Wildlife in the Garden (8) Burnet Moths- Lena Ward

I introduced Bird's Foot Trefoil (Lotus corniculatus) to my wildlife garden and it has spread in some places, particularly around the pond and between the paving stones (1).  This is the main food plant for the Six-spot Burnet moth (Zygaena filipendulae) (2) and I am very pleased to have had a small population of these handsome moths in the garden over the last few years.

Birds foot trefoil

Birds foot trefoil

Six spot burnet on Verbena bonariensis

Six spot burnet on Verbena bonariensis

This Burnet species is the commonest and most widely distributed Burnet moth in the UK.  It has bright red spots on the black background of the wings which are warning colours to deter birds, as this moth is slightly poisonous due to the cyanides in its food-plant.  The caterpillars are also conspicuous - yellow and black,

The moths are day-flying and are normally on the wing in June.  They emerge from cocoons that are easily visible and are attached to grass stems (or other slim stems) above ground level (3). The moths visit tubular flower types for honey - in my garden Verbena bonariensis is the most popular, but they also like lavender and betony.  Elsewhere I have seen them on knapweed, scabious, borage and even pyramidal orchid.  The moths disperse from my garden, flying over the tall hedges searching for flowers to visit, females for mating and new patches of food-plants for egg-laying.

Pupa of Burnet moth 

Pupa of Burnet moth

The moths mate on emergence, and it often looks as though the males are 'waiting' next to female cocoons, so that when these females emerge they are mated very quickly (4). I think it is even possible that some of the females do not inflate their wings properly because they are mated too quickly (5).  Or it could be that males mate with females that have not emerged correctly and have crumpled wings.

Emerged Burnet moths mating

Emerged Burnet moths mating

Burnet moths mating female wings not inflated

Burnet moths mating female wings not inflated

Eggs deposited on Bird's Foot Trefoil hatch out later in the summer, but the caterpillars do not mature until the following May, as they overwinter as small early instar larvae.  In my garden it is particularly interesting that when the food-plant is abundant the moth population builds up over a few years and many eggs are laid. Eventually the food-plant patches are insufficient to support all the caterpillars and the plants are eaten right down to leafless stems (6).  Larger larvae can be seen searching for suitable plants to climb up to pupate.  They move several metres from the food plant patches and can be seen attaching themselves to a stem and spinning the cocoons (7), which become papery and white as they dry out.  Other smaller caterpillars are seen moving about and presumably searching for more food-plant patches and I think these are slowly starving, although it is possible that some have been parasitized.

Food plant leaves completely eaten

Food plant leaves completely eaten

Burnet moth caterpillar spinning pupa

Burnet moth caterpillar spinning pupa

Bird's Foot Trefoil plants that have been seriously damaged do not recover quickly, so that most of the new generation of female moths must disperse for oviposition.   I have not kept detailed accounts of the population crashes in my garden, but think they are about every 3-4 years.  Probably these crashes in food-plant numbers are localized and the moths maintain their populations by moving about and exploiting new plant patches.  Although these food-plants are perennial, they occur in open habitats, often on disturbed ground and do not survive well when shaded in tall vigorously growing grasslands.

All photographs - Lena Ward ©