Monarch dieoff caused by pesticides

The major risk to Monarchs is from habitat/food plant loss, but a recent article at the Xerces Society website emphasizes the dangers of pesticides.

On January 25, 2024, volunteers stumbled upon a devastating scene: scores of dead and dying monarch butterflies scattered across the lawn of a private property adjacent to the Pacific Grove Monarch Sanctuary overwintering grove in California. While volunteers periodically encounter both live and the occasional dead monarch on the ground near the grove, several details about this incident struck them as unusual. 

Most notably, the dying butterflies were spasming, a symptom commonly observed in response to pesticide poisoning. The number of butterflies involved was also alarming, as approximately 200 out of the nearly 2,000 monarch butterflies overwintering in the sanctuary at the time were affected. While some of the monarchs were scattered across the lawn, the majority were grouped in several piles parallel to the edge of a nearby building. These grouped butterflies showed no signs of predation or rodent caching, suggesting that something else was responsible for their unusual positioning...

The analysis by USGS revealed that the monarchs had been exposed to a variety of pesticides including multiple insecticides, fungicides, and herbicides. On average, each butterfly contained residues of 7 different pesticides in their bodies. Three extremely toxic pyrethroid insecticides were each found at or near lethal levels in the tested butterflies...

Given the timing, location, and profile of pesticides detected, it is likely that the monarch’s deaths were caused by an unreported or untraceable pesticide application by a local resident or business.

The article concludes with some recommendations, including these:

Do not apply pesticides to open flowers or when monarchs and other pollinators are active.   

Keep in mind that organic pesticides are not necessarily safe for monarchs and other pollinators. For example, the organic pesticide, Bacillus thuringiensis (Bt), is highly toxic to monarchs and other butterflies. 

Remember that pesticides applied to impervious surfaces (such as driveways, walkways, patios, and building exteriors) can wash into vegetated areas and contaminate plants and soil.  

Be aware that pesticides used to control “household” pests such as termites, ants, cockroaches, and spiders can also be deadly to pollinators and other beneficial insects.

Last summer crews of workers went through our neighborhood, offering to spray homes and yards for "pests." I watched as they used long-handled sprayers to treat walls ten feet off the ground, to kill ???what.