Researchers in Nairobi are rearing thousands of small wasps that attack and kill a common pawpaw pest.
The wasps will be released in several pawpaw growing counties where the pest, papaya mealybug, is destroying fruits in farms.
The mealybug can destroy half of all pawpaw fruits in a plantation by creating a thick layer of pests themselves and white wax, which makes the fruit inedible.
It also feeds on fruits and stems, resulting in yellowing, stunting, deformed leaves.
“At least 18,000 adult moths (of the wasp) are being produced at the Kenya Agriculture and Livestock Research Organisation (Karlo) in Muguga every month,” said Ivan Rwomushana, a senior scientist on invasive species at Cabi, an agriculture research organisation that promotes environment-friendly farming methods.
He spoke in Nairobi at a meeting organised by PlantwisePlus, a global programme, led by Cabi, to increase food security and improve rural livelihoods by reducing crop losses.
The wasp, called Acerophagus papaya (A. papayae) attacks the mealybug by laying its eggs on the pest until it dies. It also feeds on the pest but does not affect the fruit.
A Cabi-led study released two weeks ago confirmed that the wasp actually works against the mealybug in Kenya. This had already been proven in other countries such as Ghana, India and Pakistan.
The researchers in 2021 released wasps in six pawpaw farms and evaluate the ability of the species to parasitise the mealybug until November 2022.
They outlined their findings in the latest issue of the Crop Protection journal.
The Cabi scientists and colleagues from Kenya Plant Health Inspectorate Service (Kephis), Kalro, and Kenyatta University, said A. papaya (the wasp) is a good candidate to control mealybug which, in Kenya, causes crop losses ranging from 53-100 per cent.
The organisations said they estimate the value of the crop losses to be Sh41,9712 per hectare annually and as such the mealybug poses a serious threat to all 10 major pawpaw growing counties in Kenya including Meru, Murang’a, Kitui, Busia and Baringo.
The mealybug originated from Central America before spreading to the Caribbean and South America in the 1990s. It was first detected in Africa in 2010 in Ghana and in Mombasa in 2016.
It has since rapidly spread to 21 counties across Kenya, reaching near epidemic levels, particularly in pawpaw-growing locations.
Currently, most Kenyan farmers control the mealybug primarily using synthetic pesticides, whose extensive use of synthetic pesticides poses health risks to humans and animals.
The mealybug also covers itself with a protective waxy coating over the body, making it difficult to target and control effectively.
The A. papayae wasp reduces the overreliance on chemical insecticides.
Wasp released at the Coast in the 2021–2022 trial established in all the six research sites, with a parasitism rate of 30 per cent or higher at all research sites within a month.
The highest parasitism rate (72.89 per cent) was recorded after more than 1,000 parasitoids were released on a farm, and parasitism was observed four months after releases, Cabi said.
Dr Selpha Miller, lead author and postdoctoral research fellow, Invasive Species Management at Cabi, said, “The high parasitism rates and female-biased sex ratios obtained with third and adult female host instars indicate that mass rearing of A. papayae should be done with these host instar stages."
“A. papayae establishment in the field and parasitising the mealybug to levels not detectable in the field has proved to be the most effective control strategy that should be implemented in Kenya and other Africa countries.”
In most of the research sites, the population counts of mealybug reduced significantly after releases to a level where it was difficult to find the pest at the research sites.
For the successful establishment of wasp, pawpaw farmers in the coastal region were encouraged to reduce the use of chemical pesticides to conserve the wasps.
The scientists added that most parasitism rates that were recorded to be above 40 per cent in this study were reported after five months of establishment.
The wasp method of eliminating the mealybug was pioneered in Pakistan.