Insects As Pollinator

Humans determine if an insect is beneficial, benign or pestiferous. Even when an insect is classified as a pest, it can have, under different circumstances, a beneficial role. A fly in a house is a pest, but that same fly is beneficial in its role as a detritivore. Termites are pests when invading homes, but are beneficial when fulfilling their ecological role in a forest.

Of all the insects in the world, only 1% of insects are pests but they are responsible for the loss of 13% of crops and 9% of forest production. The Balsam woolly adelgid, Adelges picease, has destroyed nearly all of the Fraser firs (Abies fraseri) in the southern Appalachian Mountains and is thought to be responsible for the loss of two native bird species (Alsop & Laughlin 1991). Formosan termites, Coptotermes formosanus, cause over $1 billion in structural damage per year. Frequently, pest insects are invasive species, such as the Balsam woolly adelgid and the Formosan termite. Within their native ranges – Europe and southern China, respectively – these insects are less likely to be pestiferous.

Most insects are beneficial to humans either directly or indirectly. Directly beneficial insects include pollinators and insect predators and parasites of pests. Other insects provide humans with material goods such as honey (honey bees), silk (silk moths), dyes and shellac (scale insects), and tannic acid and inks (insect galls). Without insects, weed control would be more difficult.

In Australia, an introduced species of cactus, the prickly pear (Opuntia spp.), was controlled by Cactoblastis cactorum, a moth. Predator, parasite, and parasitoid insects provide top-down management of herbivore population. During the 1800’s, cottony-cushion scale – a scale insect – was accidently introduced into California citrus groves from Australia. The citrus industry was saved when the Vedalia lady beetle, a natural predator of the cottony-cushion scale, was introduced.

Indirect benefits from insects may be more numerous and important than direct benefits. Insects indirectly benefiting humans include all insect herbivores, prey, predators, and detritivores because they are an integral part of the biotic community of ecosystems (Triplehorn & Johnson 2005). A well-functioning ecosystem provides services such as soil fertility, clean air, and clean water. Termites, cockroaches, and other soil dwelling insects, help to break down plant debris. Flies, beetles, and moths, help to decompose dead animals. Dung beetles are critical for the decomposition of animal feces. Insects are food for bats, birds, amphibians, reptiles, fish and many mammals. Aquatic insects such as mayflies and stoneflies are used to monitor the health of streams and lakes.

Insect-Pests

An important group of insect pests is those that transmit human diseases (Table 1). Malaria, responsible for 700,000-1,000,000 deaths annually, is transmitted by Anopheles sp. mosquitoes (Centers for Disease Control). The female Anopheles ingests the disease agent, Plasmodium spp. – a parasitic protist – from an infected human. After an incubation period in the mosquito of 7-30 days, transmission to an uninfected human is possible.

Insects seldom become pests in natural ecosystems, but in managed or simplified ecosystems when an insect population becomes large enough to cause harm to people, crops, animals, or possessions, insects may be categorized as pests.  Pest outbreaks in natural ecosystems usually last 3-4 years, even with no intervention. Various factors limit the length of a pest outbreak, including intraspecific competition for resources, diseases and parasites. Agricultural pests, along with pests found in homes and landscaping, are usually problematic because of human created circumstances. In managed, or simplified, ecosystems, such as cropland, orchards, or landscaped areas, the food supply for a pest may be increased while the habitat/niche of predators may be removed or reduced. The European corn borer (Lepidoptera: Ostrinia nubilalis) is a significant agricultural pest in the United States. As a result of the acreage density of corn in the Midwest, the moth larvae have an abundance of food while the prevalence of the adult parasitoid wasp, Macrocentris grandi, dependent on nectar plants for food, may be reduced.

To address pest problems, the conditions favoring pest population growth must be considered. Integrated Pest Management (IPM) consists of tactics to prevent and methods to monitor and control pests. Some common prevention tactics used in homes include window and door screens to exclude flies and mosquitoes and washing clothes and linens, vacuuming, and bathing to prevent body parasites. Using plant varieties most suitable to environmental conditions, providing habitat for beneficial insects, keeping bushes and woodpiles away from buildings, and emptying containers with standing water are all methods to reduce pest populations in landscaping. If preventative measures fail, the least toxic pesticides are chosen first to reduce the risk of losing insects that help keep pest populations under control. Figure 6 shows the thinking process recommended when using IPM. Insects are key components of healthy ecosystems and people benefit from the goods and services provided by ecosystems. If habitat for beneficial insects is provided, fewer pest outbreaks are likely to occur. Managed ecosystems need to mimic non-simplified ecosystems by containing a mosaic of plants for beneficial insects.

Pollinators

Most flowering plants (75%) require an animal pollinator. There are over 200,000 species of animal pollinators and the vast majority of these are insects. Insect pollinators include beetles, flies, ants, moths, butterflies, bumble bees, honey bees, solitary bees, and wasps. Butterflies and moths (Lepidopterans) are important pollinators of flowering plants in wild ecosystems and managed systems such as parks and yards. Butterflies and moths have different niches; butterflies are active during the day while moths are active in the evening and at night. Because the adult and juvenile forms of butterflies and moths do not eat the same food, it is necessary for an ecosystem – whether wild or managed – to contain both nectar and host plants. By ensuring the presence of host plants in an area, the adult moth or butterfly will be able to lay her eggs on the appropriate plants for the eggs to hatch and the larvae to feed. Without these host plants adult moths and butterflies may not be present even if the nectar plants are available.

Some pollination relationships are quite specific. A specialist relationship exists between the yucca moth and the yucca plant. Moth larvae only eat seeds of the yucca plant, and the plant depends on adult moths carrying pollen to facilitate sexual reproduction. These specialist relationships can be negatively impacted if there is a decline of either species. A major determinant of Monarch butterfly population size is the availability of its host and nectar plant, milkweed, Asclepias syriaca. Milkweed prevalence decreased between 1999 and 2009 in Iowa corn and soybean fields following the widespread adoption of herbicide resistant cultivars. Generalist butterflies and moths are more common than specialists. Adult Lepidopterans pollinate night-blooming jasmine, evening primrose, four o’clocks, jimpson weed, orchids, blazing star, Joe-Pye weed, petunia and zinnia. Swallowtail caterpillars feed on dill, parsley, sycamore, or willow.

While adult butterflies and moths are important pollinators, their larvae may be pests. The larvae of Cabbage White butterflies – an introduced speciesin the United States – are significant pests in Brassicaceae family plants. The Eastern comma butterfly larva is a pest of hops. The tobacco hornworm (larva of the sphinx moth) and the tomato hornworm (larva of the five-spot hawkmoth) can be significant pests of tobacco, tomato, potato, eggplant and pepper plants. Clothes moth larvae can do damage by feeding on wool, felt, silk, fur, and feathers. Grain moths may be seen flying in kitchens and feed on flour or cereal grains.

Honey bee, Apis mellifera, is an introduced, naturalized North American generalist pollinator. Honey bees are eusocial, forming a high organized society where each colony has overlapping generations, one queen who mates with multiple males a week after she emerges as an adult, and female workers who cooperate in the rearing of her full-, ¾- and half-sisters.

Bees are one of the largest groups of pollinators and can be social or solitary animals. Honey bees (Figure 1) and bumble bees (Figure 2), common eusocial pollinators, are generalists that visit many plant species to obtain nectar and pollen. Honey bees, the most important crop pollinator, pollinate over 100 different fruits and vegetables, while bumble bees, which vibrate as they pollinate, are more efficient pollinators for plants such as tomatoes. Native pollinators assist in the pollination of native crops such as blueberries, squash, pumpkin, cucumbers, and cranberries but more research needs to be done to understand how to improve pollination rates and support healthy populations of native pollinators.

Honey bees, first brought to North American by European settlers as a source of sweetener, have naturalized. A honey bee colony has a single reproductive female and 10,000 – 60,000 female workers. The workers pollinate as they forage. Today, most honey bees are found in managed colonies housed within apiaries (Figure 3). A honey bee colony lives through cold winters by clustering in a tight ball with individual bees vibrating wing muscles to generate heat. The colony will consume 18-27 kg of honey during the winter to supply the energy needed to generate temperatures up to 35^oC. In warm climates, honey bees may build their comb outside to facilitate hive cooling in warm weather (Figure4).

Managed honey bees are housed in apiaries and each colony occupies a wooden hive consisting of hive boxes and frames (a). The frames in the top box of the hive are filled with honey and harvested by beekeepers. To prevent bears from eating the brood and honey and skunks and raccoons from eating adult bees, this apiary is enclosed with an electrified fence.

The frames can be removed to inspect for pests and disease and evaluate the health of the colonies (b). Nurse bees are caring for the capped brood.

Honey bees will usually build comb inside a cavity but in warm climates they may build comb outside to facilitate cooling. This hive photographed in January in Kauai, Hawaii is at least 8 years old. During the summer months, when pollen and nectar are more readily available, the number of bees occupying the hive is significantly greater and the comb will not be visible due to the number of bees. Feral colonies are common in Kauai, unlike the mainland United States, because the varroa mite, the most significant pest of honey bees, has not been introduced to Kauai. 

Bumble bee colonies do not overwinter. Each spring, mated queens emerge from hibernation to establish colonies. Each queen begins by locating a nest site and building wax pots (for nectar and pollen) and wax cells (for eggs). She will rear the first generation of adults who will take over the foraging and nest building tasks. The queen will continue to lay eggs and the colony will grow until late summer when reproductive males and females are reared and mate. All but the mated queens die before winter.

There are approximately 17,000 solitary bee species (Berenbaum 2007). Many are active as adults for only a short time each year and pollinate a narrow range of plants (Bosch & Kemp 2004). For example, the mason bees (~ 130 species in North America) pollinate blueberries, blackberries, and cherries (Figure 5). 

After a female mason bee mates she finds a tube-like structure and builds a mud wall at the end. Her first step is to make numerous trips (~ 25) to collect nectar and pollen, which she places at the end of the tube. Next she backs into the tube and lays an egg on top of the nectar and pollen. Her final step is to build a mud wall to partition the tube. She will continue these three steps until female eggs fill the rear of the tube and male eggs fill the front. During each of her 25 foraging trips per egg, a mason bee female will visit up to 75 flowers.

Mason bees are active in the spring and are excellent pollinators of a variety of crops including apples and blueberries. This Mason bee, Osmia cornifrons is the primary pollinator of apples in Japan and was introduced into the United States in 1977 by Suzanne Batra (USDA) for orchard pollination. 

To pollinate the almond crop, ~ 1 million honey bee colonies are needed in California every February. The Maine blueberries require ~ 50,000 colonies and New York apples need ~ 30,000 colonies. With the significant decline of honey bee colonies, there is concern about honey bee survival and our dependence on honey bees for crop pollination . The cause of honey bee population decline is unknown, but many researchers suspect habitat degradation, parasites, disease, and pesticides, to be contributing causes. 

Pollinator decline has not been limited to honey bees. Declines have been observed in bumble bee species, including a 96% decline in four North American species linked to Nosema bombi, a microsporidian. Our knowledge of most native bumble bee and solitary bees is so limited that it is difficult to say conclusively if the suspected declines in populations or loss of species is occurring only at the regional level or if the declines are global.

An insect’s relationship with humans is beneficial, benign or pestiferous only because we have defined it as such. Therefore some insects can have more than one relationship with humans. Honey bees pollinate our crops but may be considered a pest because they can sting. Ants are unwanted guests if found in a house but are important de-composer organisms for the maintenance of soil fertility. Food, lumber, clean air and water and all the other goods and services derived from ecosystems would not exist without insects. Living in balance with insects and the other component of ecosystems will aid human survival and prosperity.