CABI, working with key Kenyan partners, is helping to protect livelihoods and food security by fighting the devastating papaya mealybug (Paracoccus marginatus) pest in five more major papaya growing counties in Kenya.

Scientists from CABI’s regional centre for Africa in Nairobi have recently deployed Acerophagus papayae – an environmentally friendly and safe-to-use biological control agent – in the inland Kenya counties of Machakos, Makueni, Embu, Tharaka Nithi and Baringo, where smallholder farmers are affected by the pest.

This means CABI scientists and colleagues from the Kenya Agricultural and Livestock Research Organisation (KALRO), the Kenya Plant Health Inspectorate Service (KEPHIS) and the National Museums of Kenya (NMK) are now fighting the pest – which can destroy whole crops – in total of eight counties across the country.

The work is funded by the CABI-led PlantwisePlus programme, which has over 150 plant clinics in Kenya that assist farmers with their plant health problems. Extra funding is from the Darwin Initiative project ‘Biocontrol of papaya mealybug in East Africa.’

Papaya 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 2016 in Mombasa County, Kenya.

CABI, KEPHIS and KALRO, estimate the value of the crop losses to be £2,224/ha annually. But CABI and partners are helping smallholder farmers fight back against the devastating papaya mealybug in Kenya thanks to A. Papayae, a tiny parasitic wasp.

“Strong evidence of exceptional efficiency”

Success has already been achieved by using the biocontrol agent to curb the papaya mealybug menace in Mombasa, Kwale, and Kilifi as part of Integrated Pest Management (IPM) plans.

Within two years of the first release in these counties, the biocontrol agent was causing up to 75% mortality of the papaya mealybug and average papaya harvests almost doubled between treatment and control farms. With higher yields, the average income of farmers on treatment farms increased by approximately 20%.

These plans included encouraging smallholder farmers to reduce their use of chemical pesticides so that the biological control agents can thrive and control the pests.

A CABI-led study, published in the journal Crop Protection, revealed “strong evidence of exceptional efficiency” of the biological control agent, which has also been deployed in eight locations across Uganda.

The efficacy of A. Papayae was investigated by CABI scientists and colleagues from the Kenyan institutions when it was released at six papaya farms in the coastal region of Kenya between December 2021 and November 2022.

The biological control agent established in all six research sites within the coastal counties, with a parasitism rate of 30% or higher at all research sites within a month.

The highest parasitism rate (72.89%) was recorded after more than 1,000 parasitoids were released on a farm, and parasitism was observed four months after releases. This demonstrates the potential for A.Papayae to be a successful biological control agent for papaya mealybug.

The researchers found that female parasitoids play a pivotal role in pest control through host feeding or oviposition, leading to the reduction of papaya mealybug populations.

Dr Selpha Miller, Postdoctoral Research Fellow, Invasive Species Management at CABI, author of the research into A. Papayae and, project manager of its release in Kenya, said, “The papaya mealybug is a devastating crop pest which the potential to cause crop losses ranging from 53-100%.

“But we have shown that A. Papayae has the ability, as part of an Integrated Pest Management plan, to be very effective with control of the papaya mealybug being achieved within six months of release.”

Treatment at no cost to smallholder farmers


The parasitoid mummies are placed on cards that are distributed to farmers to place on their affected crops (Credit: CABI).

Dr Selpha Miller said the roll out of the agent is being funded by partners at no cost to smallholder farmers. A. Papayae is mass-reared from the CABI-KALRO rearing facility at Muguga on the outskirts of Nairobi.

The parasitoid mummies are then placed on cards and distributed to farmers. The cards are stapled underneath papaya leaves. When the parasitoids emerge, they begin to search for the mealybugs and parasitize them.

Farmers play a role in the rapid multiplication and spread of the parasitoid on the farms by constructing Natural Enemy Field Reservoirs (NEFRs), which is based on technology pioneered by the late CABI scientist Riaz Mahmood in Pakistan.

The farmers put papaya leaves that may be having parasitoid mummies into the NEFRs for a period of 14 days. The mummies on the leaves emerge and parasitize the papaya mealybugs. NEFRs are also important as they help in breeding of other important natural enemies of papaya mealybugs and other pests.

NEFRs is a box that is made using locally available materials and provided with a shade of also locally available materials. NEFRs work by allowing the breeding of the biological control agent in a box in the field.

The technology led to an increase in the absolute and average count of the A.Papayae used to manage the papaya mealybug in Kenya as outlined in CABI-led research published in the journal Biological Control.


CABI with NEFR technology at a cotton farm in Pakistan (Credit: CABI).

Dr Johnson Nyasani, Chief Research Scientist at KALRO, said, “The use of Acerophagus papayae as a biological control agent to control the papaya mealybug is a safer-to-use and more sustainable way of controlling the pest as part of an Integrated Pest Management plan that reduces the reliance on chemical pesticides.”

Additional information

Main image: CABI is working in partnership to help more smallholder farmers in Kenya tackle the papaya mealybug pest with Acerophagus papayae (Credit: CABI).

Project page

Find out more about CABI’s work to help smallholder farmers manage and mitigate the papaya mealybug pest through the project page ‘Biocontrol of papaya mealybug in East Africa.’

Relevant stories

‘Kenyan farmer perceptions of biological control of papaya mealybug.’

‘How a tiny wasp can save the livelihoods of papaya farmers.’

‘Study shows “strong evidence of exceptional efficiency” of biological control agent against papaya mealybug pest.’

‘Natural Enemies Field Reservoirs play critical role in management of papaya mealybug, CABI study reveals.’


Study brief

CABI has published a study brief ‘Biocontrol for papaya mealybug: lessons learnt from Kenya’ which can be read here.

Media contacts

Dr Selpha Miller, Postdoctoral Research Fellow, Invasive Species Management, based at CABI’s regional centre for Africa in Nairobi

Eunice Murathe, Corporate Communications Officer,