For the first butterfly, it was the right place at the right time.
An alfalfa or sulfur butterfly (Colias eurytheme) fluttered into our pollinator garden in Vacaville to sip some nectar from a Mexican sunflower (Tithonia rotundifolia). It lingered for several minutes.
But on another day, several miles away, an alfalfa butterfly wandered into a rural nursery, and splat! Nailed by a yellow sticky trap, which is used to lure, trap, monitor and detect insect pests.
Second butterfly: wrong place at the wrong time.
"There are four to seven generations per year of alfalfa caterpillar, and each generation is closely synchronized with the hay-cutting cycle so that the caterpillar pupates before cutting occurs," according to the UC Statewide Integrated Pest Management Program (UC IPM) website. "Caterpillar populations usually result from a flight of butterflies into the field when the alfalfa is less than 6 inches tall. Extremely large numbers of adults migrating between fields are often present from June to September in the Central Valley and from May to October in the southern desert."
UC IPM points out that factors contributing to economically significant caterpillar numbers are:
- Slow and uneven growth of the crop
- Lack of natural enemies
- Hyperparasites (other parasitoid wasps attacking the natural enemy wasps reducing their numbers)
- Hot, dry weather.
Butterfly guru Art Shapiro, UC Davis distinguished professor of evolution and ecology, says 2020 proved to be a major outbreak year for the alfalfa butterfly in the Sacramento, Yolo and Solano counties. (See butterfly invasion in the Aug. 28, 2020 Bug Squad blog).
Ever seen one in a sticky trap? Often you'll see assorted--and tiny--flies, gnats, beetles, leafminers, psylids and the like. So a yellow butterfly in a yellow sticky trap really stands out.
It's a strikingly beautiful insect.
But in its larval stage, the alfalfa butterfly, Colias eurytheme--also known as the orange sulphur butterfly--is a pest.
If you grow alfalfa, you're not a fan of this butterfly, and rightfully so.
"Alfalfa caterpillars can consume entire leaves," according to the UC Statewide Integrated Pest Management Program (UC IPM) website. "The larger larvae are most destructive."
The butterfly lays its eggs "on the new growth of alfalfa that is less than 6 inches tall," UC IPM says. "Eggs hatch into green caterpillars in 3 to 7 days. Full-grown caterpillars are about 1.5 inches long and are distinguished from other caterpillars on alfalfa by their velvety green bodies with white lines along their sides."
"Caterpillar populations usually result from a flight of butterflies into the field when the alfalfa is less than 6 inches tall. Extremely large numbers of adults migrating between fields are often present from June to September in the Central Valley and from May to October in the southern desert."
We've been seeing lots of alfalfa butterflies sipping nectar on our African blue basil (Ocimum kilimandscharicum × basilicum 'Dark Opal').
Sometimes they don't notice you and you can edge toward them, camera in hand.
Sometimes they don't even notice that a honey bee is shadowing them. Honey bees are also quite fond of African blue basil.
Butterfly meet bee. Bee meet butterfly.
In the insect photography world, that's called a "two-fer"--a bonus of two insects in one photo.
Splat! Splat! Splat! What was that?
A squadron of flying insects? No, more like multiple squadrons of flying insects.
There's a major outbreak in the area of alfalfa butterflies, also known as sulphur butterflies, Colias eurytheme.
Art Shapiro, UC Davis distinguished professor of evolution and ecology who has monitored butterflies in Central California for more than 40 years and posts information on his website, says he's been receiving lots of inquiries about the sulphur butterfly outbreak. Their caterpillars are major pests of alfalfa. The 'cats can consume an entire alfalfa leaf, including the midrib.
"Outbreaks like this used to be commonplace in late summer and fall in the Valley," Shapiro says, "but they disappeared during the drought: from 2012 through 2015 there were hardly any Colias, and I even contacted ranchers and farm advisers to find out if they were using any new pesticides or had altered alfalfa culture in some way that might account for the situation." The answer: No.
"The last big outbreak around here (Davis) was in October 2012," the professor noted. "Long-time residents can recall killing multitudes of them when driving near alfalfa fields in the Valley back in the 'old days.' Now it's happening again! Mass dispersal, like we're seeing now--when they blanket urban flower gardens and mowed lawns--typically occurs when the alfalfa is cut. Females are laying eggs on clover in lawns. One is, of course, tempted to say it must somehow be related to the weather--but we really don't know."
Encounters with cars and people and--yes, predators--continue. We just noticed an alfalfa butterfly trapped in a spider web in our yard.
There will be more.
UC Statewide Integrated Pest Program: Alfalfa Butterfly
We've seen bumble bees, honey bees, sweat bees, wool carder bees and syrphid flies topple into our swimming pool, but never an alfalfa butterfly until now.
This male alfalfa butterfly--the gender identified by noted butterfly expert Art Shapiro, professor of evolution and ecology at UC Davis--bumbled into our pool early one morning. Not sure why it fell into the pool. It spiraled over the catmint, fluttered over the pool, and then dropped into the water. Not very gracefully, either.
We netted it and placed it on a flowering artichoke so it could rest a bit and dry its wings.
The alfalfa butterfly, Colias eurytheme (common name "orange sulphur" or "alfalfa butterfly") is a significant pest of alfalfa, Shapiro says on his website, Art's Butterfly World. It "overwinters as a larva almost entirely in annual vetch at low altitudes, and colonizes alfalfa only as the vetch senesces in May-June. Aside from alfalfa and annual vetches, it also breeds on a variety of clovers and sweet clovers and occasionally on lupines."
"An oddity of sulphurs is that their orange and yellow pteridine pigments are water-soluble," Shapiro told us today. "Had it remained in the water long enough, its scales would have been leached to translucency."
And, he pointed out, "Colias that sit out in the alfalfa during overhead irrigation regularly get water-spotted."
This one not only survived but so did its colors.