Invasion

August 22, 2025

@Elijah Nouvelage, Reuters

Inside the back section of my local newspaper was a USA Today article on and invasion taking place in the San Jaquin Valley, California.  This month’s newsletter by the Almond Board of California said the infestation has impacted more than 100,000 acres of almond trees and caused US$109 million to US$311 million in losses from damage to equipment and crops over a year.  The valley is one of the world’s top agricultural regions.  The invasion started last fall and took growers by surprise.  Now they are still scrambling to figure out how to repel the intruders, often turning to Roger Baldwin, a rodent expert with the University of California, Davis, where he is a cooperative extension professor.  Baldwin used to get fewer than one inquiry a year, but he said, “There are inquiries and questions about doing interviews on rat control and giving seminars on rat control, and can I write articles about this.”  His phone buzzes with requests from farmers, journalists, and others seeking expertise on how to control the rat invasion on the state’s US$4.7 billion almond industry.

When I went online, I found the brown rat (Rattus norvegicus), also known as the common rat, street rat, sewer rat, wharf rat, Hanover rat, Norway rat, and Norwegian rat, is a widespread species of common rat known to create periodic invasions in other parts of the world.  The island of Taukihepa (Maori) or Big South Cape Island (European) is an offshore island of New Zealand to the west of the southern tip of Rakiura or Stewart Island.  The island has no permanent inhabitants but is visited by Maori hunters (muttonbirders) in search of sooty shearwaters (Ardenna grisea), known in New Zealand as “muttonbirds”, that came to nest on the island in the spring.  The muttonbirders arrived in March 1964 to find the island devastated by rats.  Previously free of mammalian predators, the ecology of the island was overwhelmed in a matter of years.  Many endemic species of bird (some flightless) were driven to extinction and the surviving insect and bird life heavily reduced.  An eradication program was initiated to rid the island of rats, and the island was rat-free again in 2006.

The exact cause of the California rat invasion is unclear, but it may have been spurred by abandoned fields.  California accounts for more than 75% of the global almond production and exports 70% of its output to 100-plus countries, with India as the leading customer.  Rats have always been a bigger problem at citrus orchards but have now developed a taste for nut trees.  Baldwin and others say the shift comes from three contributing factors.  A protracted drought and diminishing water supplies in the early 2020’s left over half a million acres (202,343 ha) in California unplanted.  That also meant there was no pest control in those fields.  The drought ended in the winter of 2022-23 and began a three-year spell of at least average rainfall bringing vegetation growth and an abundance of food resources.  State restrictions on pesticides have also limited the means growers have for getting rid of rodents.  The rats are joined by five species of squirrels (family, Sciuridae), deer mice (genus, Peromyscus), and others, but rats have by far been the most destructive.  Some farmers say they have spent hundreds and even thousands of US dollars per acre on rat control over the last year.

THOUGHTS: Invasive plants and animals can quickly expand to the level of an invasion if left unchecked.  Humans have tried to control the invasion using biological and pesticide controls.  The biological fixes have a record of resulting in an invasion of their own.  Globalization has brought an influx of goods and services to every corner of the earth.  It has also resulted in the destruction of ecosystems in these same areas.  While we may never reverse these effects, we can be vigilant in choosing what to purposefully spread and curtailing any negative aspects that may result.  Act for all.  Change is coming and it starts with you.