Pests and Pollinators

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 (Triplehorn & Johnson 2005) but they are responsible for the loss of 13% of crops and 9% of forest production (Pimental et al. 2000). 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 (Corn et al. 1999). 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 (Peters 1993). 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 (Wilson & Schwarzlaender 2004). Predator, parasite, and parasitoid insects provide top-down management of herbivore populations (Naylor & Ehrlich 1997). During the 1800’s, cottony-cushion scale – a scale insect – was accidently introduced into California citrus groves from Australia (Grafton-Cardwell & Gu 2003). 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.

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.

Disease	Occurence of disease	Vector	Occurence of vector
Chagas disease	Rural areas in Mexico, Central and South America	Kissing bug (Triatoma sp., Rhodnius sp., and Panstrongylu sp.)	United States, Mexico, Central and South America
Dengue	Sub-tropics of Africa and Asia, Western Pacific, equatorial South America, Central America, Hawaii, Texas, Puerto Rico, U.S. Virgin Islands, Guam, Samoa	Mosquito (Aedes aegypti, and A. albopictus)	World-wide except Antarctica
Filariasis	Sub-tropics of Africa and Asia, Western Pacific, Haiti, the Dominican Republic, Guyana, Brazil	Mosquito (Anopheles sp., Culex sp., Aedes sp.)	Africa - Anopheles, Culex - Americas, Pacific and Asia Aedes
Leishmaniasis	Mexico, Central and South America, Middle East, Africa, southern Europe	Sand Fly (Phlebotomus sp.)	Between 50o N and 30o S latitude (absent from NewZealand, Pacific Islands)
Malaria	Sub-tropics of Africa and Asia, Western Pacific, equatorial South America	Mosquito (Anopheles spp.)	World-wide except Antarctica
River Blindness	Sub-Saharan Africa, limited area of South America, Yemen	Black Flies (Simulium sp.)	World-wide except Antarctica (fast flowing streams)
Typhus (louse-borne)	Where body lice are present	Body Lice (Pediculus humanus humanus)	Where humans are present
West Nile Virus (humans are a dead-end host)	Africa, North America, Europe, Middle East, Asia and Oceania	Mosquito (primarily Culex sp.)	World-wide except Antarctica
Table 1: Some human diseases and their insect vectors.

Insects seldom become pests in natural ecosystems, but in managed or simplified ecosystems when an insect population become large enough to cause harm to people, crops, animals, or possessions, insects may be categorized as pests (Elizinga 2004). Pest outbreaks in natural ecosystems usually last 3-4 years, even with no intervention (Rotenberry et al. 1995). Various factors limit the length of a pest outbreak, including intraspecific competition for resources, diseases and parasites (Roland 1993).

Agricultural pests, along with pests found in homes and landscaping, are usually problematic because of human created circumstances (Elizinga 2004). 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 (Triplehorn & Johnson 2005). 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 (Elizinga 2004, Peters 1993, Triplehorn & Johnson 2005). 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.

Most flowering plants (75%) require an animal pollinator (Burger 1982, Gullan & Cranston 2010). There are over 200,000 species of animal pollinators and the vast majority of these are insects (Berenbaum 2007). 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 (Berenbaum 2007). 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 (Pellmyr et al. 1996). 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, (Brower et al. 2011). Milkweed prevalence decreased between 1999 and 2009 in Iowa corn and soybean fields following the widespread adoption of herbicide resistant cultivars (Hartzler 2010).

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 (Snell-Rood & Papaj 2009, Cipollini 2002). The Eastern comma butterfly larva is a pest of hops (Dole 2003). 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 (Fraser et al. 2003). 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.

Bees are one of the largest groups of pollinators (Berenbaum 2007) 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 (Berenbaum 2007). 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 (Gullan & Cranston 2010). A honey bee colony has a single reproductive female and 10,000 – 60,000 female workers (Winston 1987). 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 (Figure 4).



Bumble bee colonies do not overwinter (Baer & Schmid-Hempel 2003). 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.

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 (vanEngelsdorp et al. 2011). The cause of honey bee population decline is unknown, but many researchers suspect habitat degradation, parasites, disease, and pesticides, to be contributing causes (vanEngelsdorp et al. 2010, Singh et al. 2010).

Pollinator decline has not been limited to honey bees (Berenbaum 2007). Declines have been observed in bumble bee species, including a 96% decline in four North American species linked to Nosema bombi, a microsporidian (Cameron et al. 2011). 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 (Berenbaum 2007).

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 decomposer 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.