Integrated Pest Management

IPM Manual for Landscape Pests in British Columbia

Chapter 8: IPM for Insects & Mites in Greenhouses & Nurseries

Learning Objectives

When you have completed this lesson, you should be able to:

Classify greenhouse and nursery facilities into two categories according to maintenance level.
List five methods that prevent insect (and mite) problems in such facilities.
Know where to look for help in identifying pests.
Describe three general monitoring methods used in greenhouses and nurseries.
Describe the use of biological controls and physical / mechanical controls in greenhouses and nurseries.
List general guidelines for using insecticides to control greenhouse and nursery insects.
Describe the role of evaluation in an IPM program for pests in greenhouses and nurseries.
Describe monitoring methods, give examples of treatment thresholds and treatments for aphids, greenhouse whitefly, black vine weevil and spider mites.

Introduction

This chapter describes IPM for pests found in conservatories, propagating greenhouses and nurseries, indoor plantings in offices and public buildings and other interior plantscapes. Common pests in such planting are black vine weevil (Otiorhynchus sulcatus) and various species of whiteflies, thrips, spider mites, fungus gnats (Bradysia spp.), aphids, mealybugs and scales.

As described in Chapter 2, using a system for categorizing sites according to maintenance level can help in planning an IPM program. As interior plantscapes and plant nurseries are generally all high value or high maintenance sites they may be divisible into only two main classes.

Facility Categories

Examples of facilities that might be included in each category include:

Class A - Highest level of service: e.g., conservatories, interior display gardens, hanging baskets, plants in offices and malls.
Class B - Moderate to high level of service: e.g., propagation benches in greenhouses, outdoor nurseries for planting stock.

The following section describes the general steps in an IPM program for indoor plant and nursery insects (and mites). It also gives examples of treatments and management techniques currently available. Outlines of IPM programs for four common pests are included at the end of the chapter. This information is intended as a starting point, to be adapted to suit specific sites and improved as new products and methods are developed.

Prevention

The elements of prevention (also called "cultural controls" in some references) for greenhouse and nursery pests involve using optimum site design and management. When designing and planting conservatory beds and other display areas, plan for long-term insect management by starting with the correct drainage, soil preparation and fertility. Select plants suited to the light levels and other environmental conditions. Follow this with management practices that keep plants healthy, including providing good air circulation and ventilation to lower the temperatures in sunny conditions, measures to raise humidity in dry conditions, as well as pruning, thinning and staking as appropriate.

Additional preventative practices include:

a) Choose pest resistant plants:

Choose plants that are poor hosts for common greenhouse and nursery pests. For example, lantana, hibiscus, geraniums, fuchsia, Poinsettias and Jerusalem cherry are preferred hosts for greenhouse whitefly (Trialeurodes vaporariorum), whereas most waxy leaved foliage plants are not. In conservatories and other interior plantscapes, plants with chronic pest problems are easy to find. A long-term pest management plan would include replacing them with less susceptible species if possible.

b) Avoid importing pests:

The majority of indoor plant pests do not come from outdoor plants. They are spread on infested plants from other greenhouses and nurseries. To avoid bringing new pests into a facility:

Inspect all planting stock before purchase or delivery. Check roots as well as foliage, to ensure plants are not bringing in new or exotic pests.
" Quarantine" new shipments of plants. Some managers set aside a separate greenhouse or holding room, where new stock is kept away from established plants for several weeks, until it is known whether they are carrying pests. If necessary, the new plants can be treated before they are moved into the main greenhouse or nursery.
Segregate stock plants from young plants to prevent spread of pests.
Grow bedding plants from seed.
Use non-soil mixes or pasteurized soil mixes for nursery stock and bedding plants to avoid soil- borne insects.

c) Keep pests out:

A few species, such as western flower thrips and aphids are common on outdoor plants and can fly into greenhouses through vents. To prevent this:

Screen all vent openings and fan intakes. Window screen mesh will keep out moths, aphids and other species. Extremely fine mesh or micro-pore screen is required to keep out thrips. Such fine mesh reduces the air flow through the vent, usually to 20-25% of the flow through an unscreened opening. This means that the surface area of vents must be increased accordingly to allow the right amount of ventilation.

Identification

Correct identification of insect and mite pests is essential because monitoring and treatment methods must be tailored to a particular species. Once a problem species is identified, then information about its life cycle can be used to pinpoint when treatments will have the greatest effect and what preventative measures would be most effective.

If you suspect an insect problem, inspect damaged leaves carefully, under magnification. A 10-15 X hand lens is useful for this, while using a binocular dissecting microscope is even better. This is because many greenhouse and nursery plant pests are extremely small and hard to see. Experienced pest managers can often identify indoor pests based on the characteristic appearance of leaf damage, however, it is always a good idea to search until the damaging insects or mites are found. This is especially important if biological controls are to be used (described below), because some only attack particular species.

To identify insects and mites, use:

reference books, insect guides (listed at the end of this chapter)
resources on the internet
staff at garden centres, community colleges, botanical gardens
professional diagnostic services

Note: Professional services and companies can send samples to the provincial Plant Diagnostic Laboratory, Ministry of Agriculture and Lands, 1767 Angus Campbell Road, Abbotsford, B.C. V3G 2M3. Phone: 604-556-3126 (there is a charge for this service).

Tips on preserving insect samples for identification include:

Put large, adult insects in a freezer to kill them, then allow the specimens to dry in the air. Store them in vials or small boxes. It is easier to handle specimens for future reference if they are mounted on a pin. Insert the pin as soon as the specimen thaws from the freezer, before it dries out.
Preserve tiny insects, mites and soft-bodied larvae in small, tightly sealed vials of rubbing alcohol. Use a fine paintbrush to pick up the specimens. The colours fade in alcohol, therefore make a note of the natural colour of the live specimen to help in identification.
Label all specimens in pencil with location, date, host plant and your name.

Monitoring

Examples of common monitoring methods for greenhouse and nursery pests are:

Visual Inspections

Regularly, usually weekly, inspect a representative sample of plants. Use a 10-15 X hand lens or headband magnifier to get a close look.

Check the undersides of leaves, growing tips and lower leaves, along stems, leaf veins and in axils of leaves.
Look for curled, twisted, rolled or discoloured leaves and buds.
Look for webbing and yellowish "stippling" (speckled appearance) on leaves that would indicate spider mites.
Look for sticky patches of honeydew that would indicate sucking insects on the foliage above.
Check for larvae around crown of plants susceptible to root weevils.
Slide nursery stock out of pots to check the root ball for larvae, especially where weevil damage appears on leaves (small notches in the leaf margins).

Indicator Plants

Where many different kinds of plants are grown, as in conservatories, using indicator plants can reduce the time spent monitoring. These are plants you would check first because they are most attractive to particular pests. As long as these plants stay pest-free, there is likely no need for other plants to be checked. If pests are found on the indicator plants, then other plants over a wider area should be inspected. To use indicator plants with confidence, you must know what pests to expect and which plants in the conservatory are likely to be the most susceptible to them. Examples of indicator plants are:

for spider mites: palms, Easter lily, diefenbachia, citrus
for greenhouse whitefly: poinsettias, hibiscus, lantana, Jerusalem cherry, fuchsia

Sticky Traps

Yellow sticky traps are used to monitor for whiteflies, fungus gnats, thrips and certain species of leaf miners. Blue sticky traps are also used to monitor for western flower thrips. Sticky traps in a greenhouse

Tips for using traps include:

Hang traps as soon as seedlings, plugs or transplants are put on benches. Place traps among nursery stock outdoors as soon as weather warms in early spring.
Hang traps level with, or just below, the tops of plants to catch whiteflies. Place them 20-30 cm above the level of the soil or greenhouse bench to catch fungus gnats.
Use enough traps to give a representative sample from the area. Usual rates are 1 sticky
trap/50-200 m2 or 1 trap/50 plants. Use the higher densities of traps to pinpoint infestations more accurately.
Inspect traps weekly and count the insects caught. Replace traps after 3-4 weeks (indoors) or 2 weeks (outdoors) or more often if they become loaded with insects, dust or debris.

It is important to keep written records of all counts, as well as notes and maps of damage from visual inspections for future reference.

For a summary of monitoring methods to use in greenhouses and field nurseries, see tables included in the Nursery and Landscape Pest Management and Production Guide (see Further Reading). For more information on monitoring indoor pests in plantscapes and conservatories, see other references listed at the end of the chapter.

When and How Often

The monitoring schedule depends on the life cycle of the problem insects or mites and on the time of year. Although most indoor plant pests may be present year around, they usually reproduce more slowly in the winter. A month-by-month timetable for monitoring common greenhouse and nursery pests in BC is included in the Nursery and Landscape Pest Management and Production Guide.

Action Decisions

In conservatories and other public facilities, the injury level often depends on how much damage the public will tolerate, rather than on the direct harm a pest might be causing to a plant or to a site. In propagation and nursery areas, the injury level may depend on whether the pest is likely to spread to other stock and what effect it could have on the growth rate or long-term health of the plant. The injury level can also vary with the age of the plants. For example, more damage may be tolerable early in a propagation cycle, when plants have time to replace damaged leaves, than would be acceptable for plants ready to sell.

Injury levels also depend on the category of site. For example:

Class A Sites: There is usually a very low tolerance for infestations of insects because of high visibility and aesthetic value of plantings.
Class B Sites: Low levels of infestations may be more tolerable, especially in plants where "clean-up" sprays can be used before plants move to sales or planting sites.

The action level depends on what type of control is planned. If biological controls are to be used, releases usually start after pest populations are present, but at very low levels. Examples of action levels used by greenhouse managers are:

Greenhouse Whitefly (Trialeurodes vaporariorum):

1 whitefly/10 traps, or 1 whitefly found on any trap, to begin releases of Encarsia parasitic wasps
5 whiteflies/trap/ week to apply a pesticide

Thrips (Thrips tabaci, Frankliniella occidentalis):

1 thrip on any trap may be used as an action level for low release rates of the predatory mite Amblyseius cucumeris
3 to 4 thrips/trap/ week for higher release rates of predators or applying a pesticide

Aphids (various species):

When one or more aphid colonies is found, begin releases of biological controls.

Action levels for treating aphids can be higher in bedding plants and nursery stock about to be planted out because there are usually enough native aphid predators present outdoors to control aphids.

Treatments

When a pest problem occurs that must be treated, the manager should re-evaluate the management program to determine how to prevent the problem in the future.

Cultural Controls

In addition to methods that produce healthy plants, sanitation methods are important cultural controls in greenhouses and nurseries.

Sanitation — Ways to remove pest breeding sites include:

Control weeds both in, and around, greenhouses and nurseries as they can be alternate hosts for whiteflies, aphids and other pests. Remove weeds under benches, in nursery pots and planting beds.
Maintain at least a 3-m wide, closely mowed strip around the outside perimeter of production greenhouses and nurseries to remove host plants for thrips, aphids, tarnished plant bugs, etc.
Drain all wet spots in greenhouses benches, floors and between nursery rows to remove breeding sites for fungus gnats and shore flies. Correct any problems with irrigation systems in benches and beds that cause some areas to be over watered.

Biological Controls

Many species of natural enemies are now sold commercially to control common pests in greenhouse and conservatories. Relatively high release rates may be needed to provide acceptable control on ornamentals. In general, the biological controls should be released as soon as monitoring shows target pests are present at low levels. Before placing an order, always consult the supplier of biological controls to make sure you order the right species and know how to use them. Suppliers can provide monitoring and release rate information as well as tips on timing releases, providing alternate food or water sources and adjusting temperatures to get the best results from their products. See Chapter 3 for a table of biological controls available commercially.

Most predatory insects are not effective in offices or malls where plants are spaced far apart within a much larger building. This is because the adult insects become lost and often fly to windows (where people using the building may consider them a nuisance). Predatory mites, however, can be effective in such situations because they cannot fly and are more likely to remain on plants. They are also so small that people do not notice them.

Physical and Mechanical Controls

Examples of physical and mechanical controls for indoor and nursery pests include:

Yellow sticky traps used in large numbers (1 trap/plant) to catch adult whiteflies, fungus gnats (Bradysia spp.) and shore flies (Ephydra spp.).
Strong sprays of water used to control aphids and some species of sawflies, such as roseslug and pearslug (Endelomyi spp.).
Removing infested leaves by hand or pruning out infested branches to manage light infestations on a few plants; this is particularly useful for leaf miners.
Hand-held vacuum cleaners used to remove adult whiteflies from the top foliage of plants.

Chemical Controls

Fewer chemicals are registered for use indoors than are available for outdoor use. Nowhere is the use of highly toxic and persistent chemicals less advisable than in interior plantscapes, malls and office areas frequented by the public. Least-toxic, ´preferred´, products are recommended because:

it is usually difficult to keep people out of such sites long enough to ensure sufficient ventilation after an application, and
commercially available biological control agents are effective for many indoor pests, but most cannot be used while pesticide residues are present on plants.

Preferred insecticides are described in more detail in Chapter 3. Those of most use in interior plantscapes include:

insect growth regulator, kinoprene (e.g., Enstar®), for aphids and whiteflies on ornamental
Bacillus thuringiensis kurstaki for various species of caterpillars
insecticidal soaps for chewing and sucking insects and spider mites
pyrethrins for thrips, aphids and flying insects
horticultural (summer) oils for scale, mites, aphids and whiteflies

Regularly scheduled, broadcast spraying is usually not recommended because aphids, thrips and spider mite populations in greenhouses commonly develop resistance to routinely used pesticides. Western flower thrips populations that are resistant to pesticides are now wide-spread in the horticulture industry.

There are times when broadcast sprays may be appropriate as a "clean-up", such as:

When pest populations are too high to successfully start a biological control program, least-toxic, non-residual products can be used to reduce pest populations before releasing biological controls.
Before bedding plants or nursery stock are sold or shipped to retailers they might be treated to remove insects (both pests and beneficial species, if their presence is a concern).

If an insecticide treatment is judged necessary, select the product according to the following guidelines. Make sure the product selected is:

effective and registered for the specific insects or mites and for indoor use, if required
applied as a spot treatment instead of broadcast application wherever possible (see exceptions above)
applied at the right time to have the greatest effect on target species. For example, timing pesticide applications to control scale would be determined by monitoring for presence of the mobile nymph, which is the stage most susceptible to pesticides.
compatible with use of biological controls, if these will be used (lists of compatible pesticides are available from biological control suppliers).

For more precautions and guidelines for using pesticides in landscapes, see Chapter 11.

Evaluation

After any type of treatment, monitor, using visual inspections, sticky trap monitoring and other methods to assess the effects on the pest population. To determine the progress of biological control programs it may be necessary to do follow-up monitoring for a month or more.

Use this information, along with other monitoring records, observations from field staff, feedback from clients, budget records, etc. to assess the effectiveness of the IPM program and find ways to improve it for next year.

IPM Examples

Following are descriptions of four common pests in greenhouses or nurseries, with notes on monitoring, action decisions and treatments that could be incorporated into an IPM program. In any IPM program, correct identification of the pests before taking action is critical, as is the evaluation step to determine how to improve the program in the future.

Aphids

Aphid midge larva attacking an aphid Aphids are small, pear-shaped sucking insects, 2-8 mm long, with long, backward projecting antennae. There are many species of aphids, but the most common one in greenhouses and conservatories is the green peach aphid (Myzus persicae). Other species common in nursery stock are the rose aphid (Macrosiphum rosae), potato aphid (Macrosiphum euphorbiae) and spruce aphid (Elatobium abietinum). Aphids suck plant sap, causing distorted foliage and buds and dropped leaves. They also excrete a sweet honeydew, which drips onto leaves and supports the growth of unsightly sooty moulds. On indoor plants green peach aphids can continue to reproduce all year around.

Monitoring

Inspect plants weekly. Look for distorted, curled foliage and shiny drops of honeydew on leaves as an indicator of sucking insects. Use a 10-15 X hand lens to look for aphid colonies, concentrating on new growth and growing tips as well as on the undersides of oldest leaves.

Action Decisions

Where biological controls will be used, the time to take action for releasing predators is usually as soon as aphid colonies are found during regular monitoring.

Timing of pesticide applications as a ´clean-up´ for aphids on nursery stock may be most economical if done when stock is ready to be shipped to retail outlets.

Treatments

Physical Control

Prune out heavily infested branches. For plants that can withstand it (and that are in a location that permits it), remove aphids by spraying the plants with a strong stream of water. It is only necessary to knock the aphids loose from the plants because this damages their mouthparts and prevents them from feeding again. Two or three sprays are usually sufficient to control aphids.

Biological Control

Natural enemies of aphids sold for aphid control on indoor plants include the aphid midge (Aphidoletes aphidimyza), lady beetles (Hippodamia convergens, Harmonia axyridis) and several species of parasitic wasps (Aphidius spp.). In conservatories, biological controls for aphids can be very effective if they are introduced as soon as the first aphids are found. Using a combination of biological control species is often more effective than relying on one species. It may be necessary to control ants (using ant traps) in conservatories to stop the ants from protecting aphid colonies from predators.

On outdoor nursery plants, the aphid midge, A. aphidimyza, is likely to be the most effective predator for release. Generally, they should be released in the spring, 2 or 3 times at 7-10 day intervals to supplement the naturally occurring populations of aphid predators outdoors.

Chemical Control

Aphids are hard to control with pesticides because spray coverage must be very thorough to reach aphids on undersides of leaves and in curled leaves. If any aphids survive they quickly generate a new colony. Aphid populations also readily become resistant to pesticides that are used often.

Insecticidal soap, pyrethrins, and horticultural oils can be used as spot sprays or to reduce aphid populations before releasing biological controls. The insect growth regulator, kinoprene, selectively controls aphids on greenhouse ornamentals. Heavy infestations in nursery stock may require treatment with diazinon or other registered pesticides as a last resort.

**Greenhouse Whitefly (Trialeurodes vaporariorum)**

Adult whiteflies are tiny (1 mm long), powdery white insects. The nymphs suck plant sap; they look like flattened, pale green or white scales on the undersides of leaves. Their feeding weakens plants and they excrete honeydew, which supports the growth of unsightly sooty moulds. Greenhouse whitefly does not overwinter outdoors in BC, therefore can only be spread between greenhouses on infested plants. Once present in an interior plantscape, it is difficult to eradicate, but biological controls can keep numbers below injury levels.

Monitoring

Hang yellow sticky traps in the plant canopy and count the adult whiteflies caught on traps weekly.
Inspect indicator plants, such as lantana, hibiscus, poinsettia, Jerusalem cherry and fuchsia, which are highly susceptible to whiteflies.

Action Decisions

In mixed plantings, where only some plants are attractive to whitefly, a suggested action level for releasing the whitefly parasitic wasp Encarsia formosa is 1 whitefly/10 yellow sticky traps.
Where plantings or greenhouse benches include a large number of plants susceptible to whitefly, Encarsia release would start when one whitefly is caught on any trap (always contact the biological control suppliers for their latest recommendations on release rates and timing).
In nurseries where there is a history of whitefly problems, it may be most effective to release
Encarsia weekly, at low rates, even before whiteflies are found on monitoring traps. Release rates should be increased after whiteflies are detected.

Treatments

Biological Control

The parasitic wasp, Encarsia formosa, attacks whitefly nymphs. They turn from white to black as the parasite develops inside. Encarsia can eliminate whitefly from greenhouses in the summer, however, they are not as effective during cool weather and low light conditions of winter. Monitor progress by comparing counts of unparasitized whitefly nymphs (white) to parasitized nymphs (black). As a general rule, Encarsia releases should continue until over 80% of the nymphs have turned black. The small black lady beetle, Delphastus catalinae is a whitefly predator that can be released along with Encarsia as soon as whiteflies are present.

Physical and Mechanical Control

Vacuum adult whiteflies from the top leaves of plants using a hand-held vacuum. Use high numbers of yellow sticky traps to physically remove adult whiteflies. Before releasing biological controls, prune out the worst infested foliage to reduce the whitefly population and make it easier for Encarsia to search the plants.

Chemical Control

Chemical control of whiteflies is difficult because of the waxy coating on the nymphs. Before releasing Encarsia, reduce whitefly populations with insecticidal soap. After releasing Encarsia, spot spray new growth (where adult whiteflies congregate) with soap. The insect growth regulator, kinoprene, can be used to control immature whiteflies on greenhouse ornamentals.

Black Vine Weevil (Otiorhynchus sulcatus)

Black vine weevil larva killed by nematodes Black vine weevils are common nursery pests in BC. They are dark grey beetles, 8 mm long, with a noticeable "snout". Adult weevils chew half-circle notches in the edges of leaves of rhododendron, azalea, yew and other ornamentals. The larvae are small, plump, white grubs that feed in the roots and crowns. Their feeding can stunt or kill plants.

Treatments

Physical Control

Adult weevils come out to feed at night and can be knocked off of plants onto ground sheets and destroyed.

Biological Control

Insect parasitic nematodes are effective at controlling the root weevil larvae in plantscapes and nursery stock. Apply when weevil larvae are present in the soil (after July), water in well and keep soil continuously moist. Nematode products vary, so always follow product recommendations for timing and release rates.

Chemical Control

Diazinon soil drenches are registered for control of larvae.

Spider Mites

The most common mite in interior plantscapes is the two-spotted spider mite, Tetranychus urticae. It has a wide host range and becomes very damaging if left unchecked in hot, dry conditions. Spider mites suck plant sap, leaving stippled leaves. Some species spin silken webs. European red mite (Panonychus ulmi) occurs on nursery roses, ornamental apples, cherry and hawthorn. Citrus red mite (Panonychus citri) can occur in plantscapes on citrus, palms, and other subtropical plants. In plantscapes, spider mites are often worst on leaves in the warmest and driest conditions, such as the top of palms nearest the glass.

Monitoring

Inspect leaves under 10-15 x magnification for webbing, stippling and mites.
Tap twigs and branches to knock mites onto a sheet of white paper on a clipboard.

Action Decisions

The predatory mite Phytoseiulus persimilis is most effective when applied at the first sign of a two-spotted mite infestation.

Treatments

Mist plants and walkways frequently to raise humidity. This slows the feeding and reproduction of spider mites, while making conditions more favourable for predatory mites.

Biological Controls

Several species of predatory mites are sold commercially to control spider mites:

Phytoseiulus persimilis is effective on two-spotted spider mites in greenhouses and protected cultures.
Galendromus (=Typhlodromus) occidentalis is a hardy native predator mite that controls European red mite and citrus red mite. It does well under high or low temperatures and is more effective than P. persimilis in tree top foliage.
Amblyseius fallacis is particularly effective on woody ornamental plants. An organophosphorous pesticide resistant strain is available from one BC supplier; it can be used where pesticides have been used to control other pests.
The lady beetle Stethorus punctillum can also be used along with predatory mites for control of spider mites during the growing season.

Chemical Control

Before releasing biological controls, spider mites populations can be reduced with sprays of insecticidal soap or fenbutatin oxide, which is compatible with predatory mites.