Autumn Larvae in Savernake Forest

For the last seventeen years standardised counts of autumn larvae have been conducted in Savernake Forest, Wiltshire. Here's the data - 

 

The table shows that autumn larval numbers mostly bumble along at a low ebb, but erupt spectacularly in good summers. These eruptions are highlighted in purple. 2025 was one of them.  It may well have been better than the data suggest, so don't take the data too literally - it merely shows trends.

The work is based on the assumption that the females lay a comparable percentage of eggs within reach each year, though there is no actual evidence for this either way... (searching is done from the ground only, using a shepherd's crook and binoculars - anything else would require a risk assessment the size of War and Peace). 

What's missing here is annual assessment of the quantity and quality of the sallow resource. These are not easy to quantify, particularly foliage quality (e.g. many of the Forest's sallows were afflicted by Melampsora Willow Rust during the wet summers of 2023 and 2024). 

Suffice it that the number of sallows in the Forest fluctuates considerably but is generally in decline, due to increasing squirrel damage ('bark stripping'), ride trimming (which renders many sallows unsearchable by removing lower limbs) and random felling during timber harvesting. 

Also, Savernake is not on particularly 'sallowiferous' soils, lying mainly on Clay with Flints of varying thickness overlying chalk. There has a paucity of sallow regeneration since the early 2000s.