Impact of Chemical Treatments on the Pollinator Insect Population of Oil Palms in an Industrial Palm Plantation: The Case of Blidouba in the South-west of Côte D’ivoire

In the humid, sun-drenched forests of West Africa, the oil palm stands as a giant of the agricultural world, its fruit providing a vast portion of the vegetable oil used in food and industry. Yet, this massive crop faces a quiet but critical challenge: it cannot reproduce on its own. Unlike many plants that rely on the wind to carry their pollen, the oil palm depends entirely on tiny insects to move pollen from the male flowers to the female ones. Without these small workers, the trees would produce almost no fruit. For decades, farmers have relied on a specific group of beetles, mostly from the genus Elaeidobius, to perform this essential task. These beetles live within the flower clusters, feeding and mating while ensuring the next generation of palms is born. However, in the vast, industrial plantations where these trees are grown, farmers also use powerful chemical sprays to fight off pests that damage the crop. The central question for anyone managing these lands is whether the very chemicals used to protect the trees might be killing the tiny insects that make the harvest possible.

Researchers in the southwest of Côte d'Ivoire set out to answer this question in a large industrial plantation managed by PALMCI. They focused on a specific site in Blidouba, where they wanted to see exactly what happens to the population of these vital pollinating beetles when three different types of insecticides are applied. The team chose three common chemical treatments used in the region: one based on oxamyl, another on lambda-cyhalothrin, and a third on thiocyclam hydrogen oxalate. To get a clear picture, they divided a large section of the plantation into separate test areas. They spent a month before any spraying began carefully counting the beetles on the flowers to establish a baseline, noting how many were visiting the female flowers and how many were living on the male ones. This initial survey revealed a bustling community of insects, with nearly ten different species present, dominated by the crucial Elaeidobius beetles.

Once the baseline was set, the team applied the chemicals to the trees using standard spraying equipment. They then began a rigorous daily count, starting just one day after the spray and continuing for four days. They checked the flowers for dead insects, collecting them to identify exactly which species had been lost. The results were stark and immediate. The chemical known as Karate, which contains lambda-cyhalothrin, proved to be the most devastating. Within the first day, it had killed nearly 42 insects per flower cluster, a mortality rate of over 70 percent. By the fourth day, this chemical was still causing significant deaths, with a mortality rate remaining near 60 percent. The other two chemicals, Oxatop and Evisect, also caused deaths, but their effects were less severe and faded much faster. Oxatop killed about 17 percent of the insects on the first day, while Evisect killed about 9 percent. By the third day, the effects of these two had dropped to very low levels, whereas Karate continued to hold a high level of toxicity.

The study also looked at how the living insects behaved after the spraying. Even on the days when the chemicals were not killing the beetles directly, the number of beetles visiting the female flowers plummeted. On the first few days after treatment, the researchers found fewer than one beetle per flower cluster in most cases, a dramatic drop from the hundreds they had seen before the spraying. This suggested that the chemicals were not just killing the beetles but were also disrupting their ability to find the flowers or move around normally. The researchers noted that the beetles that survived the initial blast seemed to have lost their way or their drive to visit the flowers, which would severely impact the tree's ability to produce fruit.

However, the story did not end in total loss. The researchers continued to monitor the plantation for two months after the spraying stopped. Slowly, the insect population began to return. By the one-month mark, the number of beetles on the flowers was climbing back up. By two months, the abundance of these pollinators had returned to levels that were broadly similar to what was observed before the chemicals were ever applied. This recovery suggests that the surrounding environment, including nearby areas that were not sprayed, acted as a refuge. Insects from these safe zones likely moved back into the treated area, and the surviving beetles reproduced to rebuild their numbers. The study concludes that while the chemicals used in industrial plantations can cause a sharp, temporary collapse in the pollinator population, the ecosystem has a capacity to bounce back if the pressure is removed. The findings highlight a clear trade-off: the immediate protection of the crop from pests comes at the cost of a temporary but severe disruption to the natural pollination process, with one specific chemical posing a much longer-lasting threat than the others.