All About our Pollinators
What is a Pollinator?
A pollinator is anything that helps carry pollen from the male part of the flower (stamen) to the female part of the same or another flower (stigma). The movement of pollen must occur for the the plant to become fertilized and produce fruits, seeds,and young plants. Some plants are self-pollinating, while others may be fertilized by pollen carried by wind or water. Still other flowers are pollinated by insects and animals, such as bees, wasps, moths, butterflies, birds, flies and small mammals, including bats.
Insects and other animals such as bats, beetles and flies visit flowers in search of food, shelter, nest-building materials, and sometimes even mates. Some pollinators, including many bee species, intentionally collect pollen. Others, such as many butterflies, birds and bats move pollen accidentally. Pollen sticks on their bodies while they are drinking or feeding on nectar in the flower blooms and is transported unknowingly from flower to flower resulting in pollination.
Note: The first year we started beekeeping in our backyard, my strawberry plants yielded three times the amount of fruit that I had in previous years.
What is Pollination?
Pollination is the fertilization of a flowering plant. Pollination occurs when pollen is transferred from the anthers of a flower to the ovules of that or another flower. Honey bees are responsible for pollinating a variety of fruits, vegetables, legumes, and nuts such as almonds.
Why Should Pollination Matter to Us?
Worldwide, roughly 1,000 plants grown for food, beverages, fibers, spices, and medicines need to be pollinated by animals in order to produce the goods on which we depend.
Foods and beverages produced with the help of pollinators include: apples, blueberries, chocolate, coffee, melons, peaches, potatoes, pumpkins, vanilla, almonds, and tequila.
In the United States, pollination by honey bees, native bees, and other insects produces $40 billion worth of products annually.
Pollinators Endangered and in Trouble
For the last ten years or so, I have been reading articles that the pollinators are in trouble - that populations are declining. Many of our pollinators are federally “listed species,” meaning that there is evidence of their disappearance in natural areas.
The U.S. National Park Service indicates the decline is related to:
The loss of habitat attributed to :
Agriculture, mining and human development.
Alternate land uses may not provide overwintering, foraging, and nesting sites for pollinators that have specific habitat needs.
Concrete, cement, and metal surfaces replace vegetated areas limit habitat for ground-nesting pollinators and provide scarce opportunities for pollinator foraging.
Fragmented habitat may be too small to meet pollinator survival needs.
The increasing availability of non-native plant species that attract pollinators away from native species that are more nutritious and better food sources.
The use of pesticides (herbicides, insecticides, fungicides)
Pesticides remain in the environment and impact our pollinators
Insecticides applied to plant seed that, as the plant matures, contaminate pollen grains that are food sources
Insecticides that don’t directly kill pollinators, but hamper the ability to forage
Herbicides that kill native plant species that are important forage plants for pollinators.
Flowering plants may occur farther north or at higher elevations as a response to warming temperatures and may become out of sync with their pollinators.
The types and distributions of pollinators may change; pollinators adapted to warmer temperatures may expand their northward range, displacing other pollinators.
Parasites and diseases
Varroa mites, Hive Beetles and Colony Collapse Disorder impacting honey bees
There is more extensive transfer of parasites and diseases due to rapid travel and commerce.
Non-native parasites and diseases infecting native species.
All About Honey Bees
So much of our agricultural productivity is dependent on the European honey bee (Apris mellifera) that it is no wonder that our attention is drawn to their plight. The U.S. has lost over 50% of its managed honeybee colonies over the past 10 years. When the honey bee suffers, so does agriculture, and so, potentially do all who depend on the bounty that comes from animal pollinated angiosperms, the flowering plants from which we derive many of our most delicious and health-giving fruits and vegetables.
While honey bees are clearly not the only hard working pollinators that deliver a bounty to humans and other animals, their recent deaths from Colony Collapse Disorder (CCD) starting in 2006 have captured the world’s attention. To date, CCD has been defined as a series of symptoms, but the cause and the cure have remained complex and elusive. CCD is not the only problem facing honey bees; in fact, in 2010 the overwintering losses were at the same unsustainable rates of over 30% but the cause seemed to be less from CCD than from other problems. Below is a list of the variety of issues facing honey bees
The Varroa mite (Varroa destructor): An external parasite that has spread from its original host, the Asian honey bee Apis cerana, to nearly all Western honey bees (Apis mellifera) worldwide. Virtually all European honey bees are highly vulnerable to Varroa mites, although some honey bee strains (VSH, Russians) show partial resistance to the mites. This mite weakens honey bees by sucking hemolymph (“blood”) from its host and by transmitting bee pathogens. A female mite reproduces by invading the cell of a bee larva just before capping. Once inside, the female lays eggs to produce offspring that feed together on the developing bee. The mother mite and her adult daughters emerge from the cell with the young adult bee host. Eventually, at high infestation rates, the mites overwhelm and kill the host colony. Beekeepers control Varroa mite populations by monitoring mite infestation rates and applying chemical treatments when mite populations become too large. Due to increased concerns over the effects of miticides on bees and mite resistance to commercial miticides, researchers are developing alternative approaches (“softer” chemical treatments, the genetics of mite resistance in honey bees, mite pheromones and hormones, and physical treatments) to control this mite.
Nosema ceranae: This microscopic fungus can weaken or even kill colonies when the majority of workers become infected. Spores of the fungus survive on wax combs and stored food inside colonies. When workers eat these spores the fungus invades the lining of the intestine. Highly infected bees cannot digest efficiently and die earlier. Beekeepers use antibiotics and disinfection of hives to control this disease.
Viruses: Thus far, more than 20 honey bee viruses have been identified. These viruses can impact bees in multiple ways, including killing developing larvae and pupae, decreasing the lifespan of adult bees, causing spasms and tremors, reducing cognitive skills, and impairing wing development so that bees cannot fly. Most honey bee colonies have multiple viruses, and the levels of these viruses can fluctuate throughout the year. Exposure to other stressors, particularly Varroa parasitization, can immunosuppress bees so that the effects of the viruses are more dramatic. The only treatment for viruses thus far is to feed the colony a solution of virus-specific RNA that enhances the bees' immune responses to these particular viruses, but these treatments only suppress the viral infections, and do not eradicate them. Other approaches that are being investigated include breeding bees with genetic resistance to the viruses.
Bacterial Diseases: American foulbrood (Paenibacillus larvae) is an infection that kills young bees (brood) inside the wax cells in which they develop. This dead brood becomes a source of infection spread by workers nursing young brood. Some bees can detect and remove the diseased brood and this stops the disease from spreading. Beekeepers also use antibiotics to prevent the disease.
Pesticides: Pesticides are usually man made chemicals designed to kill pest organisms, that may injure plants or animals including humans. Pests cause economic damage by reducing crop yields directly or by producing crop, or ornamental plant diseases, or by competing with crops, or by reducing animal and human health, or by damaging buildings and structures. Pesticides are categorized according to their intended use as well as by their chemical composition. Pesticides are widely used and are divided into insecticides/acaricides, used to control insects and mites or ticks, fungicides used to control plant diseases; rodenticides, used to control rodents; and herbicides used to prevent weeds from competing with crops, grasses or ornamental plants. Pesticides usually contain an active ingredient, with a known mechanism for killing the target pests. Pesticides vary widely in their safety to humans and the environment and are sold as a formulation with added ingredients that augment the action of the active material when mixed in water for application. More than 1200 chemicals are registered for use in the United States and are used in some 18,000 separate products sold under a variety of trade names. People who apply the more toxic pesticides must have training and a state issued license to use these materials.
Some insecticides have warnings or bee hazards on their label because they are toxic to honey bees, causing honey bee deaths. If the insecticide has a sub-lethal affect on honey bees it may result in reduced larval survival, altered foraging behavior or shortened lifespan of adult bees. The extent of the sub-lethal affects is still unknown.
Honey bees live in colonies that are often maintained, fed, and transported by beekeepers. Centuries of selective breeding by humans have created honey bees that produce far more honey than the colony needs. Beekeepers harvest the honey. Beekeepers provide a place for the colony to live and to store honey in. The modern beehive is made up of a series of square or rectangular boxes without tops or bottoms placed one on top of another. Inside the boxes, frames are hung in parallel, in which bees build up the wax honeycomb in which they both raise brood and store honey. Modern hives enable beekeepers to transport bees, moving from field to field as the crop needs pollinating and allowing the beekeeper to charge for the pollination services they provide.
A colony generally contains one breeding female, or “queen”; a few thousand males, or “drones”; and a large population of sterile female “worker” bees. The population of a healthy hive in mid-summer can average between 40,000 and 80,000 bees. The workers cooperate to find food and use a pattern of “dancing” to communicate with each other.
The Queen Bee
The queen is the largest bee in the colony. Queens are developed from larvae selected by worker bees to become sexually mature. The queen develops more fully than sexually immature workers because she is given royal jelly, a secretion from glands on the heads of young workers, for an extended time. She develops in a specially-constructed queen cell, which is larger than the cells of normal brood comb, and is oriented vertically instead of horizontally.
She will emerge from her cell to mate in flight with approximately 13-18 drone (male) bees. During this mating, she receives several million sperm cells, which last her entire life span (from two to five years). In each hive or colony, there is only one adult, mated queen, who is the mother of the worker bees of the hive, although there are exceptions on occasion.
Although the name might imply it, a queen has no control over the hive. Her sole function is to serve as the reproducer; she is an “egg laying machine.” A good queen of quality stock, well reared with good nutrition and well mated, can lay up to 3,000 eggs per day during the spring build-up and live for two or more years. She lays her own weight in eggs every couple of hours and is continuously surrounded by young worker attendants, who meet her every need, such as feeding and cleaning.
The male bees, called “drones”, are characterized by eyes that are twice the size of those of worker bees and queens, and a body size greater than that of worker bees, though usually smaller than the queen bee. Their abdomen is stouter than the abdomen of workers or queen. Although heavy bodied, drones have to be able to fly fast enough to catch up with the queen in flight. Drones are stingless.
Their main function in the hive is to be ready to fertilize a receptive queen. Mating occurs in flight, which accounts for the need of the drones for better vision, which is provided by their big eyes.
In areas with severe winters, all drones are then driven out of the hive. The life expectancy of a drone is about 90 days.
The Worker Bees
A worker bee is a non-reproducing female which performs certain tasks in support of a bee hive. Worker bees undergo a well defined progression of capabilities. In the summer 98% of the bees in a hive are worker bees. In the winter, besides the queen, all bees are worker bees. Workers feed the queen and larvae, guard the hive entrance and help to keep the hive cool by fanning their wings. Worker bees also collect nectar to make honey. In addition, honey bees produce wax comb.
The Honey Bees’ Second Shift
In addition to gathering nectar to produce honey, honey bees perform a vital second function - pollination, making them a critical component of today's agricultural market. In fact, about one-third of the human diet is derived from insect-pollinated plants, and honey bees are responsible for 80 percent of this pollination.
Without the honey bees’ pollination work, the quantity and quality of many crops would be reduced and some would not yield at all. According to a 2000 Cornell University study, the increased yield and quality of agricultural crops as a result of honey bee pollination is valued at more than $14.6 billion per year. And although other insects can pollinate plants, honey bees are premier pollinators because they are available throughout the growing season and pollinate a wide range of crops.
Honey Bee Nutrition:
Honey bee colonies are healthier and stronger with access to pollen from diverse sources of flowering plants. However, floral diversity in landscapes has been reduced by intensive agriculture (single crops, few flowering weeds, limited hedgerows) and urbanization. In recent years, the pollination of early crops (such as almonds in California in February) has further increased the demand for strong colonies at times of year with few floral sources. Furthermore, changes in climate patterns may also affect seasonal availability of flowering plants. This requires beekeepers to use artificial sources (sugar syrup, corn syrup, and pollen substitutes) to try to meet the increased nutritional demands of their colonies.
Honey Bee Pesticide Issues
Provided by: Purdue University/ Tom Campbell
Purdue researchers found that honeybees collect pollen from a wide range of plants, even in areas dominated by corn and soybeans.
A Purdue University study shows that honeybees collect the vast majority of their pollen from plants other than crops, even in areas dominated by corn and soybeans, and that pollen is consistently contaminated with a host of agricultural and urban pesticides throughout the growing season.
Christian Krupke, professor of entomology, and then-postdoctoral researcher Elizabeth Long collected pollen from Indiana honeybee hives at three sites over 16 weeks to learn which pollen sources honeybees use throughout the season and whether they are contaminated with pesticides.
The pollen samples represented up to 30 plant families and contained residues from pesticides spanning nine chemical classes, including neonicotinoids – common corn and soybean seed treatments that are toxic to bees. The highest concentrations of pesticides in bee pollen, however, were pyrethroids, which are typically used to control mosquitoes and other nuisance pests.
“Although crop pollen was only a minor part of what they collected, bees in our study were exposed to a far wider range of chemicals than we expected,” said Krupke. “The sheer numbers of pesticides we found in pollen samples were astonishing. Agricultural chemicals are only part of the problem. Homeowners and urban landscapes are big contributors, even when hives are directly adjacent to crop fields.”
Long, now an assistant professor of entomology at The Ohio State University, said she was also “surprised and concerned” by the diversity of pesticides found in pollen.
“If you care about bees as a homeowner, only use insecticides when you really need to because bees will come into contact with them,” she said.
The study suggests that overall levels of pesticide exposure for honeybees in the Corn Belt could be considerably higher than previously thought, Krupke said. This is partly because research efforts and media attention have emphasized neonicotinoids’ harmful effects on pollinators and their ability to travel and persist in the environment. Few studies, however, have examined how non-crop plants could expose bees to other classes of pesticides. Looking at Midwestern honeybees’ environment through this wider lens and over an entire season could provide more accurate insights into what bees encounter as they forage, Krupke said.
Krupke and Long collected pollen weekly from May to September from hives placed in a nonagricultural meadow, the border of a cornfield planted with neonicotinoid-treated seeds and the border of a cornfield planted with non-treated seeds. They waited to begin their collection until after growers had planted their crops to avoid the heavily contaminated dust that arises during the planting of neonicotinoid-coated seeds.
The samples showed that honeybees collect the overwhelming majority of their pollen from uncultivated plants, particularly the plant family that includes clover and alfalfa.
The researchers found 29 pesticides in pollen from the meadow site, 29 pesticides in pollen from the treated cornfield and 31 pesticides in pollen from the untreated cornfield.
“These findings really illustrate how honeybees are chronically exposed to numerous pesticides throughout the season, making pesticides an important long-term stress factor for bees,” Long said.
The most common chemical products found in pollen from each site were fungicides
All About the Native Bees
Note: Our Make a Difference page provides details on the nesting and nectar needs of our native bees.
The European honey bee receives most of the credit for crop pollination, but the number of managed
honey bee hives is half of what it was in the 1950s; and this number continues to decline primarily
because of honey bee pests and diseases. Native bees, however, contribute significantly to crop pollination and, on farms with sufficient natural habitat located nearby, may even provide all of the required pollination for some crops. In order to support the native bee community, it is essential to provide nesting sites in addition to floral resources. Unfortunately, intensively managed farm landscapes often lack the untilled ground, tree snags, plants, and small cavities that native bees require for nest construction.
Agroforestry practices can provide essential nesting habitat for bees, our most important crop pollinators. Most native bees nest underground in areas that are sunny, well-drained, and either bare or partly vegetated. Alternatively, they nest in narrow tunnels in wood, or small cavities such as abandoned rodent nests.
Native bees have very different nesting requirements from the more familiar European honey bee
(introduced from Europe in the early 1600s). Unlike the large comb-filled hives of a honey bee
colony, they are generally solitary species, with each female constructing and provisioning the nest by
herself. Only when adults emerge from their hidden nests do we see them flying about pollinating
crops and other plants. The rest of the year they are tucked away inside the cells of their underground
or plant-tunnel nests. Most solitary bees are active as adults for only a few weeks each year and most
have only a single annual generation. An exception are some social sweat bees that can have several
overlapping generations through the summer. These sweat bees are the most abundant native bees in
some studies of crop pollination and build large populations over the summer growing season.
Bees visit flowers to collect nectar (carbohydrates to power their flight) and pollen (providing protein, oils and minerals needed by bee larvae). Bees find their favorite flowers by color and scent; a colorful and aromatic native bee garden is pleasing to humans as well. Bees are everywhere; every urban yard has bees whether you know it or not, no matter how beat down or poorly tended.
Most bees don't sting, and few species defend their nest (bumblebees are an exception). Bees generally only use their stingers in defense. No need to fear being stung if you move slowly and non-aggressively. Only females are capable of stinging (males of all bee species lack this capacity).
Solitary wood nesting bees
About 30 percent of our 4,000 native bee species are solitary wood-nesters that build their nests inside hollow tunnels. These tunnels may occur in the soft pithy centers of some twigs (e.g. box elder, elderberry, or various cane berries); they may be left behind by wood-boring beetle larvae or, in the case of carpenter bees, may be excavated by the bees themselves. Another small but important set of bee species – at least one of which has been documented as an important pollinator of watermelon – tunnel into soft, above-ground rotting logs and stumps.
Solitary ground nesting bees
Most (about 70 percent) of our native bee species excavate their nests underground. These ground
nesting native bees all burrow narrow tunnels down to small chambers (the brood cells) six to 36-plus
inches under the surface. Inside these brood cells next year’s bees develop. In order to build these
nests, bees need direct access to the soil surface, often on sloped or well-drained sites.
The remaining bees – only about 45 species in the US – are social bumble bees. Bumble bees are frequently our most effective crop pollinators. They construct nests in small cavities, often in old rodent
burrows, either underground or beneath fallen plant matter, or occasionally above ground in abandoned bird nests. Queen bumble bees start new nests each spring and by mid-summer their colonies can have dozens or hundreds of workers, all visiting nearby flowers. For this reason, doing what you can to encourage bumble bee nest sites in agroforestry practices can go a long way towards supporting crops that flower during summer months.
All About Monarchs
• Monarch caterpillars need milkweed plants (in the genus Asclepias) to grow and develop into butterflies. They eat and grow rapidly, increasing their weight almost 3,000 times in 10-15 days!
• Milkweed leaves contain toxins that monarch caterpillars accumulate in their bodies. By the time they are adult butterflies, this accumulation of toxins makes them taste quite unpleasant to many predators. Predators learn not to eat them, and this helps monarch butterflies to survive.
• The monarch migration is unique. Successful migrants can navigate more than 1,500 miles to
a site unknown to them in the fall, live through an overwintering period of 5 months and then return north to reproduce in the spring for a total life span of 8-9 months.
• To accomplish their great fall migration, monarchs conserve energy during flight. Much like birds, they gain altitude by soaring in “thermals”, or updrafts of warm air. Once at the top of each thermal they glide toward their destination. In this way, monarchs make their migration at an average pace of 25-30 miles per day, quite impressive for an insect the weight of a paperclip!
• Most monarchs joining the migration each fall are three or four generations removed from those that made the journey the previous year – yet somehow, they find the same groves of trees visited by their ancestors! How monarchs navigate to these forest groves remains an unsolved scientific mystery.
Monarch Population Decline
Visit our Make a Difference page to learn what you can do in your own backyard to assist our beloved Monarch butterfly.
The monarch butterfly (Danaus plexippus) is perhaps the most recognizable and beloved insect of North America. Across its 2,000 mile migration (one-way), the monarch connects ecosystems and captivates people. Unfortunately, this national treasure has endured a decline in population of more than 80% from the 21 year average, between 1994-95 and 2014-15, across North America. This dramatic deterioration has spurred numerous conservation groups, universities, government agencies, and other organizations and individuals to take action. Restoration of deteriorated monarch habitat, as well as the conservation of existing habitat, throughout its range is vital to preserving this hallmark
species. Land managers are in a unique position to restore and protect monarch habitat. By incorporating monarch conservation into larger restoration plans, land managers can fuel the migration and create vital breeding habitat. Habitat of all shapes and sizes is valuable to monarchs, and connectivity is key. Monarchs rely on habitat corridors and “islands” to move along their migration pathways. Land managers have the ability to affect immediate change at a landscape level for monarchs.
Each fall millions of monarch butterflies migrate to overwintering sites in Mexico and to a scattering of locations along the coast of California. In the spring monarchs return to breeding areas and the cycle starts again: a two-way migration that is one of the most spectacular on the planet. Yet, this migration appears to be declining. Researchers are working to determine the causes of this decline; some theories include:
Milkweeds and nectar sources are declining due to development and the widespread use of herbicides in croplands, pastures and roadsides.
Loss of milkweed needed for monarch caterpillars to grow and develop, due to habitat conversion and adverse land management. Because 90% of all milkweed/monarch habitats occur within the agricultural landscape, farm practices have the potential to strongly influence monarch populations.
Development (subdivisions, factories, shopping centers, etc.) in the U.S. is consuming habitats for monarchs and other wildlife at a rate of 6,000 acres per day - that's 2.2 million acres each year, the area of Delaware and Rhode Island combined!
Widespread adoption of herbicide-resistant corn and soybeans (genetically modified crops) has resulted in the loss of more than 80 million acres of monarch habitat
Drought conditions in California and other areas in the western U.S., resulting in lower milkweed biomass, and reduced availability of milkweed late in the summer
Insecticide and herbicide use to control insects and weeds, with unintended consequences for monarchs
Overwintering habitat loss and degradation in California, due to development within and adjacent to overwintering groves, and decay of overwintering trees as they age
Habitat loss in overwintering sites in Mexico, due to illegal logging
Monarch Life Cycle
In order to understand the habitat needs of the monarch, it is essential to first become familiar with its life cycle. The monarch has four distinct life stages: egg, larva (caterpillar), pupa (chrysalis), and adult. First, a monarch egg is laid on a milkweed leaf. This egg hatches into a caterpillar within 3 to 6 days. The caterpillar feeds and grows over a 2-week period. Once fully grown, it chooses a safe location to form its chrysalis. After about 10 days, an adult emerges from the chrysalis and begins to feed on nectar once its wings dry. This life cycle repeats throughout the monarch migration, resulting in multiple generations across an enormous spatial range. While the journey south is comprised
of a single generation of non-reproductive monarchs, the journey north is comprised of 3-4 generations of breeding monarchs. Because some monarchs will stay in a location to breed, it
is important to have nectar sources available beyond the northern migration date range for any given
region. Monarchs typically live 2-5 weeks during the summer breeding season. The final generation
generation of the year does not reproduce and enters a state known as “reproductive diapause.” These butterflies migrate to Mexico where they overwinter, becoming reproductive again in February and March as they move north towards summer breeding grounds. Some monarchs have been known to live as long as 9 months.
The North American monarch migration is one of the largest known insect migrations on Earth. Weighing less than a gram, monarchs can travel 50-100 miles per day. The longest distance recorded traveled by a monarch in a single day is 265 miles. The monarch population of North America is divided into two main subpopulations - one located east of the Rocky Mountains and the
other to the west – although there is probably some interchange between these populations across the Rocky Mountains and in Mexico. Butterflies from the eastern population overwinter in Mexico, while those in the west overwinter at numerous sites along the California coast. There is also a nonmigratory population of monarchs that breeds year-round in southern Florida. This guide focuses on conservation and management of the eastern population, but many of the same principles can be applied for the western population, as well.
The eastern migration starts in March as butterflies from Mexico travel north into Texas and other southern states, breeding as they move northward. The butterflies produced in these areas move northward in May and June to colonize the northern U.S. states and southern Canada. Two or three additional generations are produced before the southward migration begins two months later. Beginning in mid-August and continuing into fall, hundreds of millions of monarchs fly 2,000-3,000 miles south to spend the winter in high-elevation oyamel fir forests at an elevation of 2400-3600 meters in the Sierra Madre Mountains of central Mexico. Monarchs utilize the same 11-12 oyamel fir forest sites each year, making the preservation of these forests a top priority. The journey south can take up to two months to complete. Exactly how monarchs navigate to and from the same overwintering sites each year is still a mystery. Simple orientation mechanisms and major geographic features, such as the Appalachian and Sierra Madre mountain ranges, play a role in funneling monarchs towards their ultimate destination. Visit Journey North (www.learner.org/jnorth/monarch/) to track the fall migration and monitor the arrival of monarchs in the spring. Monarch Watch (www.monarchwatch.org) has a tagging program that helps us understand the routes they take in their migration.
In their western range, monarchs winter in forested areas along the California coast, from Baja to Mendocino County. In the spring, western monarchs move inland, breeding in scattered habitats containing milkweeds throughout much of the west but primarily in California. Abundance of adult monarchs is driven by annual precipitation that supports late-season milkweeds suitable for caterpillars, and by suitable temperature regimes that allow for completion of the monarch life cycle. In November, western monarchs begin to return to their overwintering sites along the coast.
Milkweed: The Host Plant
Monarchs almost exclusively lay eggs on plants of the genus Asclepias, commonly referred to as milkweed, in the family Apocynaceae. Of the roughly 73 milkweed species native to the United
States, monarchs utilize about 30 as host plants. Milkweed gets its name from its milky sap, which contains toxic cardenolide alkaloids that protect the plant from herbivory. After hatching, monarch larvae feed on the host milkweed plant and begin to sequester these cardenolides in their tissue, making them toxic to predators. The monarch’s distinct, bright coloration functions as a warning to predators that it is toxic and should be avoided. This defense mechanism forms the basis of monarch survival, and the monarch is incredibly dependent on the availability of milkweed for reproduction.
Loss of milkweed across the monarch range is one of the greatest threats facing the North American monarch population. Urbanization, large-scale agricultural development, and the widespread use of
herbicide and herbicide-resistant crops are largely considered the primary causes of milkweed decline. Milkweed losses in the Midwest are coincident with the increased use of glyphosate herbicide for transgenic glyphosate-resistant corn and soybean crops, otherwise known as “Round-Up
Ready” plants. Many scientists argue that this is the most serious threat to milkweed, and consequently monarch, populations. Restoring milkweed is a top priority for monarch conservation, but good intentions must be met with scientific understanding in order to be successful. Before planting, land managers must determine which milkweed species are native to their area and select locally adapted plants for their project. Even for the most competent land managers, this is not always an easy task. Of the 70-76 milkweed species native to the U.S., only 30 are known to host monarch larvae. Seeds are commercially available by the pound for only 7 species, by the ounce for another 5 species, and in small packets (suitable for home gardens) for 3 more species. Roughly 18 milkweed species are currently available as plugs through Monarch Watch’s Milkweed Market. Consult the plant tables at the end of this guide for a regional list of “workhorse” milkweed species, which are both beneficial to monarchs and relatively easy to obtain.
Tropical milkweed (Asclepias curassavica) is an introduced species that is capable of persisting through the fall and can interfere with the behavior of migrating monarchs in the United States. A nonmigratory population of monarchs in southern Florida breeds throughout the year on this nonnative species. Since it is often widely available, uninformed gardeners or land managers may be tempted to plant tropical milkweed in order to promote monarch populations on their land. However, the persistence of this species can potentially encourage monarchs to linger and forego their migration, which may result in the eventual death of the monarchs due to cold temperatures or an outbreak of the specialist protozoan, Ophryocystis elektroscirrha. Parasite spores, such as those of O. elektroscirrha, on monarch adults are deposited onto eggs and milkweed and then ingested by the larvae. The parasite can reduce larval survival, butterfly size, life span, mating success, and ability to fly. The prevalence of infection by O. elektroscirrha increases with monarch density at local scales and is negatively correlated to ability to migrate. Infection prevalence is highest in sedentary monarch populations, such as those in southern Florida, with about 70 percent of individuals being heavily infected. While the benefits of planting tropical milkweed are still widely debated, land managers should use native milkweed species for restoration projects.
Tropical milkweed is not the only nonnative species related to native Asclepias that poses a threat to
migrating monarchs. Within the plant family Apocynaceae, several species of the invasive European
species, Vincetoxicum (often taxonomically placed in Cynanchum), attract female monarch butterflies
to oviposit, or lay eggs, on their stems and leaves. This leads to imminent starvation of emerging
monarch caterpillars, which are unable to survive on the foliage of this nonnative plant.
Note: Monarchs do favor the Tropical Milkweed for laying their eggs. However, you need to be responsible when using Tropical Milkweed. The plant should be cut back or removed by September 1st so Monarchs join the migration. If not, they will linger too long and perish because they are unable to fly in the cold temperatures.
Nectar plants provide the fuel necessary for monarchs to complete their massive journey. Just as with
humans, monarchs require a nutritious and balanced diet to sustain their energy and health requirements.