Exotic pests cost California growers over $3 billion every year, according to the California Dept of Food and Agriculture. Keeping these organisms in check is crucial for the nation’s food supply, and it’s a constantly evolving challenge. However, farmers today have more tools and research knowledge at their disposal than ever before. Here’s an overview of these non-native invaders, with a look at the inventive ways the agriculture industry has found to stay one step ahead of them.

What are “Exotic” Pests?

Close-up of a Gypsy Moth Caterpillar With Fuzzy Grey and Blue Hair
The gypsy moth ( Lymantria dispar ) – family Erebidae – hairy, colorful caterpillar – macro – closeup

This term simply means organisms that originated somewhere else. It is the opposite of “native.” These are diseases, microbes, plants or insects that are accidentally introduced into a new ecosystem. If conditions in their new location are biologically friendly, their populations can explode in size. They often have no predators, because the local creatures haven’t evolved to see them as a food source. There are too many individual organisms to list here (although anyone interested in the details can visit the website of the California Department of Food & Agriculture (CDFA), and read their detailed lists of invasive species.) Bugs and organisms travel around the world along with people and products — often undetected. According to the Center for Invasive Species Research at University of California Riverside, every year this state acquires an average of nine new species of bugs, and three of these typically become pests. While exotic pests are generally insects (many of which carry and introduce diseases), there are a few types of invasive exotic plants as well that cause agricultural problems. One example is salt cedar, or tamarisk, which can clog irrigation canals and negatively change the chemical composition of soil.

Who’s Fighting Against These Invaders?

Individual farmers, of course, are key in protecting against pest invasions, although they don’t stand alone in their efforts. The CDFA has a mandate to “Protect against invasion of exotic pests and diseases.” The University of California Statewide Integrated Pest Management Program partners with growers, communities and public agencies to develop sustainable ways to keep agricultural pests under control, while the California Department of Pesticide Regulation(CDPR) works to make sure pesticides are used in the safest, most effective way. CDFA encourages growers and gardeners to make use of the “Report a Pest” program, which helps the state keep track of possible problem species. Many private companies, of course, partner with public agencies to help growers prevent damage to their crops and livelihoods.

Bugs Do More Than Eat Plants

Close-up Image of Greenery Huang Long Bing Citrus Greening Disease

Insects and other invertebrates (“bugs” in general) are the most common kind of exotic pest. In addition to consuming crops, they often bring other kinds of problems as well. The Asian citrus psyllid, for example, is an insect about the size of an aphid. It carries a disease called “huanglongbing,” or “citrus greening,” which is one of the most destructive citrus diseases in the world, and it has no cure. Once a tree is infected with HLB, it will die. The insect is native to Asia, and was first discovered in the United States in Florida, 1998. Ten years later it had spread to California, most likely on graft wood. Despite quarantine efforts, it has spread throughout southern and central California. The light brown apple moth affects over 2000 species of plants, affecting their growth pattern and overall health. California and the USDA together have spent $70 million so far in efforts to control the spread of this moth. Since this pest has not yet appeared in other states, it poses a risk to California fruit growers’ ability to export their products domestically. The glassy-winged sharp shooterintroduces a pathogenic bacteria that causes a whole arsenal of diseases in many important agricultural crops.

Larger Animals

A few exotic pests show up in macro form. Nutria are 20-lb South American rodents that made their way to California from Louisiana. These animals are not yet common here; 510 total have been caught in California this year, although they now live in 30 states. They are serious agricultural pests because of direct crop consumption (including destroying 10 times the amount of crops they actually consume) — and also because their burrowing habit damages landforms and irrigation infrastructure.

Animal Resembling a Groundhog in Grass Looking Wet

How California Growers Protect Their Crops

The California Department of Pesticide Regulation (CDPR) recommends a diverse, step-by-step approach to managing invasive and damaging species. This is sometimes termed “Integrated Pest Management.” Before turning to chemical pesticides, prevention is the essential first step, followed by using all less-toxic methods of pest management. Only then are chemical pesticides introduced. This least-toxic first approach is having an effect: a June 2019 news release from CDPR [shows that use of the most toxic pesticides has dropped in California since 2017. Below is a look at the hierarchy of pest management approaches:


The state may institute quarantine, regulating imported produce to agricultural areas and creating inspection checkpoints to keep out accidental “hitchhiking” pests.

Cultural Controls

In some instances, established pests can have their lifecycles disrupted when crops are not repeatedly grown in the same soil. Also, it is sometimes possible to interrupt pest reproduction by changing planting schedules, or adding additional mulch.

Physical Barriers

Row covers during a sensitive part of the growth cycle or other barriers can be effective at preventing insect and bird problems. Many citrus farmers are building screen houses around their trees, in order to protect them from the Asian citrus psyllid.

Beneficial Predators

In the case of HLB, scientists began to experiment with a tiny wasp (Tamarixia radiata) that preys on the psyllid. Scientists at Cal Poly, in partnership with Cal Poly Pomona, the California Department of Food and Agriculture, Citrus Research Board and the Citrus Pest & Disease Prevention Program, are breeding and releasing this wasp. In 2018, more than 3 million of these predator wasps have been released. Often, this approach to controlling pests is the most effective.

Large Close-up Image of a Wasp Pests on Field
Wasp (Tamarixia radiata)

Crop Destruction

In some cases, the only workable way to prevent the spread of pests involves destroying affected crops, which is a last resort that no grower wants to face. In Florida, many acres of citrus trees have been destroyed, in an effort to control HLB. California has mostly found other methods of control, so widespread destruction has not been needed — but it’s still a serious concern. The LA Times has an interesting in-depth article on this problem, published in March 2019.


Pesticides are the class of chemicals that repel or eliminate pests of every type. There are disinfectants and fungicides to eradicate microorganisms, mold, mildew and fungi. There are insecticides to kill insects, rodenticides to kill mice and rats, molluscicides to kill slugs and snails, and herbicides to kill weeds. Each of these categories contains many different products. They are all closely regulated by the California Department of Pesticide Regulation (CDPR).

Some pesticides are applied to the soil before planting, while others are used on the plants themselves. Application occurs during various times in the growth cycle, depending on the substance and the pest involved. The most toxic pesticides are applied so as to ensure that little or no residue remains at the time of harvesting.

Organic and Synthetic are on a Continuum

Many pesticides — over half of those used in Calfornia — have active ingredients that are approved for organic use. Most synthetic pesticides (as well as most that are accepted for organic use) are considered to be at the low end of the toxicity spectrum. There are not necessarily any clear dividing lines between natural and synthetic pesticides. Minerals such as sulfur and copper, for instance, are potent at fighting soil diseases, and many types of oils are used for controlling soft-bodied insects.

Farmer spraying pesticide in paddy field

Labels Carry Vital Information

Pesticide buyers will find three different levels of toxicity on the labels of products available to them: Caution (least toxic), Warning (higher toxicity) and Danger (most toxic). Labels are also required to list the exact pests that the substance is effective against, together with detailed instructions about safe usage.

More than 77,000 farms grow food in California, accounting for two-thirds of the fruits and nuts consumed in the United States. Over 25 percent of the state’s produce is exported around the world. California’s growers know that they play a crucial role in making sure that people everywhere have enough to eat. Invasions by new exotic species of pests is an inevitable reality, but today’s farmers have enough tools available to them to be able to hold their own.

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