The Science Behind the Study
In February 2024, we launched a three-year field trial in partnership with the University of Aveiro’s Department of Biology, CESAM, and EntoGreen, a
Portuguese company specialising in insect biotechnology, specifically Black Soldier
Fly (BSF). The trial is part of the InsectERA Agenda Mobilizadora, a national programme funded by the European Union that explores the role of insects in sustainable agriculture and the circular economy.
The study is being conducted on a parcel in Valença do Douro, at 400 metres altitude, planted over 40 years ago. These are old vines — not yet centenarian, but mature enough to express the kind of vine age that truly matters. They were chosen for a practical reason: the gentler terrain and better road access allow for more rigorous
scientific measurement than steeper, more remote sites. The intention is to apply what we learn in Valença do Douro further upstream — in our centenarian vineyard, Vinha das Silvas, on the schist slopes above the Rio Torto. Science begins where it can be done best. The ultimate focus is always the oldest vines.
At the centre of the study is something called frass — an insect-derived organic fertilizer. Frass is rich in organic matter, contains chitin residues from the insects themselves, and is nutrient-dense. It is produced through the bioconversion of larvae
of Hermetia illucens, the Black Soldier Fly. These larvae are extraordinary bioconverters: fed with olive pomace (the solid residue from olive oil production), they process this by-product and leave behind a fertilizer rich in nitrogen, phosphorus, organic matter, macro- and micronutrients, chitin, and diverse microbial activity.
The circular logic of this system is important to us. The olive pomace that feeds the larvae comes from the same agricultural landscape that surrounds our vineyards. What returns to the soil is not a synthetic input, but a living, biologically active material — the by-product of a by-product, transformed into something that nourishes the oldest vines of the Douro.
What the Results Show
The trial runs until 2027, and we are deliberately cautious about drawing conclusions before final results. That said, the interim report from January 2026, produced by the University of Aveiro, contains findings we believe are worth sharing.
Across all fertilised plots, organic treatments — including insect frass — stimulated measurable enzymatic activity in the soil. Enzymes are the engine of soil biology: they break down organic matter, release nutrients, and sustain the microbial communities that underpin vine health. Seeing this activity increase is a meaningful signal.
Perhaps even more relevant for the long-term health of these vineyards: soil water retention capacity increased substantially compared to the baseline measurement taken in January 2024, when compared to data recorded in July 2025. In a semi-arid region where summer droughts are becoming more frequent and severe, the soil’s ability to retain moisture is not a minor detail — it is a question of resilience. The increase in organic matter, confirmed through loss-on-ignition data, shows that the soil is actively changing.
As for the vines: physiological measurements indicate healthy levels of chlorophyll and carotenoids across all plots, consistent with well-nourished plants. No negative impact on vine health has been identified. So far, the biological indicators are positive.
Why Old Vines, and Why Now
There is a reason this study begins with old vines rather than young ones. The 40- year-old parcel in Valença do Douro is not ancient — but it represents what the Douro achieves when a vineyard is allowed to mature: deep root systems exploring schist soils, natural resilience to drought, and a relationship with the soil built over decades, not seasons. These are the qualities we aim to understand, protect, and reinforce.
Our ultimate concern, however, lies with truly old vineyards — the centenarian vines of Vinha das Silvas, above the Rio Torto, planted over a century ago and carrying genetic heritage that modern viticulture has largely lost. As members of The Old Vine Conference, a global non-profit organisation dedicated to the recognition and protection of historic vineyards, we believe that caring for old vines is a form of agricultural conservation.
The frass study is the science behind that conviction: a rigorous attempt to understand what these vines require from their soil and how we can provide it without resorting to synthetic solutions. We begin in Valença do Douro because conditions allow us to do the science properly. What we learn there, we intend to bring to the Silvas.
Looking Ahead
The trial will continue until 2027. We will share final conclusions once they are published. Until then, we extend our thanks to the team at the University of Aveiro — particularly the researchers from the Department of Biology and CESAM — and to Entogreen, for their partnership, rigour, and willingness to bring their science into
these very old vineyards.
The Douro has always been a place where survival required ingenuity. The vineyards of Valença do Douro have grown here for over forty years. The centenarian vines of Vinha das Silvas for more than a hundred. Our intention is to give them every possible advantage for the decades ahead.
The University of Aveiro field trial — “Assessment of the impact of frass on plant and soil health and productivity in Douro vineyards” — runs from February 2024 to 2027. It is conducted in partnership with the University of Aveiro’s Department of Biology, CESAM, and EntoGreen, within the framework of the EU-funded InsectERA Agenda Mobilizadora.
