After roaming among passengers waiting to board a flight to Los Angeles at Daniel K. Inouye International Airport on a recent weekday morning, Trapper the beagle sniffed out a piece of contraband.

Krystyna Jennings had a banana in her cooler bag, which is not allowed to be taken from Hawaii to the mainland.

Trapper, part of the U.S. Department of Agriculture’s “Beagle Brigade,” had done his job.

In the midst of holiday travel, the USDA is reminding the public of what can and cannot be transported to the mainland due to tropical pests in Hawaii that pose a threat to mainland agriculture.

All travelers from Hawaii to the mainland are required to pass USDA inspection before checking in and to declare fresh fruits, vegetables, plants or other agricultural products. Failure to do so can result in delays and civil penalties of $100 to $1,000 per violation.

“Just one infested plant or piece of fruit could cause millions of dollars in damage, expensive eradication efforts and lost trade revenue,” said Alan Dowdy, associate deputy administrator of USDA Plant Protection and Quarantine, in a statement. “Invasive insects and plant diseases cost our nation about $40 billion each year and hurt a wide variety of businesses — from farming to tourism — which can result in hundreds of thousands of lost jobs. Consumers pay more for food and other related goods when farmers lose crops to pests and diseases.”

Jennings had to relinquish her banana but was not fined for the offense.

Among the most frequently intercepted fruits are apples, bananas, oranges and mangoes. Berries of any kind, including fresh coffee berries, are prohibited, as well as sugar cane, cactus plants, lei with sea grapes, and soil or any plants in soil.

The USDA’s top concern is the prevention of fruits and plants carrying Hawaii’s coconut rhinoceros beetles, light brown apple moths, and oriental and Mediterranean fruit flies from reaching the mainland.

While orchid and plumeria lei are fine, those with Mauna Loa flowers, a day lily, and blue jade vines are prohibited because of the bean pod borer. Mock orange leaves, a popular substitute for maile in lei, are also prohibited because the Asian citrus psyllid, which poses a huge threat to the citrus industry, lays eggs on them.

The USDA conducts two screenings at the Honolulu airport before passengers make it to the gate.

The first stop is an agricultural inspection station outside of the check-in counter that X-rays luggage — an extra step — before travelers check their bags in with their airlines. This initial step is required for all passengers traveling from Hawaii to the mainland, according to Chris Tokumaru, USDA plant health safeguarding specialist.

On Hawaiian Airlines, luggage goes through agricultural inspection and travels on the same conveyor belt to check-in. Passengers are asked to make sure their luggage passes through before heading to their gates.

Once the luggage clears screening, USDA inspectors put on a custom-colored sticker, which is different for each airline. Passengers without the required stickers will be sent back.

Most passengers caught with restricted fruits are simply unaware and plan to eat them on the plane, said Tokumaru, or are bringing them as gifts. While a fruit purchased in Hawaii may have originally been grown on the mainland, once in the isles, he said, it’s potentially been exposed to pests here.

Coconuts and fresh pineapples are permitted after inspection. Fresh papayas, bananas and lychees are also permitted but must be treated at a USDA-approved facility and packed in sealed boxes that are properly marked and stamped, not carried loosely.

The second check occurs in tandem with the Transportation Security Administration screening of carry-on bags. USDA inspectors are positioned at a station behind TSA, with screens that view the same images as items being scanned. Inspectors look for the shape and density of restricted fruits.

If fruits somehow make it past these two checkpoints, the Beagle Brigade serves as the last line of defense at the gate. Passengers waiting for flights to the mainland are surveyed randomly.

Honolulu has six beagle and canine handler teams working for the USDA who make more than 1,000 interventions a year. All of the dogs were rescued or adopted from shelters, and are trained for detection work in Georgia.

Beagles are ideal, according to Tokumaru, because of their acute sense of smell, strong food drive and size.

When restricted fruits are found, they are placed into a labeled, brown paper bag noting the time, date and location and eventually thrown out. The higher-risk fruits — and fruits that cannot be identified — go to the USDA’s plant inspection station at the airport, where they are cut apart, examined and pests extracted, then destroyed.

On a recent weekday Mary Beust, acting supervisor for the plant inspection station, stood before a table of cut-open fruits that included apple bananas, calamansi, papayas and rambutan. On average the station intercepts 150 to 300 insect-laden fruits per month, in passenger luggage and hand-carried bags, outgoing mail and cargo.

Beust also had specimens of male and female coconut rhinoceros beetles and their larvae in bottles at the station. The beetle, native to Southeast Asia and first detected in Hawaii in December 2013, is a pest the USDA definitely wants to prevent from reaching the mainland.

It attacks coconut palms by boring into the crown or top of the tree, damaging tissue and feeding on the sap. The beetles could potentially damage more than 30 types of palms as well as banana trees. Beust held up part of a ravaged palm.

“This is an example of the damage a pest can cause,” she said.