There is a growing focus across the wine industry on reducing waste and improving sustainability. In Marlborough alone, tens of thousands of tonnes of organic by-products, such as grape marc and wine lees, are generated each year. While much of this material is returned to land, a portion still ends up in landfill, carrying both environmental and economic costs.

One emerging solution lies in an unlikely place: insect frass. The by-product of insect rearing, frass is a mixture of insect excreta, undigested feed and shed exoskeletons. In a circular system, organic waste streams are fed to insects such as mealworms or black soldier fly larvae. The insects convert this waste into protein-rich biomass, valuable for animal feed, and frass, which can potentially be returned to the soil as a fertiliser or soil amendment.

A recent experiment conducted by scientists at the Bioeconomy Science Institute (BSI) Maiangi Taiao, and funded by Bragato Research Institute and Marlborough Research Centre in collaboration with Callaghan Innovation, set out to explore whether frass derived from wine industry waste could play a meaningful role in viticulture. The results provide early but interesting insights into how this material behaves in vineyard soils, and where it might fit in future nutrient management strategies.

Image 1: Mealworm larvae feeding on a diet of grape marc, wine lees and mussel shells, credit: BSI.

Closing the loop in Marlborough

Marlborough produces large volumes of grape marc; around 82,000 tonnes annually from the 2025 harvest alone, as well as wine lees and, from other industries, mussel shells. While much of this material is spread on land, a portion is still sent to landfill, carrying both environmental costs and lost nutrient value.

Insect bioconversion offers a circular pathway to retain these nutrients within the region. In the Living Lab study, mealworms were fed a diet composed of grape marc, wine lees and crushed mussel shells. The resulting frass therefore represents a concentrated form of nutrients that originated within the region’s wine and seafood industries.

From a sustainability perspective, there is a clear appeal. Rather than exporting waste or the nutrients it contains to landfill, frass allows those nutrients to be returned to vineyard soils. This reduces dependence on imported fertilisers, particularly phosphorus, which is a finite resource, while avoiding greenhouse gas emissions associated with landfill disposal. However, the question remains whether frass behaves in a useful and predictable way in vineyard soil.

 

Is frass useful in vineyard soil?

To address this, researchers established a controlled lysimeter experiment using a sandy soil (Classification: Selwyn), representative of drought-prone vineyard soils in Marlborough. The study examined two main variables: how much frass was applied, and how it was applied.

Frass was tested at two relatively high rates, equivalent to approximately 15 and 30 tonnes per hectare, and applied either to the soil surface or incorporated into the top 5 cm. Soil cores were then monitored over six months to track changes in nutrient availability and water-holding capacity.

While these application rates exceed what would typically be used in mature vineyards, they provide useful boundary conditions to understand how frass behaves in soil systems.

Figure 1. View of the different treatments in the lysimeters following the one-month sampling. Credit BSI.

A nutrient-rich amendment

Frass is a concentrated source of plant nutrients. Compared with many organic amendments, it contains relatively high levels of nitrogen, phosphorus, potassium, calcium and sulphur. Therefore, adding frass to soil significantly increased the concentrations of these nutrients.

Frass acted as a slow-release nitrogen source, maintaining elevated levels of plant-available nitrogen over the six months. Even after this time, around two-thirds of the potentially mineralisable nitrogen remained in the soil, indicating a sustained supply beyond the duration of the experiment.

This slow-release behaviour is particularly relevant for perennial systems such as vineyards, where a steady nutrient supply is often preferable to the peaks and troughs associated with soluble fertilisers. In principle, frass could reduce the need for multiple fertiliser applications across the growing season.

Phosphorus levels also increased significantly, and consistent with its behaviour in soil, remained relatively stable over time. Other nutrients, including potassium and calcium, increased initially but declined gradually, likely due to leaching under the experimental conditions.

For growers, the key point is not just that frass supplies nutrients, but that it supplies them in a form and timeframe that may align with plant demand.

 

Application method matters

In vineyard systems, surface application is often preferred to avoid root disturbance. However, the experiment showed that surface-applied frass resulted in lower levels of mineral nitrogen compared with incorporated treatments. This suggests that nitrogen losses, likely through ammonia volatilisation, were higher when frass was left on the soil surface.

In contrast, incorporating frass into the soil helped retain more nitrogen, leading to higher nutrient availability over time. Incorporation into the soil is beneficial for nutrient retention, requiring shallow cultivation or the use of star tillers/finger weeders when applied under vine. Surface application, while operationally simpler, may be less efficient in terms of nitrogen use.

Where incorporation is possible, such as during vineyard establishment or replanting, it may enhance the value of frass as a nutrient source. In established blocks, alternative strategies may be needed to optimise its use, such as timing applications to minimise losses or integrating it with other soil management practices.

 

Water holding potential is complex

Beyond nutrients, the study also explored whether frass could improve soil water-holding capacity; a relevant question for Marlborough’s free-draining soils.

Initially, adding frass increased the total amount of water the soil could hold when saturated, but did not significantly change water content at field capacity. In some cases, higher application rates even appeared to reduce plant-available water, because more water was held tightly and thus unavailable to plants.

However, after three to six months, soils treated with higher rates of frass held more water at field capacity, suggesting a potential improvement in water storage as the material decomposed and interacted with the soil.

It is important to note that these results come from a controlled system without plant roots or soil fauna, both of which play critical roles in soil structure under field conditions. As such, the real-world effects on vineyard soils may differ, potentially in positive ways as the organic matter in frass contributes to aggregation and pore structure.

 

Getting the rate right

The amounts of frass used in the experiment delivered very high nutrient loadings. At the higher rate, frass supplied several hundred kilograms of potentially available nitrogen per hectare, far exceeding the requirements of mature grapevines, which typically need only modest nitrogen inputs.

This reinforces the principle that more is not necessarily better. In practice, frass would need to be applied at much lower rates in vineyards, calibrated to match vine demand and avoid excessive nutrient accumulation or leaching.

The study highlights the importance of developing application guidelines through field trials, where plant uptake, soil processes and environmental conditions can all be taken into account.

 

Where to next?

As with many emerging amendments, the promise of frass comes with a degree of uncertainty. Lysimeter studies provide valuable controlled insights, but field validation is essential.

Future work will need to answer several practical questions. What are the optimal application rates for different vineyard stages? How does frass interact with cover crops, irrigation and existing fertiliser programmes? What are the longer-term effects on soil biology and structure? And critically, how consistent is frass composition if the insects are fed different waste streams?

As the wine industry continues to focus on sustainability and resource efficiency, solutions that close nutrient loops and reduce waste are likely to gain traction.