A Plague of Cactus - Excerpt from an article in bioGraphic
Across Kenya’s wildlife-rich Laikipia Plateau, a thorny enemy is advancing. But a tiny sap-sucking insect may help save the region’s animals and people.
Before the sun has peeked above the horizon, Philip Nangoo Larpei, a Maasai elder in his 60s or 70s (he hasn’t kept track), is already outside checking his livestock. His wife milks the family’s cows and goats, filling plastic jugs. After the milking, one of Larpei’s grandsons, a boy of around seven, releases some two dozen cows from a corral and leads them down a slope in search of grasses to forage. Some of the cows’ lips are visibly cut and distorted by cactus spines, which poke from their skin like toothpicks.
I walk with Larpei as he trails behind his grandson. He scans the rolling terrain in search of elephants, which frequently roam through these fields in northern Laikipia, Kenya. As soon as I take my eyes off the ground, several cactus spines poke my arm and ankle. I curse the cactus—an invasive plant—as I try in vain to pluck the tenacious spines while simultaneously navigating the thicket before me. Native shrubs, forbs, and grasses are in short supply on this dry, compacted reddish soil.
“These cattle used to have plenty of grass to eat right here, around our home, but now they have to walk farther and farther to find anything worth eating,” Larpei says later in his backyard as his wife delivers a pot of chai. Flies flock to our cups. Wafts of cow manure suffuse the morning air.
As Larpei sees it, the cactus must go, or he will lose his livelihood and have to move his family to elusive greener pastures. Some of his neighbors have already left. If they can’t get rid of the cactus, he says, “in 10 years this land will be gone. There will be no one living here, nothing left to graze on.”
What’s happening in Kenya’s Laikipia region is a case study in how degraded land—damaged by unsustainable grazing practices and harmed further by climate change—is giving invasive species a leg up. Sometime in the late 1940s or early 50s, British colonial officials planted Opuntia stricta, a cactus native to the Americas, around their outpost in the tiny town of Dol Dol, a few miles from Larpei’s homestead. The shrubby plant’s attributes—bright yellow flowers, purple bulbous fruit, and protective spines—apparently made it both ornamental and functional, as a living fence or hedge.
For half a century Opuntia stricta, also known as prickly pear, coexisted with other vegetation on this part of the Laikipia Plateau, which lies between snow-capped Mount Kenya in the southeast to the rim of the Great Rift Valley in the west. But by 2005, the cactus’s advance had become a full-blown invasion, thanks largely to a complex collusion of factors: human population growth in the region and a shift from a nomadic to a more sedentary lifestyle, overgrazing and tree-cutting for fuel production, dispersal of the plant’s seeds by wildlife, and more frequent droughts.
These interconnected assaults have caused a decline in productive grasslands, including native forage for wildlife and livestock. Now, roughly 60 percent of the rangelands of northern Kenya are “severely degraded,” according to Tom Lalampaa, CEO of the Northern Rangelands Trust, an organization that supports about 40 community conservancies across 10 counties in northern and coastal Kenya. The region’s compacted soil, which is increasingly overrun by invasive species and stripped of native grasses, is a far cry from the healthy grasslands of Lalampaa’s youth, when human and livestock populations were much smaller, and when elders more effectively enforced sustainable rangelands-management practices among the younger herders.
Many invasive species plague Kenya, but Opuntia stricta stands out for its tenacity: It spreads through a variety of methods, and it is painfully hard to kill. Small chunks of a pad that break off from a plant can sprout roots and grow independently. Remnants of the cactus that people toss into gullies get washed into creeks or rivers when the rains come, soon to be reborn into menacing plants miles downstream. And elephants, baboons, and even human children are accelerating the cactus’s spread by eating the fruit and defecating the seeds, and by breaking off the plant’s stems when trying to access the fruit or nearby grasses, leaving chunks to take root and grow into new plants. “You can say it’s very clever, this Opuntia,” says David King’ori, Laikipia County’s environment director.
Partly because of its rich mix of humans, livestock, and wildlife, Laikipia County is ground zero for the Opuntia stricta invasion. The exact extent of the cactus’s conquest here is unclear, but remote sensing conducted by the U.S. Forest Service of a hard-hit area called Naibunga revealed that the species is present on about 90 percent of the region’s landscape, and it dominates the vegetation on about 12 percent of the land.
In addition to a growing number of cactuses, Laikipia County also hosts the largest populations of endangered large mammals in Kenya: roughly 7,000 elephants, Kenya’s second largest population; half of the country’s rhino population; and many Grevy’s zebra, reticulated giraffe, and wild dogs. Like Larpei’s livestock, many of these species have been impacted by the region’s degraded grasslands.
In recent years, the prickly pear invasion has caused a spike in human-elephant conflicts, according to Antony Wandera, a Northern Rangelands Trust wildlife ecologist. Several thousand conflicts between people and elephants are reported in Kenya each year. A short supply of native grasses, shrubs and trees has forced elephants to travel farther in search of food and water, including into human settlements, where they come seeking prickly pear fruit and other forage such as maize crops. Roughly five or six elephants are now killed each year after run-ins with humans, and elephants kill one or two people a year in the area, Wandera says. While these numbers pale in comparison to the impact of poaching on the region’s elephants, the escalating strife doesn’t bode well for either humans or elephants.
Elephants’ penchant for prickly pear fruits has made them a major dispersal agent of the seeds. Recent studies have revealed that a single elephant can deposit some 3,000 seeds in a day. And with an intestine that measures up to 20 meters long, an elephant can transport those seeds dozens of kilometers before dropping them in a new location. Shirley Strum, an anthropologist who has been monitoring the spread of Opuntia stricta since 2005, has found that the cactus tends to grow densely along elephant corridors and has been spreading in a southwesterly direction at a pace of about 2 kilometers per year.
While elephants move prickly pear seeds farther than other mammals, they aren’t the only species to eat and transport the invasive cactus. Olive baboons (Papio anubis) also rely on prickly pear fruits when native fruits aren’t available, and supplement their diets with them at other times. Strum, who first started studying the Opuntia invasion because of her work with olive baboons, has seen the primates exercise this preference first hand. “One of my favorite baboons once harvested 32 fruits in 30 minutes,” she says. Not only does the baboons’ taste for prickly pear fruits help the invaders, it also threatens the survival of native plants that depend on the primates to disperse their seeds.
While the human residents of Laikipia County aren’t above harvesting and utilizing the fruit of the prickly pear, there is little debate about whether or not the invasive plant should be encouraged to persist. It’s not just that Opuntia cacti are crowding out the native grasses and shrubs that livestock need to survive, or that the cactus-caused spike in human-elephant conflicts has many people worried. Pastoralists also lose cows, goats, sheep, and even camels to cactus-related infections and abscesses (in mouths, throats, and intestines), caused by either trying to eat the Opuntia fruit or by trying to access grass that grows within cactus thickets. Some livestock have gone blind, their eyes impaled by cactus spines. And many people, especially bare-footed children, have suffered foot infections from stepping on the spines.
Almost everyone wants the demonic cactus dead. But eradicating it is a massively thorny challenge.
One promising answer is a solution that nature herself created: a homely scale insect about the size of an eraser head. Called a cochineal (Dactylopius opuntiae), it is an unlikely warrior—a relative of mealy bugs that sucks the sap out of Opuntia stricta’s leaf-like fleshy pads, making the plant wither and produce fewer fruits and seeds, which ultimately leaves it unable to reproduce.
Cochineal insects were first used as biological controls for invasive Opuntia cacti species in South Africa more than a century ago. Since then, they have also been used effectively in Australia and elsewhere. Each cochineal species comes in several “biotypes,” each of which feeds on only one species of plant and does not move to other species. That’s crucial, because introducing one non-native species to control another has a history of going ecologically awry. But those stories—think cane toads and kudzu—involve generalists, species that can eat a variety of things. In recent years, a biotype of the Dactylopius opuntiae cochineal has been applied and monitored as a biocontrol against the invasive Opuntia stricta cactus in Kruger National Park in South Africa. By 2003, after six years of treatment, Opuntia biomass had plummeted by about 90 percent.
Today, in Kenya, the cochineal is the best weapon to date against the invasive plant. But so far, efforts to use it cannot keep pace with the gonzo cactus’s spread. Northern Rangelands Trust (NRT) wants to change that, by expanding the biocontrol program and ramping up related efforts to rehabilitate rangelands. “The scale of the challenges are landscape, and the scale of the planning also needs to be landscape,” says Kieran Avery, NRT’s director of natural resource management.
In 2010, a handful of pastoral communities in the center of the Opuntia invasion sought help battling the prickly beast from NRT and Arne Witt, a South Africa-based invasion biologist. Witt, who coordinates invasive species management at the Centre for Agricultural Biosciences International (CABI), led painstaking efforts to secure permits from the Kenyan government to use cochineal insects in the Laikipia region. That process took nearly four years, allowing the cactus to spread much further than it would have, Witt says.
With the permits finally in hand, Witt has helped NRT establish 16 greenhouses throughout Naibunga, a 180-square-mile area, to breed the bug.
Outside one of the greenhouses, covered by white plastic sheeting that flaps in the breeze, Margaret Nyambura Mamai, 60, stands amidst a group of giggling children. As manager of the 11-by-11-foot greenhouse, located in front of her homestead, she is responsible for growing and releasing cochineal insects into the surrounding landscape.
Inside, 100 metal buckets hold prickly pear leaf pads. Many of them are partially coated with a white, cotton candy-like substance, a protective membrane that forms around the tiny black insects. Every three to four weeks, the amount of time it takes for the cochineal insects to infest a good quantity of cactus pads, community members, who are paid by NRT, carry the buckets to nearby fields and place the insect-infested pads at the base of healthy cactus plants. Wind will disperse the female bugs, which are flightless, to adjacent plants.
As I walk with them, Mamai and two other women carry buckets full of infested cactus pads to a nearby field. The earth is dotted with a mix of healthy green and partially shriveled cacti, alongside some perennial short grasses. The women suddenly break out into a high-pitched call-and-response song, swaying to the rhythm as they place infested cactus pads into the thicket. They later tell me that the song, which they wrote, is a plea to the invasive cacti to go away and leave their children, their land, and their livestock alone. It is also an appeal to NRT, the Kenyan government, and other potential funders to bring them wheelbarrows and other equipment, as well as more money to help destroy the beastly cactus.
After the women have distributed the infested cactus pads, one of them uses a panga, a machete-like knife, to remove some green pads from healthy cacti and take those chunks back to the greenhouse. There, they add a couple of insect-infested pads into buckets of new, healthy, pads so that the cochineal can enjoy a steady diet of prickly pear flesh and lay thousands of eggs—the next generation of warriors.
Evidence from test plots on a private conservancy shows that it takes up to two years for infested cactus plants to die, or at least stop reproducing. The insects work most effectively during the dry season, as rainfall can wash them off the cactus and kill them. Thanks to the impacts of climate change in the region, dry periods are occurring more frequently—a boon for the bugs. But increased droughts are also further degrading grasslands, thus spurring the spread of prickly pear. The irony in this situation doesn’t escape Joseph Putunoi, NRT’s rangelands officer, who oversees the biocontrol program in Naibunga. “Rain is a two-edged sword,” he says.
Still, the bugs are making progress. A recent study confirmed that the prickly pear cacti in Laikipia’s bio-controlled areas were producing far fewer fruits in 2016 than they had been before the program was launched in 2014—an average of just 15 per cactus versus the previous 68 per cactus.
Despite these successes, the road to cactus eradication is still long. Given the uphill battle the tiny insect warriors face, and the vastness of degraded rangelands in the region, locals are not relying solely on the bug to solve their woes. NRT is training community members to also manually cut down the invasive cactus plants. To be effective, this requires completely removing the plants, including the roots and chunks of stems and pads, and destroying them by burying them deep underground or burning them.
Naibunga and other conservancies in the northern region have also been changing land-management practices to revitalize precious grasslands—planting native seeds and trees, temporarily blocking off patches of land from livestock-grazing to replenish the soil, and filling in gullies caused by land erosion.
“Unless everyone invests in landscape restoration and rehabilitation programs, including grazing-management techniques, there’s always going to be a specter hanging over our heads,” says David Kimiti, a rangeland ecologist at Lewa Wildlife Conservancy. The rangelands, he says, can only be revived if communities, private ranchers, and the Kenyan government work together to share precious resources.