Early butterflies

The warmth of recent days has triggered the first real wave of spring butterflies. Most of the species on the wing at the moment, notably peacock, comma, brimstone and small tortoiseshell, will have spent the winter as adults, hibernating in some suitable crevice, garden shed or evergreen bush. All these early season butterflies will be seeking out suitable nectaring opportunities. Those that have overwintered as adults will have to replenish energy reserves spent getting though the long cold winter months. As such, many will range widely, seeking out nectaring opportunities with flights that may well bring them into your garden. This is one reason why gardens with spring-flowering plants are particularly beneficial to butterflies, and indeed to other insects (such as bumblebees) on the wing at this time of the year.

Butterflies do not take nectar from just any old flower but actively select those that offer the best returns. We are not just talking about the amount of nectar on offer but have also to consider the amount of energy that a butterfly has to expend in getting that nectar. Each nectar-producing flower is a bit like a petrol pump but one which a visiting butterfly will typically drain on each visit. It takes time for the fuel pump to be refilled so a butterfly does not want to be visiting a pump that has been recently emptied by another visitor. The amount of nectar left in the pump is known as the standing crop and the size of the standing crop will depend on how quickly the flower produces nectar and when it was last emptied. Flowers in which the nectar is held at the end of a long tube (known as the corolla) tend to have higher standing crops because only long-tongued visitors can access them. Flowers with short corollas can be accessed by most insects, including those with short tongues, and so tend to have smaller standing crops available.

What does this mean for our visiting butterflies? Well, those butterflies which are large in size and which therefore expend more energy just getting around, will seek out flowers with the biggest standing crop. Fortunately they can access these because of their long tongues (tongue length has been found to correlate well with body size and wing loading). Such long-tongued species also prefer to forage on plants where the flowers are clumped together rather than spaced apart. This is why you see red admirals, small tortoiseshells and peacocks feeding so readily on Buddleja davidii later in the year, while shorter-tongued species (typically smaller in size and with lower flight costs) can afford to visit solitary flowers, like bramble and dandelion, with the risk of reduced standing crops.