Plant life is subject to the harms of an enormous range of diseases, pests, and other ills - some relatively harmless, and others capable of withering entire crops and destroying whole forests, too. Here are just a handful of the pests that make protecting plants such a tough job.
1. Ggypsy moths (Lymantria dispar dispar)
Image courtesy of U.S. Forest Service
Native to Europe and Asia, the gypsy moth was accidentally introduced to U.S. terrain in the late 1860s by E. Leopold Trouvelot. The French astronomer, artist, and amateur entomologist, who lived near Boston, was cultivating moths gathered in France when some specimens escaped. Twenty years later, outbreaks of the pest began cropping up in the region, and it’s been spreading its North American domain ever since.
Larval gypsy moths damage trees by gobbling down their leaves and needles; the process of refoliation depletes the trees' energy stores, leaving them more susceptible to other pests and diseases. As a U.S. Forest Service Report explains, flightless adult female moths build protected egg masses containing up to 1000 eggs on the surface of trees, which can keep eggs cosy through low and even into freezing temperatures until it's time to hatch. Then, the larvae either stick around or "balloon" to a more suitable host tree by dangling in the wind on silk-like thread. For up to 12 weeks, the developing caterpillars will consume a tree's needles or leaves at a devastating rate. Just a couple of generations of gypsy moths can keep a host tree from ever properly re-growing its foliage, often killing it.
Gypsy moths pose a threat to hundreds of species of plants, but oak and aspen trees are the moths' most common hosts - particularly in the Northern lake states and the Appalachian and Ozark mountains. The U.S. Forest Service has also described the gypsy moth as “one of North America's most devastating forest pests,” but researchers still don't know what long-term effects the moths might have on forest vegetation.
2. Fusarium head blight (Fusarium graminearum)
Also called "head scab," fusarium head blight disease (FHB) has been causing North American wheat, barley, and other grass crops to become blighted seemingly overnight for almost a century; the blight caused losses of over US$3 billion to U.S. wheat and barley farmers between 1990 and 2003. The grain-bearing spikelets of plants infected with Fusarium graminearum, the most common and destructive of several Fusarium species that affect crops, will often exhibit “premature bleaching” and shriveling - a pretty clear signal to farmers that FHB has struck and produced the mycotoxin deoxynivalenol (a.k.a. vomitoxin), which, according to a study in Interdisciplinary Toxicology, “affects animal and human health causing acute temporary nausea, vomiting, diarrhoea, abdominal pain, headache, dizziness, and fever.”
FHB has been managed pretty well in many regions using fungicides, prediction algorithms that allow farmers to pre-treat crops that are likely to become affected, and the planting of resistant crop strains; as of the ‘90s, though, the fungus has been gaining traction again in previously controlled areas: Outbreaks have been reported in the Eastern and Midwestern United States and in Eastern and Central Canada, too.
3. Corn smut (Ustilago maydis)
This fungus attacks the tassels, nodal shoots, stalks, leaf mid-ribs, and seeds (or kernels) of corn plants with localized infections - it doesn't take over the whole host plant - and converts plant tissues into tumour-like growths of up to a foot in diameter. Once mature, these thick-walled growths open to release the fungus' powdery spores, which can then infect nearby plants.
In Mexico, the blight (which is known as huitlacoche) is harvested and used as an ingredient in various dishes, but mostly, corn smut causes major headaches for farmers around the world by rendering crops unusable.
4. Rice blast (Magnaporthe spp)
The U.S. Rice Producers Association reports that rice, a staple crop for about half the world's people, is responsible for producing about a quarter of all human energy on the planet, and takes up close to 11 percent of the world's arable lands with its cultivation. Rice blast - which, according to Plantwise, is "currently the most important disease of rice worldwide" - can wipe out rice nurseries and crops, with losses from neck blast alone reaching 70 percent in some fields.
The disease exhibits a host of symptoms throughout rice plants, including lesions, rot, stunting, and plant death. Resistant and semi-resistant rice strains have helped protect against this threat, but variability among the disease's different pathogens continues to make rice blast hard to combat for farmers and researchers alike. According to the California Environmental Protection Agency, the high temperature and humidity levels in Asia and the southeast U.S. can be credited for the frequency of rice blast in those regions.
5. Locusts (Schistocerca gregaria)
Desert Locusts are similar to other short-horned grasshoppers in the superfamily Acridoidea, but have some key differences: These animals, which have been causing periodic plagues for thousands of years, will often change their behavior and form massive swarms of adults and juveniles that can migrate over long distances, leaving destruction in their wake. The UN's Food and Agriculture Organization notes that, seeing as an adult locust can eat about 2 grams - or just about its own body weight - each day, a square-kilometre swarm of 40 million locusts can consume "the same amount of food in one day as about 35,000 people."
6. Sudden oak death (Phytophthora ramorum)
First discovered on U.S. shores 20 years ago in California, sudden oak death is a disease that has, according to University of California scientists David M. Rizzo and Matteo Garbelotto, "reached epidemic proportions...[and] been found in nearly all woody plant species in mixed evergreen and redwood forests from central California to southern Oregon." Researchers are still working to pin down a lot about this fungus-like pathogen (including its origin); however, we do know that infected plants don't always die, and that some instead live on as breeding grounds for a disease that causes oozing above-ground cankers, spreads its spores through splashing and running rainwater, and has wiped out tens of thousands of trees to date.
7. Pine beetles (Dendroctonus ponderosae)
Currently, though, sudden oak death hasn't even approached the level of destruction caused by pine beetles. The bugs, which are found all along the west coast of North America, have already infested 32 million acres of lodgepole pine forest in British Columbia alone - about the size of 24 million football fields, or the whole state of Alabama - releasing "an estimated 270 tons of carbon [and] converting the forest from a carbon sink to a large net carbon source," according to the BBC. This native species has always had its population booms, but researchers credit climate changes as well as our attempts to minimize forest fires, an important factor in healthy forest ecosystems, for the bark-burrowing beetle's catastrophic rise in recent decades.
8. Root-knot nematodes (Meloidogyne spp)
There are around 15,000 species of nematodes, roundworms that are found pretty much everywhere on Earth and account for about 14 percent of all plant loss worldwide, or almost US$100 billion annually, according to the American Phytopathological Society. Root-knot nematodes are especially destructive to crops, causing galls and other abnormal growth in plants' root areas. The hundred or so Meloidogyne species of nematodes cause varying degrees of damage to plants depending on climate, plant species, and local soil conditions, but the galls caused by widespread species like M. incognita, which make roots unable to properly absorb nutrients and moisture, can lead to whole fields of wilted, un-sellable crops.
9. Stem rust fungus (Puccinia graminis)
Forms of rust, including those that cause stem, black, and cereal rusts in grain crops, pose a major threat to wheat production worldwide, have caused severe epidemics in African wheat crops, and have been spreading through the continent and into Asia and the Middle East over the past several years, according to Advances in Agronomy. The Los Angeles Times reported in 2009 that the stem rust fungus known as Ug99 could destroy upwards of 80 percent of the world's wheat crops in the near future as it spreads by air and human carriers from Africa. Oregon State University professor Jim Peterson described the fungus to the Times as a "time bomb" that's already started counting down: "It moves in the air, it can move in clothing on an airplane. We know it's going to be here," he said. "It's a matter of how long it's going to take."
10. Hemlock woolly adelgid (Adelges tsugae)
According to the U.S. Department of Agriculture's Forest Service, the aphid-like hemlock wooly adelgid (HWA) has infested almost half of native-range hemlock trees in the Eastern U.S., has been found in areas reaching from southeastern Maine to northeastern Georgia and over to eastern Tennessee, too, and is spreading into new hemlock populations at a rate of almost eight miles per year. Infested nursery stock have also brought the pest to Ohio, Vermont, and Michigan, where quarantines and eradication efforts have been launched to help keep HWA populations from growing.
HWA does its damage by inserting its "long, piercing-sucking mouth parts" into the base of hemlock trees' needles, the U.S. Department of Agriculture explains, and feeding on nutrients stored in the needles' xylem ray cells, leading to needle discolouration and loss, desiccation, and branch dieback. Recent research also suggests that this process causes a hypersensitive response in the tree, which can create "false growth rings" around infested tissue that restrict the tree's ability to transport water inside itself. Depending on moisture availability in an area and other local stress factors, HWA can be fatal within 4 to 15 years to hemlocks of all ages (sometimes even causing a mortality rate of 95 percent).
As a "foundation species," hemlock trees help to define the structure of a forest and regulate its ecosystem dynamics, and the health of about 2.3 million acres of U.S. forest is tied closely to that of hemlock populations that mostly dominate them. It's understandable, then, that the USDA's Forest Service calls HWA "the single greatest threat to the health and sustainability of hemlock as a forest resource in the eastern United States [with] impacts comparable to those of the gypsy moth, Dutch elm disease, and chestnut blight [and] the potential to remove a major ecological component from eastern forests that is important for maintaining clean water and supporting wildlife."
11. Asian longhorned beetles (Anoplophora glabripennis)
USDA Animal and Plant Health Inspection Service experts suspect that this pest first traveled from its native regions of Japan, China, and Korea (where it has similarly destroyed whole forests) to the U.S., Europe, and other areas in solid wood packing materials. The bugs first showed up in the U.S. on several Brooklyn, New York hardwood trees in 1996, according to Cornell University's New York Invasive Species Information (NYIS) project, before cropping up in Chicago in 1998 and in several New Jersey counties through the early 2000s. Their reach has extended even further in the years since, bringing them to the states of Massachusetts and Ohio, among others. According to the NYIS, "millions of acres of hardwoods could be killed [by the beetles], potentially causing more damage than the combined impact of Dutch elm disease, chestnut blight, and gypsy moths."
Asian longhorned beetles are especially deadly to maple, birch, elm, and other hardwood trees because of the big bugs' consumption-heavy life cycle: As the Journal of Integrated Pest Management explains, adult females (which are up to 1.5 inches long) bore holes through trees' outer bark and into the softer cambium layer underneath. These craters not only protect an individual egg from being crushed, they're also a nutrient-rich place for the beetles to pass their larval and pupal stages. Unfortunately for the trees, these craters are hard to seal back up again.
Options for fighting the infestation are mostly limited to the removal of infected trees and the quarantining of potential ones, and some towns and cities have reported their eradication of the beetles in the past few years. At present, however, Asian longhorned beetles still put up to 61% of urban trees in the U.S. at risk, with a potential economic loss of around US$669 billion, according to the Journal of Integrated Pest Management.
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