PlantwisePlus Blog

Classical biological control (CBC) is a widely used type of biological control that is proven to be effective in managing invasive pests. This method involves releasing natural enemies like parasitoids or fungi that can keep pest populations under control. However, additional technology, such as natural enemies field reservoirs – or NEFRs – can improve CBC even further.

A CABI-led study has looked into how establishing NEFRs in farms in coastal Kenya can help fight pests such as the papaya mealybug, an invasive insect species that affects a wide range of plants.

What are NEFRs?

The late CABI scientist Riaz Mahmood developed and pioneered NEFRs as an inexpensive and accessible way for farmers to control pest populations. These on-farm rearing structures support and promote the growth of natural enemies. Many studies have shown that non-crop habitats can also attract these natural enemies, thereby increasing their populations in surrounding crop fields.

NEFRs consist of simple fixtures that provide a roof over trays or bins. These structures can be made of woven palm leaves and wooden poles, for example. Farmers can also construct these using other materials available to them. NEFRs also help farmers with tall papaya trees, where it can be difficult to attach biocontrol agents to leaves.

According to Dr Ivan Rwomushana, a co-author of the study, “NEFR can be thought of as an elegant, self-sustaining, non-polluting, and inexpensive technology with a great potential to manage papaya mealybug.”

How NEFRs support the fight against papaya mealybug in Kenya

CABI scientists measured the effectiveness of NEFRs in controlling papaya mealybug through experiments on farms in Kenya’s coastal counties, Kilifi, Kwale, and Mombasa. These three areas have established a reputation for their papaya crops and are facing papaya mealybug infestations.

Papaya mealybug (Paracoccus marginatus) causes major crop loss. In Bangladesh, papaya mealybug causes an average economic loss of US$700 per hectare per year and 1,700 individual job losses in Ghana. Papaya mealybug is native to Central America but has become invasive in Southeast Asia, the Indo-Pacific Islands, and West Africa.

Papaya mealybugs can infect different crops, such as papaya, citrus, and cassava. The pests are also known to cause physiological changes in the plants they infest by injecting toxic substances into them. Infested plants then have their vital resources depleted. Additionally, papaya mealybugs can secrete honeydew onto the plant’s surface, which leads to sooty mould growth. This then impairs photosynthesis.

One of the most successful natural predators of the papaya mealybug is Acerophagous papayae. Researchers have used this parasitoid in other countries, such as Ghana, to control papaya mealybug. They looked at how the establishment of NEFRs could reduce papaya mealybug populations through the cultivation of A. papayae. 

NEFR experiments

The Kenya Standing Technical Committee on Imports and Exports (KSTICIE) granted CABI researchers permission to use A. papayae for papaya mealybug control. The study used eleven farms on the coast, each with three different set-ups.

The first setup, Treatment A, involved farms that did not previously have A. papayae released but had NEFRs established.

Then, Treatment B had farms with both A. papayae released previously and NEFRs established.

The last treatment, Treatment C, was the control setup. A. papayae was released previously, but there were no NEFRs.

The treatments with NEFRs used low-cost materials such as makuti (woven palm leaves) and makonge (sisal stem) to make the structures. Papaya leaves and fruits infested with papaya mealybugs filled the trays halfway. To ensure the papaya mealybugs didn’t escape the NEFRs and attack nearby trees, the legs of the NEFRs were placed in oil. Papaya mealybugs that had been parasitized, other papaya mealybugs still developing, and their immature predators were on the plants. All the NEFRs were replenished after 15 to 30 days.

The team collected data between April and June of 2023. They selected trees from the farms, sampling mid and lower-leaf sections. The team also collected fruits from fruit-bearing trees. The researchers counted papaya mealybugs at all stages of their development, including egg masses, adults, and mummies.

Decrease of papaya mealybug populations due to NEFR establishment

The researchers observed a decrease in papaya mealybug populations in Treatment A by almost 50%. There was an initial surge before the decline, as it took a little while for the CBC to start working. Treatment B showed a sharp and steady decrease of 62.8%. Treatment C was the only setup that showed increased pests, with an increase of 37.6%.

Although Treatment C already had A. papayae released, papaya mealybug populations still rose. The researchers state that a phenomenon called a lag period, which is the period before the parasitoid can fully take effect, can explain this. It may also demonstrate that having the NEFRs on the farm helps the A. papayae establish quicker. Researchers in Guam observed this phenomenon previously, where results did not materialize until one year after introducing parasitoids.

Some farmers can misinterpret this as ineffective biocontrol since the results are not immediately seen. In actuality, Treatment C showed increased populations of A. papayae but at a slower growth rate. The establishment of NEFRs greatly lessened or even removed this lag period.

NEFRs also decreased the overall number of papaya mealybugs and increased A. papayae. This is because NEFRs promote parasitoid growth, which leads to more wasps controlling papaya mealybug populations. Similar results in other studies back the results of this research. In Palau, papaya mealybug populations decreased after parasitoid release.

The broader impact and implications of using NEFRs

Using NEFRs alongside classical biological control also led to a higher number of other papaya mealybug natural enemies. Researchers found multiple types of natural enemies in the experiments, with the highest number recorded in Treatment B and the lowest in Treatment C. The diversity of these natural enemies also increased.

The NEFRs can conserve these beneficial insects, allowing them to be released back into the farming system. For example, researchers observed that the beetle, Cryptolaemus montrouzieri, also known as the mealybug ladybird, can be an effective predator because of its rapid reproduction and development, adaptability to different climates, and ease of rearing.

In addition, the observed beneficial insects could potentially also be natural enemies of other harmful crop pests.

However, while NEFRs have shown early success in helping control the papaya mealybug, their long-term impact remains unclear. In some regions, their use has been discontinued or no longer promoted. Further research is needed to understand if they can offer long-term benefits in challenging environments. They may help in areas where pests are harder to control due to factors like climate, but this remains to be proven.

Supporting smallholder farmers

PlantwisePlus and its partners support smallholder farmers in fighting the devastating papaya mealybug in Kenya. The rollout of the biocontrol agent is at no cost to smallholder farmers.

By constructing NEFRs on their farms, farmers are playing a role in the rapid multiplication and spread of the wasp.

Within two years of the first release in Kenya’s coastal 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.

Now, CABI and partners are releasing the wasp in five more major papaya-growing counties in Kenya.

PlantwisePlus recognizes the urgent need to increase farmers’ uptake of lower-risk plant protection products. By working closely with national and local government entities and farmers, the programme finds opportunities to reduce pesticide-related risks. This includes identifying and implementing classical biocontrol methods like this one to reduce the impact of priority pests.

“If you go to the coast, farmers are so happy because they are now able to sell their papaya and earn some income, something that they were not able to do before. And this is what PlantwisePlus is all about,” says CABI’s Dr Selpha Miller, an author on the paper.

Read the paper in full:

Stephen T.O. Othim, Selpha Opisa, Ivan Rwomushana, Belinda Luke. Unleashing nature’s defenders: Farmer-managed natural enemies field reservoirs (NEFRs) enhance management of the invasive papaya mealybug (Paracoccus marginatus) in coastal Kenya, Biological Control, Volume 193, 2024, https://doi.org/10.1016/j.biocontrol.2024.105528.

Find out more

PlantwisePlus Knowledge Bank Technical Factsheet: Paracoccus marginatus (papaya mealybug)

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

CABI and partners step up fight against devastating papaya mealybug on more farms in Kenya

How a tiny wasp can save the livelihoods of papaya farmers

Controlling the papaya mealybug pest – progress made in coastal Kenya

PlantwisePlus in Kenya

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PlantwisePlus gratefully acknowledges the financial support of the Directorate-General for International Cooperation (DGIS), Netherlands; European Commission Directorate General for International Partnerships (INTPA, EU); the Foreign, Commonwealth & Development Office (FCDO), United Kingdom; and the Swiss Agency for Development and Cooperation (SDC).