Small-scale Tanzanian agroforestry systems: limited by insects, knowledge, and too much pesticides

  • Vannmelon
    Andrii Zastrozhnov/Shutterstock

Thomas Corodius Sawe’s PhD shows that the crops of small-scale Tanzanian farmers are limited by pollination, and that their yields can be substantially increased if the pollination services are improved. He also found that pesticide use is abundant, and that there is a need for basic education in plant biology among farmers.  

Small-scale Tanzanian agroforestry systems: limited by insects, knowledge, and too much pesticides

Pollinating insects, which are necessary for a majority of the world’s food crops, are declining in both abundance and diversity. This threatens pollination services for wild plants and important food crops.

The effects on small-scale farming

The importance of insect pollination services for food production has been documented for crops that are available on the global markets, and which stem from large-scale farming systems. However, little effort has been directed towards understanding the role of insect pollinators in small-scale farming systems in developing countries.

“This is concerning, as these systems feed a substantial part of the world’s population,” PhD candidate Thomas Corodius Sawe says.

Melon plant flower.
Thomas Sawe

Insect pollination in Tanzania

Sawe has studied crop pollination by insects in a small-scale agroforestry farming system in the Kilimanjaro and Arusha regions in northern Tanzania. Insect pollination is essential for fruit development in watermelons, and watermelon is an important cash crop to local farmers in his study area.

“My two main research questions have been to examine how the crops are limited by insect pollination, and how various agriculture practices and environmental variables affects pollinators.”

In addition, he has interviewed local farmers and asked them to what degree they are aware of pollinators and estimated their potential vulnerability to changes in pollination services. Both in terms of declines in household income and food availability.

Limited by pollinators

In his project, Sawe has both done experiments and observational studies of the relationships between flower visits by insects and fruit quantity and quality.

“My results showed that watermelon crop yield was limited by pollination services, i.e. they do not receive enough visits.”

His findings indicate that local farmers can double the number of marketable fruits and increase sugar content of the watermelons by approximately 10%, if the watermelon flowers are sufficiently pollinated throughout the blossoming period.

Increasing inputs of fertilizer and watering had little effect on crop yield, compared to enhanced pollination.

Ph.d.-candidate Thomas Sawe out in the field.
Thomas Sawe

Poor knowledge

There is a lack of understanding among farmers how crop production works in this region.

“The general notion is that the farmer sows and fertilizes, and then ‘God does the rest’,” Sawe comments.

Only 7% of the local farmers were aware of pollinators and their importance for crop pollination, although 67% of crops grown by local farmers for household food and income depended on insect pollination to a moderate to essential degree. Watermelon crops contributed nearly 25% of household income and were grown by 63% of the interviewed farmers.

Education and advice needed

“It is critically important that small-scale farmers understand the role of pollinators and their importance for agricultural production,” Sawe says.

He further recommends that agricultural policies to improve yields in developing countries should include measures to improve pollination services, such as education and advisory services to local farmers on how to develop pollinator friendly habitats in agricultural landscapes.


A farmer spraying pesticides on his field.
Thomas Sawe

Pesticides reduce insect activity

Pesticide spraying reduced visitation rates by 50% from the lowest to the highest observed frequencies of pesticide application.

“What surprised me the most was the amounts of pesticides that they used,” Sawe comments.

The general perception is that agroforestry systems are less affected by pesticides than large-scale productions which are more common in Europe and the Americas.

However, this proved not to be the case.

Extensive pesticide use

“The farmers in this area use a lot of pesticides, and more than expected. They also combine different types, and sometimes the farmers add other chemicals, such as diesel or gasoline to make it more effective.”

He further explains that not only are some of the chemicals not approved for pesticide use, but also that mixing them can result in unforeseen synergetic effects.

“This is worrying, and there is a need to document what effects these chemicals may have on the environment and the food chain.”

“The responsible management authorities need to address this issue, and develop a sustainable strategy for managing pests and ensuring increased agriculture yield,” he concludes.

 Thomas Corodius Sawe will defend his thesis “Crop pollination by insects in small-scale agroforestry farming in Tanzania” Thursday 19 December, 2019.

Published 17. December 2019 - 13:06 - Updated 18. December 2019 - 12:36