ecology.
New investments have been initiated to understand what is driving cassava whitefly outbreaks and super abundance, whitefly systematic and control through development of whitefly resistance using novel and conventional breeding strategies and biological control.

However, presenting a paper on joint efforts to combat whitefly-transmitted cassava viruses in Sub-Saharan Africa, Dr Joseph Ndunguru from Mikocheni Agriculture Research Institute (MARI) said that effective management of the whiteflies requires availability of reliable and affordable capacity to diagnose them.

He explained that a regional cassava virus disease diagnostic project was initiated in 2008 to enhance human and infrastructural capacity of national research programmes (NARs) in seven countries namely Tanzania, Kenya, Uganda, Malawi, Rwanda, Mozambique and Zambia.

Dr. Ndunguru who is coordinating the project also said that similar initiatives are being implemented in eight West African countries namely Cote d’lvoire, Ghana, Togo, Benin, Nigeria, Burkina Faso, Cameroon and DRC through the West African Virus Epidemiology (WAVE) project.

“Through this collaboration, robust standardized molecular diagnostic tools have been developed and shared among the scientists in the network countries” he said.

He further explained that virus characterization and disease surveillance have and are being conducted in the project countries generating both disease prevalence and virus distribution maps for use during informed decision making.

He said that molecular disease diagnostic laboratories have been strengthened by providing equipments and reagents along with training scientists on different aspects of disease diagnostic.

“With these capacities in place, scientists in the network countries can respond effectively to threats from emerging viruses and their associated insect vectors in Sub-Saharan Africa” he said.

For his part, Prof. Stansly Philip from University of Florida in USA, named the Bemisia tabaci and traleurodes vaporariorum as the principal whitefly pests of vegetable crops worldwide.

Presenting a paper on biological management of whiteflies in vegetable crops: Lessons from green house to open field, at the symposium, he said that both have been subjects of classical biological control, that is, introduction of natural enemies from one place to another.

“Modern augmentative biological control could well be thought of as originating with mass production or release of Encarsia Formosa for control of Tabicia vaporariorum in green house vegetable production” he said.

He further explained that “This parasitoid continue to be the principal natural enemy used against green house whitefly and is somewhat effective against the more intractable Bemisia tabaci”.

For his part, Dr Polaszek Andrew from department of life sciences, Natural history Museum, UK, explained that scientists and pest control practitioners have been studying the natural enemies of the Bemisia tabaci-complex for decades especially the parasitoids.

He said the study is in the hope of recognizing a tangible pest control benefit from their activity. Parasitoids have been used successfully as biological control agents of whiteflies on several occasions around the world.