2.3 Common Mammal Pests
2.4 Common Insects
2.5 Common Mites
2.6 Common Plant Diseases
2.7 Plant Distress
2.8 Beneficial and Nonbeneficial Insects
2_Plant-Pests-and-Diseases
Plant Pests and Diseases
Overview
Title Image: Two-spotted spider mite. Credit: Patrick Beauzay, North Dakota State University; licensed CC BY-NC-SA 3.0.
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Introduction
Lesson Objectives
- Identify common weeds, insects, mites, and plant diseases.
- Recognize plant distress and possible causes.
- Expand on the differences between beneficial and non-beneficial insects.
- List different parasitoids, predators, and/or pathogens that can be used to manage insect and mite pest populations in crops.
Key Terms
beneficial insects - insects that manage plant damaging insects and mites
pest - organisms that cause problems for people, especially by damaging crops
weed - a plant that interferes with the management objectives at a particular location; a plant growing where it is not wanted—under certain situations, the plant may not be totally undesirable
Introduction
It is important to recognize plant distress as early as possible to increase the chance of mitigation with some type of management or intervention. There are weeds, insects, mites, and diseases that can harm plants and inhibit growth or cause death. These pests can also be controlled by “fighting fire with fire”. Biological control is the beneficial action of parasites, pathogens, and predators in managing pests and their damage. Biocontrol provided by these living organisms, collectively called “natural enemies,” is especially important for reducing the numbers of pest insects and mites. Use of natural enemies for biological control of rangeland and wildland weeds (e.g., Klamath weed, St. Johnswort) is also effective. Plant pathogens, nematodes, and vertebrates also have many natural enemies, but this biological control is often harder to recognize, less well understood, and/or more difficult to manage. Conservation, augmentation, and classical biological control are tactics for harnessing natural enemies’ benefits.
Common Weeds
Ecologists consider organisms to be ‘weedy’ if they have a suite of characteristics that would make them likely to appear and survive in disturbed habitats: high reproductive potential, extensive dispersal abilities, rapid growth rate, and ability to reproduce even under adverse conditions. However, weed is not used just by ecologists. The definition of this term is both utilitarian and subjective: “A plant that interferes with the management objectives at a particular location. It is a plant growing where it is not wanted. Under certain situations, the plant may not be totally undesirable.” This definition focus on plants but undesirable organisms (pests) come from the entire spectrum of life, including the non-plant organisms considered in this book: cyanobacteria, green algae, red algae, diatoms, dinoflagellates and fungi.
Weeds are ‘pests’, organisms that are undesirable for reasons that are specific to a particular situation and, as noted above, their undesirable nature depends upon situation. The following is a list of “pests,” given specific contexts:
- Honeybees are pests when they nest in houses.
- Earthworms are desirable in agricultural situations but are pests in native habitats where they can alter conditions and disrupt the native community.
- Dandelions are weeds in lawns, where some consider them unsightly, but are useful to pollinators and to foragers who eat their leaves and flowers.
- Sandbur is highly undesirable on beaches because of its sharp fruits that are painful to step on and stick to clothing.
- Dyer’s woad clearly is (was) useful as a source of dye, but if one is managing land for other purposes then it is a weed.
- Weeping willow trees are desirable as ornamentals, but their roots can clog drainage systems.
- The nitrogen fixing bacterium Rhizobium is desirable if it invades plant roots in habitats where nitrogen is relatively scarce. When nitrogen is abundant Rhizobium actually decrease plant growth because they are being fed by the plant.
As mentioned above, given the flexible and utilitarian definition of weed a key feature of weeds is that they must have characteristics that someone might consider undesirable. Here are some examples:
- Dandelions interrupt the smooth continuous texture of a lawn giving what some consider an undesirable look.
- Sandbur produces fruits that are very painful to step on or get caught in a stocking.
- Burdock, beggar’s tickseed, stickseed, and many others produce fruits that stick on clothing.
- Box-elder trees produce an unappealing form that readily sheds branches, and its abundant fruits clog gutters.
- Poison ivy (Figure 8.2.1), wild parsnip, giant hogweed, St. Johnswort are toxic to many humans.
The foremost undesirable feature of agricultural weeds is that their presence in cropland deters the growth of whatever crops are being grown, an interaction that generally would be labelled competition, which a concept that is easy to cite but is often much more difficult to pin down. The basic idea is that if two species utilize the same resources (water, light, nutrients) then the presence of a competitor can reduce the availability of these resources and thereby diminish the growth of the crop. The phenomenon of decreased crop growth when in the presence of weeds is well established, but the exact mechanism is elusive and certainly may vary between different weeds. Note that the best competitor for any particular crop plant is another individual of the same type because it requires the same resources and acquires them in the same manner. Thus, intraspecific competition (between individuals of the same species) may be hard to separate from interspecific competition (crop vs. weed). Keeping track of resources is often challenging and other interactions might account for the pattern. Competition could be due to factors such as allelopathy (the weed produces chemicals that deter the growth of the crop) or perhaps because the weed attracts insect pests/disease organisms that may affect the crop. For these reasons, competition is often defined without the specific requirement of resource depletion. Most agricultural weeds are plants that deter the growth of crop species for undetermined reasons. There are also weeds that can outcompete food sources for livestock. Horsenettle (Figure 8.2.2) are avoided by livestock due to their toxicity. They have deep roots and prickly leaf blades, making them difficult to eradicate by hand. Weeds can overwinter pathogens like henbit (Figure 8.2.2) being a host for soybean cyst nematodes or corn earworms.
Given the loose general understanding of the term weed, it is important to distinguish what can be classified as an agricultural weed. Here there is large overlap with characteristics of ecological weeds:
- Weeds seeds germinate readily with few or no specific germination requirements.
- Weeds grow quickly and are effective competitors, since resource acquisition is strongly tied to plant size.
- Weeds readily reproduce, allowing for their perpetuation on a site through production of a substantial ‘seed bank’ in the soil. Although most weed seeds germinate readily, not all do, and some seeds remain viable for tens of years, meaning that ungerminated weed seeds remain in the soil for prolonged time periods.
The characteristics listed above also would be desirable features of a crop plant: germinate readily, grow quickly, reproduce abundantly. However, a weed also comes with the notion of being undesirable in some context. Many plant benefits are not widely known, have been lost over generations, or yet to be discovered. Indeed, several crops are thought to have been domesticated from weeds (e.g. wheat, sunflower, barley, carrot).
While the above apply to annual weeds that die off after one season, there are also perennial weeds that regrow each year, which reproduce readily from fragmented roots which may be produced by tilling. Why till? because most crops are annuals and they would have a very hard time competing against already established plants. But note that tilling ‘plays into the weed’s strength’: it is a form of disturbance which is generally something that weeds require.
Another feature that is commonly associated with weeds is that they are non-native, meaning they were introduced into an area where they previously had not been present, generally because of human activity (both intentional and unintentional). While some would consider any non-native to be a weed, most would require that the introduced species must be invasive, meaning it spreads from where it was introduced and readily competes for resources with other plants.
The reason many introduced species become invasive is generally because ‘natural controls’ (herbivores, disease) are not simultaneously introduced with the weed or because of environmental differences. For instance, Dyer’s woad, native to Asia and Siberia, was once highly valued in Europe but became a weed in the United States. Most indigo is now made synthetically, but until the 20th century the primary sources of blue dye (indigo) were Dyer’s woad and several other unrelated species. Dyer’s woad was introduced in the eastern U.S. and showed only a modest invasive nature as it spread westward and was restricted to repeatedly disturbed sites like the sides of roads. Arriving in California and Utah in contaminated alfalfa seed, its behavior changed substantially, becoming much more invasive and entering habitats that were much less disturbed, replacing native plants. Part of this behavior is probably due to the fact that woad’s native habitat is much more similar to the arid west than to the more mesic eastern and central part of the country.
Black Locust is also an example of how the term weed is subjective. Black locust might be considered a weed in the northeast United States but not in the areas where it is native. Black locust is a tree native to the central and southeastern United States, but not to the northeast. After it was intentionally introduced to the northeast it became invasive, as it has in several other parts of North America. What had limited its spread in its native environment is unknown.
Another interesting pattern is seen in Phragmites australis—or common reed. Apparently, a variety of this species was native to North America and was present when Europeans arrived. This native variety of the species is not invasive and does not form large monotypic stands. Ecologists noted that the behavior of the plant changed drastically in the 20th century, with the plant becoming much more invasive. What actually happened was that a European variety of the same species had been introduced, and this was the plant was exhibiting the invasive behavior. The two varieties (non-invasive North American and invasive European) are very closely related (same species) and very difficult to distinguish. This example leads scientists to believe that minor genetic changes can be responsible for invasive behavior.
However, it is important to remember that “non-native” is not required as a characteristic of something that is termed a “weed.” Lots of weeds are “natives,” such as ragweed, giant ragweed, sunflower, Jerusalem artichoke, milkweed, New England Aster, and goldenrod. Most of these have not invaded new habitats but have continued to occur in the disturbed habitats found in any particular region.
Common Mammal Pests
Agricultural land management does not occur in a bubble. There are many animals that interact with crops and cause significant damage. Deer, birds, and small rodents commonly feed on plants in the ground and their fruit, flowers, and bark, as well as harvest grains and vegetables in storage. Like smaller animals, mammals are able to adapt to chemical sprays and technologies, and they can continue damaging crops after management has been done.
Common Insects
Insects are commonly viewed strictly as pests. However, without insects, earth would be a very different place. It is important to differentiate between insects that can be managed responsibly and insects that are adding value and welcome to stay. Pests are organisms that cause problems for people, especially by damaging crops. Most insects encountered daily are not pests; rather, they are harmless or beneficial. Beneficial insects are any of a number of species of insects that perform vital ecological functions such as pollination, pest control, decomposition and maintenance of wildlife species.
Types of Beneficial Insects
In general, there are two kinds of pest management beneficial insects: predators and parasitoids. Predators feed directly on other insects by chewing with their mandibles or by piercing and consuming the body liquids. Predators must kill and consume more than one prey to complete their development, and are free-living as immature and as adults. The action of predators is often obscure. Many predator species are small and hidden on the plant. Parasitoids or parasites must have a host insect to complete their development. These types of insects lay their eggs in or on other insects. Once the eggs hatch the larvae become predators and eat the insect. Parasitoids are free-living only as adults.
Attracting Beneficial Insects
The first step in attracting beneficial insects is providing optimum habitat for their basic needs. As with most things, diversity is the key. Providing a diversity of plants with varying times of flowering, plant architecture and flower color increases the amount of insect population. Some beneficial insects utilize diverse habitats for shelter and cover. Other beneficial insects consume the nectar and/or pollen from flowering plants for added energy.
Beneficial insect habitat may be provided throughout the landscape by planting a diverse mix of herbaceous plant species or it may be provided by planting habitat areas within the landscape. As a general rule, 5% to 10% of the field, garden, etc. should be designated and planted to beneficial insect habitat for optimum results.
Insect Pests
Insect infestations reduce yields and lower the quality of harvested garden vegetables. Three to seven generations of many insect pests attack garden vegetables during the growing season. All plant parts may be injured by insects. Some insects bore into roots, seeds or stems. Many suck large quantities of plant sap. Others destroy crops by chewing on the succulent foliage, stems or fruits. Plant diseases are carried by certain insects called vectors.
Safe, effective and economical control measures can minimize the loss from insects. Control can be maintained all season by a combination of cultural practices, mechanical control, biological control and chemical applications. To maintain control, follow these practices, as they apply to the vegetables in your garden:
- Anticipate insect pest problems.
- Remove other vegetation and debris that harbor insects from vegetable garden beds.
- Turn under spent plants when the vegetables have been harvested.
- Inspect plants regularly for insect infestations and spray insecticides when needed.
- Apply insecticidal sprays when the young, most vulnerable stages of insect pests are beginning to hatch or emerge.
- Observe use restrictions and avoid applying insecticide on garden vegetables within the minimum number of days between last application and harvest (pre-harvest interval).
- Select insecticides that take a short time to control insects during the harvest period that have a short pre-harvest interval.
Most insects develop from an egg and, upon hatching, have a form different from that of the adult. The series of form changes as an insect develops from egg to adult is called metamorphosis. The young insect is covered with a more or less firm skin called the exoskeleton. As the insect feeds, it grows inside this skin, but it cannot increase in volume because it is restricted by its exoskeleton. A new elastic exoskeleton then forms under the old rigid exoskeleton. The old exoskeleton splits along the back and the insect crawls out of its old skin and expands to its new size. After exposure to air for a short time, the new exoskeleton becomes hardened and the insect is ready to resume activity and grow some more. The process of shedding the old skin is called molting. Molting occurs several times over varying periods of time until the final stage is reached. With each molt insects change their form to varying degrees, depending on the kind of metamorphosis that insects may have. Most vegetable garden insect pests have either gradual (Figure 8.2.7) or complete metamorphosis (Figure 8.2.8). Examples of gradual or incomplete metamorphosis, in which the very young resemble the adults, include plant bugs, grasshoppers, stink bugs, squash bugs, aphids and leafhoppers. Examples of pests with complete metamorphosis are Mexican bean beetles, cabbage loopers, hornworms, flies, June beetles, cutworms and armyworms.
Gradual metamorphosis (Figure 8.2.7). Generally, these young insects resemble the adults. In proportion to the rest of the body, the legs and head become relatively smaller in each instar. This is because the head and legs do not grow as fast as the rest of the body. In insects that are winged, there is also a gradual development of the wings with each molt. There are no more molts after the fully developed, winged, adult emerges. Not all of these insects develop wings. The young are called nymphs. Nymphs and adults inhabit the same places and eat the same kind of food.
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Complete metamorphosis (Figure 8.2.8). All four stages of development — egg, larva, pupa and adult — are present. All increases in size occur during the larval stage. Some people erroneously think that small flies will grow to be big flies. At the end of the larval stage, the insect transforms into a pupa, which does not feed or move about. It is sometimes called a resting stage, but inside the pupal skin drastic changes are taking place. More alteration of form is going on during the pupal stage than during any other period of the insect’s development. Out of the pupa emerges the fully formed adult, complete with wings. No further molts occur. The larvae and adults of these insects may live in different habitats, eat different food, have different kinds of mouthparts and have many other differences. The larval stage of some orders of insects are called maggots, grubs or caterpillars.
Soil Insects
Many garden insect pests live in the soil during one or more stages of their life cycle. These insects are adapted to feeding in or on the planted seeds, roots or lower stems of plants.
The length of time the individual insect lives in the soil varies from two to three weeks for some flies, to three years for some wireworm species. These insects may either occur as large numbers of newly hatched larvae or as partially grown over-wintered larvae with a ravenous appetite at the time you plant your garden. The plants can be severely damaged or even killed in just a few days following planting.
Anticipate problems with soil insects. Inspect the plant bed soil thoroughly as you cultivate the bed.
Seed Corn Maggot
Description: Small, white maggots without legs or a distinct head, about 1/3 inch long, that feed externally and internally on roots and seeds.
Damage: Death of small plants may result from maggots feeding on roots.
What to do: Avoid planting spring turnips and radishes in soil that is high in partially decomposed organic matter. Do not plant in wet soil.
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Onion Maggot
Description: Small white maggots without legs or distinct head, about 1/3 inch long, that bore through underground stems and bulbs.
Damage: Thinning of stands often results from plant death caused by the maggots tunneling in small bulbs. Even if they are not totally destroyed in the garden, damaged bulbs will rot in storage.
What to do: Avoid planting onions in an area high in partially decomposed organic matter. Cull onions should be removed from the garden after harvest.
Cabbage Maggot
Description: Yellowish white; legless larva; blunt at the rear end and pointed at the front; about 1/4 to 1/3 inch long. The adult fly lays eggs in the soil around the base of the plant, and the eggs hatch into maggots that burrow down to adjacent roots.
Damage: The maggots are destructive in seed beds and in young transplants. They feed on the roots and stems just below the surface; seedlings wilt, turn yellow and die. Infested cabbage rarely produces a head. Maggots are also reported to introduce a fungus causing blackleg and to spread bacterial soft rot.
What to do: Protect seedlings from egg-laying adults by covering with floating row cover or netting to exclude the fly. Don’t plant in cold, damp soil. In the spring, wait until the soil warms up and is sufficiently dry. Add any organic matter to the soil in the fall to reduce soil's attractiveness to egg-laying spring cabbage maggot flies.
Common Mites
Spider mites (Figure 8.2.12) are tiny arthropods that are found on the underside of a leaf. They feed on the plant with piercing-sucking mouth parts. Spider mite infestation will cause yellowing and necrosis of plant tissue. They infect plants from the bottom and move upwards. Spider mites can cause economic damage to soybeans and corn. Hot, dry seasons make crops susceptible to spider mite infestation. Predatory mites that feed primarily on pest spider mites include Amblyseius spp., Neoseiulus spp., and the western predatory mite, Galendromus occidentalis.
Common Plant Diseases
Microorganisms pathogenic to insects occur commonly among protozoa, bacteria, fungi and viruses. The most common natural diseases in garden insect pests are caused by the latter two groups of microorganisms. However, they are usually effective in reducing pest numbers only after pests reach high population levels. Such pest levels are too destructive to vegetables to await the spread of the disease. Because of the increased interest and research in diseases for controlling pest insects, it is expected that more biological control agents will be made available in the future.
Plant Distress
Symptoms of poor plant vigor and health may include slow growth, discoloration of leaves, wilting or drooping foliage, leaf drop, and/or discolored roots. For plants to produce the expected yield, preferred products, or desired environmental outcomes they must be adapted to the site on which they are growing, provided with the appropriate amounts of nutrients, water, and sunshine, and protected from unchecked animal, weed, insect, and disease pests.
Plant diseases are the most significant impediment to the production and quality of food. The identification of plant diseases at an early stage is crucial for global health and wellbeing. The traditional diagnosis process involves visual assessment of an individual plant by a pathologist through on-site visits. However, manual examination for crop diseases is restricted because of less accuracy and the small accessibility of human resources. To tackle such issues, there is a demand to design automated approaches capable of efficiently detecting and categorizing numerous plant diseases. Precise identification and classification of plant diseases is a tedious job due because of the occurrence of low-intensity information in the image background and foreground, the huge color resemblance in the healthy and diseased plant areas, the occurrence of noise in the samples, and changes in the position, chrominance, structure, and size of plant leaves. The PlantVillage Kaggle database (Figure 8.2.13.) is the standard dataset for plant diseases and challenges in terms of intensity variations, color changes, and differences found in the shapes and sizes of leaves.
Beneficial and Nonbeneficial Insects
Beneficial and Nonbeneficial Insects
Beneficial insects are insects that manage plant damaging insects and mites. Nonbeneficial insects do not benefit the plant organism, but rather cause damage or plant death. Proper identification of pests, and distinguishing pests from natural enemies, is essential for effective biological control. Carefully observe the mites and insects on your plants to help discern their activity. For example, some people may mistake syrphid fly larvae for caterpillars. However, syrphid fly larvae are found feeding on aphids and not chewing on the plant itself. If you find mites on your plants, observe them with a good hand lens. Predaceous mites appear more active than plant-feeding species. In comparison with pest mites, predaceous mites are often larger and do not occur in large groups.
Preserve existing natural enemies by choosing cultural, mechanical, or selective chemical controls that do not harm beneficial species. Remember, only about 1% of all insects and mites are harmful. Most pests are attacked by multiple species of natural enemies, and their conservation is the primary way to successfully use biological control. Judicious (e.g., selective, timing) pesticide use, ant control, and habitat manipulation are key conservation strategies. Biological control’s importance often becomes apparent when broad-spectrum, residual pesticides (those that persist for days or weeks) cause secondary pest outbreaks or pest resurgence. An example is the dramatic increase in spider mite populations (flaring) that sometimes results after applying a carbamate (e.g., carbaryl* or Sevin) or organophosphate (malathion) to control caterpillars or other insects.
Eliminate or reduce the use of broad-spectrum, persistent pesticides whenever possible. Carbamates, organophosphates, and pyrethroids kill natural enemies that are present at the time of spraying and for days or weeks afterwards their residues kill predators and parasites that migrate in after spraying. Neonicotinoids (e.g., dinetofuran, imidacloprid) and other systemic insecticides that translocate (move) into blossoms can poison natural enemies and honey bees that feed on nectar and pollen. Even if beneficials survive an application, low levels of pesticide residues can interfere with natural enemies’ reproduction and their ability to locate and kill pests.
When pesticides are used, apply them in a selective manner. Treat only heavily infested areas with “spot” applications instead of entire plants. Choose insecticides that are more specific in the types of invertebrates they kill, such as Bacillus thuringiensis (Bt) that kills only caterpillars that consume treated foliage.
For most other types of exposed-feeding insects, rely on contact insecticides with little or no persistence, including azadirachtin, insecticidal soap, narrow-range oil (horticultural oil), neem oil, and pyrethrins, which are often combined with the synergist piperonyl butoxide.
In situations where you wish to foster biological control, use of nonpersistent pesticides can provide better long-term control of the pest because they do less harm to natural enemies that migrate in after the application. To obtain adequate control, thoroughly wet the infested plant parts with spray beginning in spring when pests become abundant. To provide sustained control, repeated application may be needed.
For certain harder-to-control pests where contact-only insecticides are inadequate, other choices include spinosad, a fermentation product of a naturally occurring bacterium. This insecticide persists about 1 week and it has translaminar activity (is absorbed short distances into plant tissue). Spinosad can be toxic to certain natural enemies (e.g., predatory mites, Trichogramma wasps, and syrphid fly larvae) and bees when sprayed and for about 1−4 days afterwards; do not apply spinosad to plants that are flowering.
Dig Deeper
An extended list of plant diseases can be found at the University of Tennessee Institute of Agriculture's manual with disease description and control recommendations.
https://weedid.wisc.edu/ca/weedid.php
In Tennessee, pasture weeds are commonly grouped into cold-season and warm-season groups.
"Warm Season Weeds" by Jason P. de Koff, Tennessee State University. Copyright © Tennessee State University. Used with permission.
"Cool Season Weeds" by Jason P. de Koff, Tennessee State University. Copyright © Tennessee State University. Used with permission.
https://extension.tennessee.edu/publications/documents/pb1580.pdf
https://soillab.tennessee.edu/
http://ipm.ucanr.edu/PMG/PESTNOTES/pn74140.html#TABLE4
Tables 1, 2, and 3 give examples and links to further information about parasites that can be used to manage insect pests.
Table 2. Some Important Parasites of Insects. |
Table 3. Some Important Insect and Mite Predators. |
Attributions
A novel deep learning method for detection and classification of plant diseases by Waleed Albattah, et. al. is licensed CC BY 4.0.
"Biological Control and Natural Enemies of Invertebrates" by the University of California Statewide Integrated Pest Management Program. Copyright © UC Statewide IPM Program. Used with permission.
Common Beneficial Insects and their Habitat by the United States Department of Agriculture is in the Public Domain
Inanimate Life by George M. Briggs is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.
Plant Productivity and Health by the United States Department of Agriculture, Natural Resources Conservation Service is in the Public Domain.
Spider Mites in Corn by North Dakota State University is licensed CC BY-NC-SA 3.0.
"You Can Control Garden Insects 2002" by Karen Vail, Frank A. Hale, and Jennifer Chandler, University of Tennessee Extension. Copyright © University of Tennessee. Used with permission.