Why are there fewer butterflies some years?
These are old photos. We aren’t seeing quite as many butterflies this year, 2017. Why? Nature itself is a huge reason for temporary declines in populations. It’s normal and these populations recover.
1) A late cold snap killed some caterpillars and chrysalises that had overwintered in the wild, freezing them after they emerged from diapause but before they could become adults. Once a caterpillar/chrysalis has come out of diapause, it is vulnerable to freezing temperatures. A long warm spell followed by a hard freeze can kill them.
2) Severe drought in our area prevented some host plants from growing and some from growing well.
Caterpillars depend on their host plants for their very lives. If the plant is too small or dehydrated enough, caterpillars will die.
If host plants are sparse, butterflies have a more difficult time finding one on which to deposit her eggs. When they do lay eggs, they often lay several on a small plant, sometimes resulting in all larvae starving since the plant cannot support too many caterpillars.
3) The lack of host plants resulted in a lack of caterpillars and chrysalides which resulted in a larger percentage of butterfly (caterpillar/chrysalis) predation and parasitoid deaths.
4) After the drought, day after day after day of rains prevented the few butterflies that were in our area from flying about and laying as many eggs as normal. Cloudy days and rain keeps them inactive.
Although cool nights and inactivity can prolong a butterfly’s life, the inability to fly and lay eggs for days and weeks is more likely to result in the butterfly dying a natural death without laying all her eggs.
The following years:
Because of the lack of food for predators and parasitoids, their numbers will decline the following year. This creates a safer environment for butterflies the following year, often resulting in butterfly population numbers climbing again.