The Survival of The Wood White Butterfly

The Wood White is a butterfly that lives in woodland glades and clearings and lays its eggs on various plants in the vetch family (related to Peas).

A female Wood White might lay up to 60 eggs if the weather is fine, and the eggs will hatch after 10-20 days (depending on the temperature). Many eggs are eaten by insects or attacked by tiny parasitic wasps, so that only a small percentage of those laid even hatch into the first larval stage. More about Parasites soon!

The caterpillars are eaten by birds even though they are very well camouflaged.

Dr Martin Warren studied a number of Wood White eggs

During a three year study, he followed their progress through the four larval stages to the pupa stage. The larvae of the Wood White seem to stay in one small area for the first three stages of their life, then in the fourth stage they begin to move off, spreading out to feed and eventually finding somewhere in the dead leaf layer on the ground to form pupae. When they start to move about, they become extremely hard to follow. Added to this, the pupae are almost impossible to find!

The study of the hatching rate of pupae was done by putting the pupae in marked places and counting how many of them survived until the Wood White butterflies hatched out.

The table below shows the results of the study of Wood White survival over a period of three years. The instars are the stages in the caterpillar’s development - see the section on Butterfly and Moth life-cycles.

So of the 201 eggs laid in 1978 (see the middle column), only 7 adults emerged successfully the following year. The % mortality of the eggs is the % of eggs that did not survive to hatch that year and that figure was 44.78%.

Mortality is calculated as the percentage of each stage that dies without reaching the end of the next stage.

                                             Data from Martin Warren, Butterfly Conservation

Look at the table and draw a bar graph to compare the % mortalities at the six stages for the three different years. The % mortalities should be on the vertical axis of the graph.

Complete the table below to show the overall success rate of the Wood White over the three years:

Questions for you to answer

1.What was the average success rate from egg to butterfly over the 3 years?

2.What was the average mortality rate over the three years?

3.Which stage in the life cycle of the Wood White has the highest mortality?

4.As a large number of fourth instar larvae disappear, how might this explain the figures?

5.What are the main predators of Wood White larvae?

You can download and print off this activity here.

Egg Hatching success in the Brown Hairstreak Butterfly

The Brown Hairstreak butterfly flies in late July/August and lays its eggs on the young shoots on the edges of Blackthorn thickets and hedges.

The eggs do not hatch until April or May, at least a quarter of them dying through disease or as a result of hedge cutting. Even though the caterpillars are very well camouflaged, a large majority of them are eaten by spiders, harvestmen and carnivorous insects when they are small and by birds like tits and warblers when they are larger. The older larvae change colour before they crawl down to the ground to pupate.

Once the larvae have pupated under the soil or in the leaf litter, they fall prey to mice, shrews, and ground beetles

The dangers of the Brown Hairstreak pupa stage

Jeremy Thomas in his superb book 'Butterflies of Britain and Ireland' (published by Dorling Kindersley in association with the National Trust) describes the dangers of the pupa stage of a Brown Hairstreak's lifecycle.....

'Small mammals in particular find the speckly brown chrysalis irresistible, and very large numbers are eaten. Up to four-fifths of the whole colony was killed during the pupation period on my study site in Surrey. The main culprits appear to be mice and shrews. In captivity, both show an extraordinary ability to sniff out the chrysalises.Once one is unearthed, a shrew pounces on it in a frenzy of excitement and squealing, tearing and scattering the case into tiny fragments, while gobbling up the sticky contents.

Common Shrew photo: www.david.element.ukgateway.net

'A mouse is more sedate. It sits on its hindquarters, holding the chrysalis in both hands, as a squirrel might a nut. It then neatly nibbles the chrysalis until not even the hard cuticle remains, and then washes its paws and face before scurrying off to root up another one.

Wood Mouse photo: www.david.element.ukgateway.net

'Both shrews and mice could find and eat up to one chrysalis a minute in my captive pen. Large beetles, by comparison, managed about one chrysalis a week. In this case the shell is slit right around the edge, as if cut by an old-fashioned tin-opener, and the jagged halves are prised apart to enable the beetle to feed on the rich contents.

Only rarely have I found this damage in the wild, and it appears that ground beetles pose very little danger to Brown Hairstreak chrysalises in comparison to small mammals, which cause great damage.'

Hatching success of Brown Hairstreak eggs

Here is a table of results from a three year study in Oxfordshire of the hatching success rate of Brown Hairstreak eggs. Eggs were found on young Blackthorn shoots in the early spring and were marked and watched to see whether and when they hatched.

The caterpillar leaves its empty egg shell on the twig after it has hatched and burrowed into the opening bud of the Blackthorn shoot. Usually this shell remains and is visible but occasionally it disappears. So of the eggs that were not found to have hatched. a few had disappeared (maybe predated by insects). Others had been shattered or were parasitised or remained unhatched because they were sterile or diseased.

This data sheet can be downloaded as a pdf here.

Have a look at this results table and try to complete this table below:

Then try to answer these questions:

1.What is the main cause of the hatching failure of Brown Hairstreak eggs?

2.Overall what was the % hatching success of the eggs?

3.What was the % failure rate in the eggs?

4.What are the main causes of mortality in the Brown Hairstreak caterpillars once they have hatched a. when they are small and b. when they are larger?

5.What is the main cause of mortality in the pupae of the Brown Hairstreak?

Here is a link to an extension activity about predation of butterflies and moths by different groups of predators.