Monitors counted 38 monarch eggs and larvae across 4 solar farms in Minnesota, and more turned up in the partial shade between the panel rows than in the open ground beside them

A milkweed stem was growing in the band of shadow thrown by a tilted solar panel, and something was sitting on it.

Not a bee. Not a moth.

A monarch egg, pale and smaller than a sesame seed, pressed against the leaf.

This was happening at working solar farms in Minnesota, on stems planted where gravel or turf usually goes.

So what was the shade between the rows actually doing for the butterfly?

What the shade between the rows is doing

A monarch does not choose a spot at random. The female lands on a milkweed stem, tastes it with chemical sensors in her feet, and lays a single egg only when the plant and the conditions suit her. Out in full sun on a July afternoon in the upper Midwest, a milkweed leaf surface can climb well above air temperature, and monarch eggs and small caterpillars are sensitive to heat.

Partial shade collects between two rows of tilted panels. Acting more like a roof than a wall, an array filters the harshest midday light while leaving enough for milkweed to grow. That cooler strip is the most likely reading of what monitors found, but temperatures under the panels were not measured, so the cooling explanation stays proposed rather than demonstrated.

What the counts show is simpler. The butterflies were not avoiding the panel rows: they were breeding in them.

The ground under the panels was seeded on purpose

The sites sat in Anoka and Ramsey counties, each ranging from 18 to 68 acres, and each had been seeded with a native pollinator mix a few years before the monitoring began. Milkweeds went into the ground alongside the panel footings, part of a mix chosen to bring back plants that native insects grew up with.

In many solar developments the ground below the array is gravel, bare soil or turfgrass. These sites were planted instead with a diverse spread of native wildflower species, and it takes several growing seasons for that planting to thicken into anything like prairie. By the time of the survey, dozens of flowering plant species were in bloom across the sites. Bees, butterflies, moths, flies and wasps had all arrived.

The monarchs came too, and they stayed long enough to breed.

The numbers behind the finding

A national monitoring coordinator surveyed four photovoltaic installations across June, July and August of 2021, measuring the abundance and species mix of milkweeds and flowering plants along with use by monarchs and other pollinators. Paired transects were walked at each site, one in full sun beside the array and one in partial shade between rows.

In total, 38 immature monarchs, eggs and caterpillars together, were recorded over the season, with reproduction detected at all four sites. They appeared on partial shade transects at 10 of 12 site visits and on full sun transects at only 2 of 12. Both the monarch count and the number per milkweed plant were significantly higher in the shade.

The lead monitor put it this way: “These results indicate that pollinators utilized habitat regardless of solar panel presence.” For pollinators as a whole the panels made no measurable difference; for monarch eggs and larvae the shaded rows held more of them.

Why this matters for a butterfly losing ground

Up to 99 percent of native northern tallgrass prairie in the United States has disappeared since European settlement. Milkweed, the only plant a monarch will lay eggs on and the only food its caterpillar eats, largely vanished from the agricultural Midwest along with it.

Solar development now sits on a lot of former cropland. The partners behind this monitoring cited estimates of 300,000 to 400,000 acres under ground mounted solar in the United States, with projections of 3 to 5 million more acres by 2035. That is a large amount of fenced ground that could be bare gravel or living prairie, and the choice is made at seeding time.

For more on what happens when panels change the light reaching the soil, a New York study found a 24 percent reduction in fall light changed what vegetables could grow beneath an array.

What is still missing and where the edges are

Four sites are a beginning, not a final answer. With a sample that small, no statistically significant differences were detected in the plant communities in full sun and partial shade, and the seed mixes under the panels differed from those planted alongside. Invasive plants also appeared and required follow up management.

Establishment under panels lags behind open ground. A separate assessment of eight seed mixes at three utility scale Minnesota solar sites found native plant coverage under and between the arrays rose from 10 percent one year after planting to 58 percent after three years, while full sun control areas rose from about 10 percent to 70 percent. Siting decides much of the rest.

Under Minnesota’s habitat friendly solar standard, an owner may claim benefits for pollinators and songbirds only by following state guidance, which steers projects away from remnant prairie. The shaded rows are drawing native bees as well, which is why a Georgia array study found wildflower seeded ground carrying far more native bees than the turf beside it.

The farms were built to make electricity. The seed mix was a separate decision laid on top of that, and on a stem in cool shadow on a July afternoon, a monarch found what it needed and left an egg on a leaf.