This event has the potential to trigger a bloom of desert annuals in the southwestern United States, perhaps followed by an outbreak of Painted Ladies by early March. This depends on whether enough of the heavy rainfall predicted for the coastal regions of southern California on Friday, January 29 crosses the San Jacinto Mountains to substantially water the deserts to the east.
From the U.S. National Weather Service on January 28:
An ongoing heavy precipitation event continues to unfold across central and southern California this afternoon as an atmospheric river swings through the region. Up to 12 inches of rain has already fallen along coastal areas, with more than 4 feet of snow across the central Sierra Nevada. The focus for heavy rain going into tonight will shift to coastal sections of southern California. Up to 3 inches of rain is expected to fall here, with locally higher amounts. Flash flooding and debris flows will be possible near recent burn scars, as soil cannot retain much rain in a short amount of time. Flash Flood Watches are currently in effect. Meanwhile, heavy snow is expected to continue through Friday morning across the central and southern Sierra Nevada. A widespread additional 1 to 3 feet of snow can be expected, making travel extremely dangerous and impossible at times.
If not for this atmospheric river, 2021 would probably not have been a potential year for an extensive Painted Lady outbreak. The Pacific Southern Oscillation is currently in a strong La Niña phase, which brings sinking air and dry conditions to the southwestern deserts:
La Niña—the cool phase of the El Niño-Southern Oscillation climate pattern—was firmly in place across the tropical Pacific in December 2020. Forecasters estimate a 95% chance La Niña will last through Northern Hemisphere winter. La Niña can influence seasonal climate in the United States. Conditions so far have not looked especially La Niña-like, but winter is far from over.
The atmospheric river itself is impressive - appearing on satellite photos as a curving loop of clouds extending from east of Hawaii across the Pacific into central California, then continuing northeastward into eastern Washington, southeastern British Columbia, and southern Alberta. The leading edge of the loop is forecast to move eastward over the next 24 to 48 hours, which will bring the potential for rainfall to southern California and Arizona. The loop itself is propelled by circulation around a strong low-pressure system that has been spinning to the west of the Oregon/Washington coast for the past three days.
Events such as this one suggest the potential for Painted Lady outbreaks to be triggered even during years which would otherwise not be favorable for them to occur. Will there actually be a large outbreak this year? Observers in California and the Southwest, please keep watching for Painted Ladies, and let us know what you see!
--Royce Bitzer, January 28, 2021
Fresh Painted Ladies have been proliferating in Nebraska since about July 18, suggesting a recent mass emergence. This outbreak has been most closely monitored in the eastern part of the state, but large numbers have been observed throughout Nebraska. Meanwhile, here in central Iowa, 150 miles/240 km to the east, only a few have occasionally been spotted in the past several weeks. Will they eventually arrive here, and if so, when? I keep watching the skies for migrants and the flowers for many sudden arrivals, but so far, I've seen almost none.
Almost none, because this afternoon I spotted a single Painted Lady, the first I had seen in just over three weeks. It was nectaring - or trying to nectar - in our small prairie patch on the Iowa State University campus, but wasn't having much luck because a particularly aggressive and persistent Monarch chased it several times and drove it away. So I have an observation, but no photo to accompany it.
Shortly after seeing the butterfly, I realized that the weather was turning warmer and more humid, and the wind was increasing from the south. Could this type of weather change suggest that Painted Ladies might arrive soon, riding the wind from some source region somewhere to the south? Or will Painted Ladies arrive here simply by expanding eastward from what may be a source region in Nebraska?
Because migrating butterflies often take advantage of winds blowing toward the direction to which they are inclined to go, wind flow patterns often reveal the course of a migration. The mid-day wind flow patterns from EarthWindMap on July 17 and 18th showed widespread southerly wind flow across western Iowa and eastern Nebraska on the 17th, and across eastern Nebraska through all of Iowa on the 18th. If the Painted Ladies had come from a widespread area somewhere to the south or south-southwest, one would have expected a strong front of arrivals in both states at about the same time. The current pattern of abundance suggests possibly a more localized irruption or a wind-borne migration from a relatively small source region. Observing flight and wind directions, looking at relative butterfly abundance in different locations over time, and examining patterns of daytime ground clutter on radar maps may provide helpful information about how this irruption develops and spreads.
-- Royce Bitzer, July 24, 2020
Although Red Admirals were first seen migrating southward in central Iowa in early September, their migration remained sporadic and sparse here until the end of the month. This year, unlike during 2017, most of the migrating Vanessa have been staying to the north, in Minnesota, Wisconsin, and Michigan, for much of September. That is, until the weather patterns changed during the past week....
Larger numbers of Red Admirals first appeared in Ames on September 30, when an unseasonably warm and humid air mass over the Midwest led to the development of gusty south winds that kept the butterflies, along with some roosting Monarchs, from proceeding further southward. Some of these butterflies, seen feeding on fallen apples in an orchard, were an intermixture of fresh and partly-worn individuals, perhaps from two different generations.
A cold front moving eastward into this warm, humid air mass spawned strong to severe thunderstorms during the afternoon and evening of October 1, which was followed by cool overcast for the next two days. October 4 then dawned mostly sunny, a cool, crisp day with light to moderate winds alternating between northeasterly and southeasterly. And that's when the migrants appeared again - a nearly equal mixture of Red Admirals and Painted Ladies, along with three Monarchs, flying generally south-southwest from 1 to 3 p.m. through cool air that never got warmer than 60 F.
October 5 was another thundery day of heavy rain, followed by a cool sunny day on the 6th, when the Vanessa butterflies were once again out and about and showing signs of moving. Then the migration really picked up on the 7th. As the air warmed above 60 F after 10 a.m., the butterflies started moving, and by 12 p.m., the southward migration was definitely underway. Being mostly Red Admirals this time, the migrants arrived on average once every four minutes across a 100-foot east-west line, with sometimes two or three coming in a minute around the 1 p.m. peak. The migration then gradually decreased and ended for the day shortly after 3 p.m. At that point, I went back to the orchard that I had visited on September 30 and found about 30 Red Admirals there along with a few Polygonia comma, all feeding on fallen apples (pictures at https://www.inaturalist.org/observations/34054397). They remained active there until after 4:45 p.m., when the area was becoming shaded and the butterflies started departing for the evening.
Although migrating Vanessa butterflies are not as obvious as Monarchs, when they are as abundant as they have been here recently, they are readily spotted flying generally southward. Go out to an open area on clear, crisp days with nearly-calm to moderate northerly or westerly winds and watch for dark or orange butterflies smaller than Monarchs flying from near the ground to eye level to 30 feet/10 meters overhead. Even if you do not see any flying, look at the fallen fruit beneath fruit trees for Red Admirals and other butterflies taking advantage of the feast.
-- Royce Bitzer, October 8, 2019
One month after the last Painted Lady irruption in the final days of June, another generation, perhaps even larger, suddenly emerged in central Iowa on July 29 and 30. I went out on a survey and collecting trip in Story and Boone Counties on the 30th, and found hundreds of fresh butterflies milling around and flying across Highway 17 and other local roads between Boone and Luther. There was also a large emergence between Polk City and Ankeny, with perhaps 200 along a stretch of highway between these towns. Hundreds were also observed along Interstate 80 in eastern Polk, Jasper, and Poweshiek Counties, but there were very few along I-80 farther east in Iowa and Johnson Counties, suggesting a somewhat localized irruption so far. Many of the butterflies seen along I-80 were flying north, but others were flying in various different directions.
Farmers in our area continue to have problems with "thistle caterpillars" feeding on their soybeans. While I was out monitoring yesterday, I met one grower who said that he had had to spend $30,000 this year for spraying his soybean fields for Painted Lady caterpillars. He was also disappointed to hear from me that there could be yet another generation of larvae before a final brood of butterflies emerges in early September to fly south.
This irruption began earlier in the month in the Omaha, Nebraska area, sometime between July 18 and July 23, as reported by @langabee [https://www.inaturalist.org/people/langabee ] from weekly surveys there. From those reports, I was anticipating another irruption in central Iowa, while wondering why it wasn't happening here for another week to 10 days after beginning in eastern Nebraska. Perhaps some observers in western Iowa can fill in the blanks here.
-- Royce Bitzer, July 31, 2019
There were several first-of-season Painted Lady observations reported through the CarolinaLeps newsgroup in early to mid-July. The first was observed on July 9 in Winston-Salem, North Carolina by Gene Schepker. Two Painted Ladies, along with 7 American Ladies, were observed by Harry LeGrand on July 10 in North Carolina State University farm fields south of Raleigh. Another first-of-season Painted Lady was observed on July 14 by Carolyn Seaton in Lancaster County, South Carolina. Painted Ladies typically tend to first arrive in the Carolinas in early to mid-summer, and are often considered noteworthy when they are spotted there.
-- Royce Bitzer, July 31, 2019
Large numbers of Painted Ladies have been emerging in central and western Iowa and in the Omaha, Nebraska area (and most likely elsewhere in Iowa and the upper Midwest). In central Iowa, the numbers of adult butterflies were relatively low up to and on June 27. However, large numbers of mature 5th-instar larvae were observed feeding on thistles near Dawson, Iowa on June 17, and larvae were also reaching pest levels in some Iowa soybean fields - suggesting a huge irruption was about to occur. Then, beginning on June 29, the next generation suddenly emerged in huge swarms, with hundreds along a typical mile of Iowa roadside. Painted Ladies had not yet emerged in northeast Iowa as of July 3, but larvae were present. suggesting that the irruption there is just a matter of time, and is likely to be accompanied by other butterflies arriving from more southerly regions.
These are the abundant offspring of a previous generation of butterflies that arrived in Iowa this spring from the western U.S., perhaps from regions such as Colorado or New Mexico, where large outbreaks had occurred from early April to mid-May. The butterflies emerging now are perhaps the fourth or fifth generation since the first ones appeared in the southwestern U.S. and northwestern Mexico late last winter.
Red Admirals continue to become more abundant in the upper Midwest and southern Ontario, despite several intervening interludes of cold weather. They migrated in on days when warmer, moister air was flowing up from the south. The first of two migration waves occurred on April 15-16, resulting in an abrupt increase in both butterflies and observers noticing them for the first time this year. Locally, I observed Red Admirals flying consistently northward on the 16th, as well as a threefold increase of Red Admirals in territories late that afternoon, as compared with a previous survey a week earlier. After a cooler period, a second and perhaps larger wave arrived in various parts of the Midwest from April 21 -24, again resulting in a further increase in the local population. Observers in Minneapolis, southern Wisconsin, southern Michigan, and southern Ontario reported large increases in Red Admirals, and some substantial northward migrations occurred in southern Ontario, raising the question of whether Red Admirals were actually crossing Lakes Erie and Ontario.
A subsequent period of cooler weather has quieted their activity recently, with far fewer seen during a local survey on April 29th than during a previous one on April 23. Whether this is due to the butterflies sheltering from the cool weather or to an actual decrease in numbers remains to be seen.
-- Royce Bitzer, April 30, 2019
There was an extensive northward migration of Painted Ladies in southern Oregon from at least between April 21 to April 25, 2019. Three different observers recorded migration rates ranging from between 68 northward in 2 hours on April 23 to 300 northwestward in 15 minutes on April 25. These butterflies are likely part of a second generation that originated farther south somewhere in California. From other reports I've received, conditions in California and southern Nevada have been so favorable that two generations of Painted Ladies grew and thrived there on an abundance of nectar and larval food plants.
-- Royce Bitzer, April 30, 2019
As I was writing here last month, I’ve been following the enormous Painted Lady migration that’s been happening in California and the Southwest since late January to early February. While this migration continues northward, it’s now also time to turn more attention eastward as wind and weather patterns have become favorable for Red Admirals and American Ladies to migrate northward into the upper Midwest, the eastern states, and southern Ontario. Painted Ladies may be arriving in our area somewhat later this spring, perhaps from the west, or perhaps from a smaller population that overwintered in Texas.
Red Admirals have arrived in the upper Midwest, and are already here in Iowa – they seem to have come with the warm southerly wind last Saturday, April 6, resulting in sightings in central Iowa's Story and Boone Counties on that day. They were also spotted in Omaha, Nebraska on April 2, and in Kansas City, Missouri on April 5. Chip Taylor, the director of Monarch Watch, spotted his first two in Lawrence, Kansas on April 6th, while suggesting that Red Admirals could have arrived there a few days earlier. A first-of-the-season Red Admiral was also seen on April 8 in Kingsville, Ontario, near Point Pelee National Park.
Since yesterday, it's likely that any further northward Red Admiral movement will be literally stopped cold (at least temporarily) by an enormous low-pressure system passing through the northern Great Plains, Minnesota, and Wisconsin. This storm has brought us two days of high winds in central Iowa, and heavy snow in states to our north. To our south, temperatures were in the 60s F in southern Iowa and in the low 80s in Kansas. Under these conditions, any additional Red Admirals arriving from farther south might stop migrating and accumulate in these areas, resulting in a sudden increase in the number present.
-- Royce Bitzer, April 11, 2019
Painted and West Coast Ladies are not the only butterflies in the western United States to be undergoing an outbreak. California Tortoiseshells have also been present to abundant in some places in northern California and Oregon where Painted Ladies probably have not yet arrived in any large numbers. They look like this: https://bugguide.net/node/view/1447798. While they are interesting butterflies in their own right, and worthy of further study, they are not Painted Ladies, so please be careful not to confuse the two when reporting Painted Lady sightings. Also, according to a March 18 posting on DesertLeps, California Tortoiseshells have also become abundant in the area of southern California around the Santa Rosa and San Jacinto Mountains National Monument, replacing the Painted Ladies, which are surprisingly not that common there now and seem to have mostly departed the area. However, Painted Ladies were still abundant in southern California on March 16 near the Mescal Range in San Bernardino County, CA, about 138 miles (222 km) to the north-northeast; no California Tortoiseshells were observed there.
-- Royce Bitzer, March 19, 2019