Study Shows Common Fungicide Impairs Silkworm Silk Production

By Daniel Nicodemo, Ph.D.

The increasing use of agrochemicals worldwide has caused environmental problems such as the reduction of the population of non-target organisms, including wild insects and bees used commercially for honey production and pollination services. For these cases, great attention is given to honey bees, but another beneficial insect rationally exploited by man has also had its performance compromised possibly due to intoxication with pesticides: the silkworm.

Diseases, malnutrition, and inappropriate management are factors that often negatively impact the performance of the silkworm (Bombyx mori) and may even lead to the death of managed populations. However, even in the absence of these problems, silkworm rearers have reported significant problems of decreased cocoon production. It is important to remember that sericulture, as the process of extracting silkworm silk is known, is performed by small farmers who grow mulberry plants to feed the caterpillars. In Brazil, mulberry plantations have been surrounded by huge areas with monocultures such as sugarcane, soybean, and corn, which demand a large volume of agrochemicals for cultivation, considering the conventional systems of agricultural production.

In a recent paper published in the Journal of Economic Entomology, the negative effect of a commercial fungicide widely used in agricultural crops on mitochondrial bioenergetics and the production of silkworm cocoons is highlighted. My colleagues at the College of Technology and Agricultural Sciences at São Paulo State University and I treated mulberry plants with the fungicide pyraclostrobin at doses usually indicated by manufacturers for other commercial crops. As its main effect, this fungicide, as well as other strobilurins, inhibits fungus’s respiratory chain and interferes with the transfer of electrons in cells in the course of mitochondrial respiration.

For the tests, mulberry leaves were offered to silkworm caterpillars 30 days after application of the fungicide. We evaluated in vitro and in vivo mitochondrial bioenergetics of mitochondria from the head and intestines of the caterpillars, as well as their feed intake and mortality rate and the weight of fresh cocoons and cocoons shells. At doses of 50 micromolar (in vitro) and 200 grams per hectare (in vivo), pyraclostrobin inhibited oxygen consumption, dissipated membrane potential, and inhibited ATP synthesis in the silkworms’ mitochondria. Pyraclostrobin acted as a respiratory chain inhibitor, affecting mitochondrial bioenergetics. Therefore, it was verified that the main effect expected for fungi also occurs in silkworm caterpillars.

Meanwhile, the fungicide did not interfere with the silkworms’ food consumption, but it negatively affected mortality rate and weight of cocoons at the dose of 100 grams of pyraclostrobin per hectare.

The lethality of any substance is an important variable to determine the direct impact on the ability of individuals of a given species to stay alive due to exposure to a given product and its dose, concentration, or both. However, nonlethality does not mean that the substance is not harmful, as there may be damage endured in various ways when still alive. According to the present study, even though the lethality of the caterpillars was relatively low when mulberry leaves were contaminated with pyraclostrobin, impairments occurred in the bioenergetics of mitochondria isolated from both of head and intestine of the caterpillars, negatively influencing the energy production of these organelles and cocoon production.

Daniel Nicodemo, Ph.D., is an assistant professor at São Paulo State University in the College of Technology and Agricultural Sciences. Email: nicodemo@dracena.unesp.br