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Extension Entomology

Category: Lawn and Garden

Rose Sawflies

Posted on by Sharon Schroll

–by Dr. Raymond Cloyd

 

There have been numerous inquiries regarding insects feeding, and completely devouring rose plants. These are sawflies, and there are at least two species that attack roses this time of year: the rose slug (Endelomyia aethiops) and bristly rose slug (Cladius difformis). Rose slugs are the immature or larval stage of sawflies, which are black to yellow-colored wasps.

Rose sawfly females create pockets or slits along the edges of rose leaves with their saw-like ovipositor (egg-laying devise), and insert eggs. Larvae hatch from eggs and resemble small slugs. Larvae are 1.2 cm long when full-grown and yellow-green, with an orange head (Figure 1). Larvae eventually fall onto the soil surface and pupate. Rose slugs overwinter as pupae in earthen cells created by the larvae. Bristly rose slug larvae are pale-green and 1.5 to 2.0 cm in length. The body is covered with numerous bristle-like hairs (Figure 2). There is generally one generation per year in Kansas for both species.

Figure 1. Rose Sawfly Larvae Feeding on Rose Leaf (Author-Raymond Cloyd, Kansas State University)

Rose slug larvae feed on the underside of rose leaves; resulting in the leaves appearing skeletonized (Figures 3 and 4). The larvae eventually they create notches or holes on the leaf margins. Bristly rose slug larvae feed on the underside of rose leaves and also cause leaves to appear skeletonized. However, the larvae may chew larger holes than the rose slug.

 

Figure 2. Bristly Rose Slug Larva Feeding On Leaf Underside of Rose (Author-Raymond Cloyd, Kansas State University)

Figure 3. Damage on Rose Plant Caused By Rose Slug Larvae Feeding (Author-Raymond Cloyd, Kansas State University)

Figure 4. Damage on Rose Leaf Caused By Rose Slug Larvae Feeding (Author-Raymond Cloyd, Kansas State University)

Small infestations of either the rose sawfly or bristly rose slug can be removed by hand and placed into a container of soapy water. A forceful water spray will quickly dislodge sawfly larvae from rose plants, and they will not crawl back onto rose plants. There are a number of contact insecticides containing various active ingredients that may be effective in suppressing populations of both sawflies. However, the bacterium, Bacillus thuringiensis subsp. kurstaki (Dipel® or Thuricide®) will have no activity on sawflies as this compound only works on caterpillars.

 

 

 

 

European Fruit Lecanium Scale

Posted on by Sharon Schroll

–by Dr. Raymond Cloyd

European fruit lecanium scale (Parthenolecanium cornii) is noticeable on the twigs and branches of certain trees and shrubs in landscapes. Damage associated with this scale, which depends on the extent of the infestation, includes: plant stunting and wilting. The European fruit lecanium scale is a soft scale so honeydew (a sticky, clear liquid) will be produced during feeding. Honeydew attracts ants and serves as a substrate for black sooty mold. Moreover, honeydew can drip onto vehicles parked underneath infested trees leaving unsightly residues.

 

Figure 1. Close-Up of European Fruit Lecanium Scale (Author-Dan Potter, University of Kentucky).

 

Figure 2. European Fruit Lecanium Scale on Branch (Author-Dan Potter, University of Kentucky).

European fruit lecanium scales are dark brown and 1/8 to 1/4 inches in diameter when mature (Figures 1 and 2). Some scales may have white or dark markings on the body. European fruit lecanium scale overwinters as an immature on twigs and branches with maturation occurring in spring. Females lay eggs underneath her body from May through June. Eggs hatch into small tan-colored crawlers. The duration of egg hatch can last several days depending on temperature. Crawlers migrate to leaf undersides and feed on plant fluids until late summer. At that point, the crawlers migrate back onto twigs and branches to complete their development the following spring. There is one generation per year in Kansas.

European fruit lecanium scale management involves timely applications of insecticides. Applications need to be made when crawlers are present because the crawlers are the most vulnerable life stage to insecticide sprays. Mature scales possess a shell-like covering, which protects them from exposure to insecticides. Repeat applications will be required because the eggs do not all hatch simultaneously with eggs hatching over a three to four-week period. The best time to apply insecticides is late June through early July when crawlers are feeding on leaves, which enhances their exposure to any spray residues. There are a number of insecticides, with contact activity, that may suppress populations of the European fruit lecanium scale. However, many have broad-spectrum activity and consequently will kill many natural enemies (parasitoids and predators). In general, most out-breaks of scale insects are caused by the indiscriminate use of pesticides (insecticides and miticides). Therefore, always read the label and exercise caution when applying any pesticide. In the winter, dormant oils can be applied to kill overwintering scales by means of suffocation.

 

I need to acknowledge Jeff Otto and Matthew McKernan of Wichita, KS for bringing to my attention that European fruit lecanium scale was active.

 

 

Euonymus Scale

Posted on by Sharon Schroll

–by Dr. Raymond Cloyd

We are receiving inquiries regarding euonymus scale (Unaspis euonymi) crawlers on landscape plants such as evergreen euonymus (Euonymus japonica) and Japanese pachysandra (Pachysandra terminalis). Euonymus scale overwinters as a mated female on plant stems. Eggs develop and mature underneath the scale, and then hatch over a two- to three-week period. The newly hatched crawlers, noticeable migrating along the stem, start feeding near the base of host plants. Crawlers can also infect adjacent plants by being blown around on air currents, resulting in infestations not being detected until populations are extensive and damage is noticeable later on in the season. Leaves eventually become spotted with yellow or white areas. Plants located near foundations, walls or parking areas are more susceptible to euonymus scale than plants growing in open areas that receive sunlight and air movement. Moreover, the variegated forms of euonymus are more susceptible to euonymus scale than the green forms.

 

 

Figure 1. Euonymus Shrub Heavily-Infested with EuonymusScale (Author-Raymond Cloyd, Kansas State University)

Heavy infestations of euonymus scale can ruin the aesthetic appearance of plants (Figure 1) resulting in complete defoliation or even plant death. Females are dark brown, flattened, and resemble an oystershell whereas males are elongated, ridged, and white in color (Figures 2 and 3).

 

Figure 2. Female and Male Euonymus Scale on Leaf (Author-Raymond Cloyd, Kansas State University)

 

 

Figure 3. Female and Male Euonymus Scale on Leaf (Author-Raymond Cloyd, Kansas State University)

Males are typically located on leaves along leaf veins and females reside on the stems. There may be up to three generations per year in Kansas.

Cultural practices such as pruning-out heavily infested branches—without ruining the aesthetic quality of the plant—is effective in quickly reducing euonymus scale populations. Be sure to immediately discard pruned branches away from the area. If feasible, avoid planting Euonymus japonica in landscapes since this species is very susceptible to euonymus scale. Winged euonymus (Euonymus alata) is less susceptible to euonymus scale, even when adjacent plants are infested. Insecticide applications conducted from May through June, when the crawlers are most active, will help alleviate problems with euonymus scale later on in the season. Insecticides recommended for suppression of euonymus scale populations include: acephate (Orthene); pyrethroid-based insecticides [bifenthrin (Talstar®), cyfluthrin (Tempo®), and lambda-cyhalothrin (Scimitar®)]; potassium salts of fatty acids (insecticidal soap); and horticultural (petroleum or mineral-based) and neem (clarified hydrophobic extract of neem oil) oil. Check plants routinely for the presence of crawlers, which will help time insecticide applications. Three to four applications, in general, should be performed at seven to 10-day intervals although this is contingent on the level of an infestation. Euonymus scale is a hard or armored scale, so, in most cases, soil or drench applications of systemic insecticides such as imidacloprid (Merit®) are not effective in suppressing euonymus scale populations. However, the systemic insecticide, dinotefuran (Safari® or Zylam®) may provide suppression of euonymus scale populations when applied as a drench to the soil due to the high water solubility (39,000 ppm) of this systemic insecticide.

Euonymus scale is susceptible to many natural enemies (e.g. parasitoids and predators) including: braconid and ichneumonid wasps, ladybird beetles, green lacewings, and minute pirate bugs. However, natural enemies may not provide enough mortality (‘killing power’) to substantially impact “high” populations of euonymus scale. Furthermore, insecticides such as acephate (Orthene®), and many of the pyrethroid-based insecticides, such as; bifenthrin (Talstar®), cyfluthrin (Tempo®), and lambda-cyhalothrin (Scimitar®) are directly harmful to natural enemies, so applications of these pesticides may disrupt any natural regulation.

 

I need to acknowledge Jeff Otto of Wichita, KS for informing me that euonymus scale was active in South-Central KS.

Carpenter Bees

Posted on by Sharon Schroll

–By Dr. Jeff Whitworth and Dr. Holly Schwarting

Carpenter Bees have been very active all across the state for the preceding 7-10 days and the noticeable activity may continue for another week or two.  These large (3/4 to 7/8 inch) black and yellow bees are most often mistaken for bumble bees.  However, carpenter bees, even though around the same size and flying at about the same speed, have bare abdomens while bumble bees have hairy abdomens.  Carpenter bees thus have shiny, dark blue to black abdomens while most bumble bees have hairy yellow abdomens.

Most folks do not want to get close enough to any large bee to make these distinctions, and they are even more difficult to distinguish when they are flying.  But, if you have large black and yellow bees hovering around any wooden structures and they are ‘dive bombing’ or buzzing around intruders into their area, i.e. you, neighbors, pets, etc. they are most likely carpenter bees.  It is the males that are buzzing intruders and they can be distinguished by their ‘bald faces’ which appear to have a yellow triangle in the middle of a black face.

These males cannot sting; they are just very territorial because they are waiting for a female bee to emerge from one of the holes in the wood so they can mate with her. This dive bombing behavior will continue until all the new females have mated, then the males will die and the noticeable activity will cease.  These females will then excavate new holes or extend established ones located in older, untreated, unpainted wood where they provision cells with nectar and pollen and then deposit eggs.  The larvae feed on these provisions throughout the summer, and then pupate.  The next generation of adults typically does not become active until the following spring.  While carpenter bees are not social bees, populations may build up in favorable locations and over years of repeated excavating, can weaken even structural wood.  For more information on carpenter bee biology and management, please visit: https://www.bookstore.ksre.ksu.edu/pubs/MF2946.pdf

Termites vs Ants

Posted on by Sharon Schroll

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Termite and ant colonies have been very active over the past week or so and are producing reproductives or ‘swarmers’.  We have seen flying/fluttering individuals every place we have stopped throughout north central Kansas, as long as it was between about 10am and 4pm.  Thus, we have received many calls regarding the differences between reproductive ants versus reproductive termites, in both cases often just referred to as ‘swarmers’.  This swarming behavior seems to be initiated about the same time each year for both ants and termites as the same type of warm, wet weather evidently triggers both.  Thus, it is imperative to be able to distinguish the two as they do very different kinds of damage and consequently require different management plans.

Termite reproductives, or swarmers, are dark brown to black, with transparent or translucent wings of equal size, and the dark body is cigar shaped, having no noticeable body divisions or waist.  Termite antennae are straight and lack a club on the end.  Ant reproductives, or swarmers, are also dark brown to black with transparent or translucent wings, but the fore or front wings are a little longer than the bottom or back wings.  Ant antennae are elbowed, coming out perpendicular to the head then bending forward at a 90 degree angle.

For more information on ant identification, biology, and control, please visit:  https://www.bookstore.ksre.ksu.edu/pubs/MF2887.pdf

For more information on termite identification, biology, and control, please visit: http://www.bookstore.ksre.ksu.edu/pubs/mf722.pdf

 

 

European Elm Flea Weevil

Posted on by Sharon Schroll

–by Dr. Raymond Cloyd

   

We are seeing damage on elm (Ulmus spp.) trees caused by the larval stage of the European elm flea weevil (Orchestes alni). Larvae are cream-colored, legless (Figure 1),

Figure 1. European elm flea weevil larva

and found in the mines of leaves. Adults are 3.0 mm in length, red-brown in color with black spots or markings on the abdomen or wing covers (Figure 2).

Figure 2. Adult European elm flea weevil.

 

The mouthpart is shaped-like a snout (Figure 3)

 

Figure 3. European elm flea weevil adult (note the snout-like mouth)

since they are weevils and the hind legs are thickened and enlarged, which allows the adults to jump when disturbed. Adults are initially active in May, and after mating, females lay eggs in the large mid-veins of new leaves. Eggs hatch into larvae that tunnel through the leaf as they feed (which is occurring now), creating serpentine-like mines that enlarge as larvae mature (Figure 4).

Figure 4. Serpentine mines created by European elm flea weevil larvae.

Larvae eventually transition into a pupal stage, and then adults emerge in May and June. Adults primarily feed on leaf undersides creating small holes on young leaves (Figure 5).

Figure 5. Feeding damage caused by European elm flea weevil adult.

 

The feeding damage caused by both the larvae and adults will not kill an elm tree; however, extensive feeding may ruin the aesthetic appearance. Adults overwinter under loose bark and in leaf litter under previously infested trees. There is one generation per year in Kansas. Nearly all elm species are susceptible to feeding by the European elm flea weevil especially Siberian elms (Ulmus pumila) and certain elm hybrids with Asian parentage.

Management of European elm flea weevil involves maintaining proper tree health by means of watering, mulching, pruning, and fertilizing. Insecticides may be used to minimize damage; however, insecticides may be difficult to apply to large trees. Insecticides must be applied in May and June in order to suppress adult populations. A number of insecticides may be used including: acephate (Orthene), imidacloprid (Merit), or carbaryl (Sevin). However, if damage is not extensive, especially on large trees, then there be no rationale for using insecticides. For more information regarding European elm flea weevil management contact your county or state extension specialist.

 

 

 

 

 

Clover Mite

Posted on by Sharon Schroll

–Dr. Raymond Cloyd

We have received inquiries regarding homes being invaded by populations of the clover mite, Bryobia praetiosa. This is the time of year when clover mites can be found entering homes, apartments, and commercial buildings, with large numbers crawling around and causing people to “freak out.” In general, clover mites enter buildings from the sunny-side or southwest exposure. They can aggregate in large numbers in the corners of buildings (Figures 1 and 2). Clover mites are primarily considered a nuisance pest because they do not bite humans. However, clover mites will leave a red stain when purposely or accidently crushed.

Figure 1. Cluster Of Clover Mites In Corner Of Building

 

Populations of clover mites only consist of females since males have never been found (we may need to look harder). Adult clover mites are slightly larger than a pinhead (1/30-inch long), red in color, with extremely long, pink front legs that may be used to distinguish clover mites from other mite pests. Clover mites overwinter as eggs in protected locations and there is usually one generation per year. Adults feed on over 200 plant types including: clover, grasses, ivy, honeysuckle, apple, and elm. Clover mite populations can be extensive in well-fertilized turfgrass near foundations, and their feeding will cause turfgrass to appear silvery or frosty.

Figure 2. Extensive Population Of Clover Mites Entering Building

The management of clover mites involves the following: 1) remove turfgrass near building foundations; 2) place an 18 to 36-inch wide band of an inorganic mulch around the foundation; 3) mow and trim turfgrass as short as possible; 4) avoid over-fertilizing turfgrass, especially with water-soluble nitrogen-based fertilizers; 5) remove weeds growing around the foundation; 6) remove or limit the growth of ivy or other host plants growing around the foundation or walls; 7) use plants near the foundation that are not typically attractive to clover mites, including: marigold, petunia, geranium, arborvitae, and/or yew; and 8) caulk or seal cracks or openings in the foundation or around window seals. Clover mites inside a home or building can be vacuumed up, however, be sure to avoid crushing them. Applications of insecticidal soap (potassium salts of fatty acids) will kill clover mites on contact. You can trap/capture clover mites on saran wrap (Figure 3) or a hardened surface coated with a sticky substance and position by openings such as window seals. If necessary, consult with a pest management professional for recommendations regarding perimeter treatments of pesticides (miticides) to keep clover mites from entering homes or buildings.

Figure 3. Clover Mites Captured On Saran With Sticky Substance

 

 

European Pine Sawfly

Posted on by Sharon Schroll

–Dr. Raymond Cloyd

Yesterday (April 17, 2017) European pine sawfly, Neodiprion sertifer larvae were detected feeding on my “indicator pine” in Manhattan, KS (I was totally excited!). Young caterpillar-looking larvae are 1/4 inch in length and olive-green in color with a black head (Figures 1). Mature larvae are >1.0 inch long with green stripes. The larvae are gregarious or feed in groups on needles of a variety of pines, especially Scotch, red, and mugo pine. When disturbed, each individual larva will arch their head and abdomen (last segment of an insect body) back, forming a “C-shape” (Figure 2), which is a defensive posture to ward-off predators.

Figure 1. Young European Pine Sawfly Larvae

Eventually, larvae will strip the needles of mature foliage, leaving only the central core, which is white and then turns brown (Figure 3). In general, larvae complete feeding by the time needles emerge from the candelabra. Therefore, those really is only a minor threat of branch or tree death resulting from sawfly larval feeding. However, the loss of second- and third-year needles will be noticeable in landscape trees; thus ruining their aesthetic appearance. In late spring, larvae drop to the ground and pupate in brown, leathery cocoons located at the base of trees. Adults, which are wasp-like, emerge in fall and lay eggs in needles prior to the onset of winter. There is one generation per year in Kansas.

Figure 2. European Sawfly Larvae In A Defensive Posture (Arching Head And Abdomen Back)

Sawfly larvae look-like caterpillars; but, they are not caterpillars (Order: Lepidoptera). Sawflies are related to ants, bees, and wasps (Order: Hymenoptera). The primary way to distinguish a sawfly larva from a caterpillar is by the following: 1) sawfly larva have prolegs (fleshy abdominal legs) on every abdominal segment whereas caterpillars are missing prolegs on the abdomen and 2) caterpillar larva have hairs or crochets on their feet whereas sawfly larva do not have hairs or crochets on their feet.

Figure 3. Feeding Damage To Pine Caused By European Pine Sawfly Larvae

Sawfly larvae are not caterpillars, therefore, the bacterial insecticide, Bacillus thuringiensis subsp. kurstaki (sold as Dipel) will not directly kill sawfly larvae. Dealing with sawfly larvae involves hand-picking (you can wear gloves if you wish) or dislodging larvae from plants by means of a forceful water spray. If necessary, there are a number of insecticides that may be applied to suppress European pine sawfly populations including: acephate (Orthene), azadirachtin, carbaryl (Sevin), spinosad (Captain Jack’s DeadBug Brew and Conserve), and any pyrethroid insecticide (e.g., bifenthrin, cyfluthrin, and lambda-cyhalothrin). Be sure to read the insecticide label to make sure that sawflies are listed. For more information regarding European pine sawfly management contact your county or state extension specialist.

 

Ash/Lilac Borer: Don’t Get “Bored-Down” By This Caterpillar Borer

Posted on by Sharon Schroll

–by Dr. Raymond Cloyd

 

        The time of year has come to be thinking about dealing with the ash/lilac borer (Podosesia syringae). First, you need to understand that this is not the same insect pest as the Emerald ash borer (Agrilius planipennis), which was recently discovered (March 31, 2017) in Doniphan County (Kansas now has 7 counties under quarantine). Emerald ash borer is a wood-boring beetle whereas the ash/lilac borer is a wood-boring caterpillar. Ash/lilac borer adults are generally active from mid-April through early-May. Adults are brown, clearwing moths that resemble paper wasps (Figure 1). Peak moth activity commonly occurs from May through June; however, this depends on temperature. Adult females lay tan-colored, oval-shaped eggs in cracks and crevices, or wounds at the base of plant stems. A single female may live approximately one week and lay up to 400 eggs. Below are the major life history parameters and management strategies affiliated with the ash/lilac borer:

Figure 1. Ash Lilac Borer Adults Mating

 

* Larvae cause plant damage by creating tunnels and feeding within the bark (cambium). Moreover, larvae may bore further into the wood and feed within the sapwood and heartwood.

 

* Feeding by the larvae restricts the flow of water and nutrients causing shoot or branch dieback. Ash/lilac borer primarily feed near the base of plant stems creating swollen areas or cracks at the base of plants, and where major branches attach to the trunk.

 

* Evidence of larval feeding includes the presence of light-colored sawdust that accumulates at the base of infected trees or shrubs.

 

* Ash/lilac borer overwinters as a late-instar larva located in feeding tunnels or galleries.

* To determine if trees or shrubs are or have been infested with ash/lilac borers check for the presence of brown papery pupal cases that protrude from the bark (Figure 2). These are the pupal cases where adults emerged from.

 

* In Kansas, there is generally one generation per year.

Figure 2. Pupal Cases of Ash Lilac Borer Protruding From Tree Trunk

 

* The way to avoid problems with ash/lilac borer is to minimize “plant stress” by properly implementing cultural practices, such as; irrigation (watering), fertility, pruning, and mulching. Stressed plants are more susceptible to attack than so called “healthy plants.” A two to three foot wide mulched area around the base of trees and shrubs prevents injury from lawn mowers and weed-trimmers. In addition, avoid pruning plants in late spring through early summer (under usual weather conditions), because this is when adults are typically present and the volatiles emitted from pruning cuts may attract adult females.

* Insecticides containing the active ingredients, permethrin or bifenthrin can be applied to the bark, at least up to six feet from the base, in order to prevent ash/lilac borer larvae from entering plants after eggs hatch. Clear-wing borer larvae crawl on the bark searching for entry points, which exposes them to insecticide residues.

 

* Pheromone traps are commercially available for capturing adult males (Figure 3), which helps to determine when females will be laying eggs. Pheromone traps help in timing insecticide applications. Insecticide spray applications should begin seven to10 days after capturing the first adults. Be sure to also check pheromone traps two to three times per week and record the number of newly captured adult males.

 

Figure 3. Pheromone Trap Used To Capture Ash Lilac Borer Adult Males

 

* For more information regarding ash/lilac borer management contact your county or state extension specialist.

Dormant Oils

Posted on by Sharon Schroll

–by Dr. Raymond Cloyd

Dormant oils are applied during winter in order to deal with insect and mite pests that survive the winter in overwintering life-stages, such as eggs or mature females. Instead of waiting until spring to initiate “control” measures, the application of dormant oils may be helpful in reducing costs associated with pesticide inputs (in this case, insecticides and miticides) later in the season (spring through fall). The advantages of applying dormant oils include: 1) a wide range of activity against most species of mites and scales—even the eggs; 2) minimal potential for resistance developing in insect and/or mite pest populations; 3) less direct and indirect harmful effects to beneficial insects and predatory mites compared to pesticides with long-residual activity; and 4) relatively low toxicity to humans and other mammals. The disadvantages of dormant oils include; potential phytotoxicity during the growing season and minimal residual activity or persistence.

Dormant oils are typically derived from paraffinic crude oil, and are the heaviest of the petroleum-based oil sprays with a low unsulfonated residue (Figure 1).

figure-1-dormant-oil-product

Figure 1: Dormant Oil Product

The unsulfonated residue is an assessment of the phytotoxic compounds remaining after distillation and refining. An unsulfonated residue >92% indicates a highly refined product with less potential for phytotoxicity. Dormant oils generally have a unsulfonated residue value <92%.

Dormant oil applications are primarily directed at killing overwintering pests including certain mites and scales (Figure 2),

figure-2-dormant-oil-product-label-information

Figure 2: Dormant Oil Product Label Information.

before they become active in the spring and are capable of causing plant injury. Applications are made during winter so as to minimize phytotoxicity to ornamental plants. A 2% to 4% application rate is generally recommended in late fall to early spring. Dormant oils have contact activity and either suffocate; by blocking the breathing pores (spiracles), or directly penetrate and disrupt cell membranes of exposed insect and mite pests. However, dormant oils have minimal residual activity once residues dissipate, so thorough coverage of all plant parts is essential.

Since dormant oils are applied to all plant parts, the overwintering stage of the insect or mite pest must be located on the plant. However, not all insect and mite pests overwinter on plants. For example, dormant oil applications are not effective against the twospotted spider mite (Tetranychus urticae) because the mite overwinters as a female in plant debris, mulch, or other non-plant protected places. In contrast, the spruce spider mite (Oligonychus ununguis) overwinters as an egg on plants, primarily evergreens such as arborvitae, hemlock, juniper, and pine; so the spruce spider mite is susceptible to dormant oil sprays.

Dormant oils are effective in killing the overwintering stages of scales, especially first and second instars or nymphs (=crawlers). For example, euonymus scale (Unaspis euonymi) overwinters as second instar nymphs or mature females; both life stages are susceptible to dormant oil applications. However, certain scales that overwinter as eggs such as oystershell scale (Lepidosaphes ulmi) and pine needle scale (Chionaspis pinifoliae) are more tolerant of dormant oil applications because the eggs are generally stacked or piled on top of each other. Subsequently, dormant oils may not penetrate and contact the bottom layer. As a result, additional insecticide applications are typically required after egg hatch.

An issue when using dormant oils is the potential for plant injury or phytotoxicity. Some plants, such as arborvitae, beech, redbud, and certain maples (Japanese, red, sugar, and amur), may be harmed by dormant oil sprays. Furthermore, the needles of Colorado blue spruce (Picea pungens) may be discolored or change from blue to green as a result of a dormant oil application. Phytotoxicity may be a problem when >4% application rates are used and/or when applications are performed in early fall before dormancy or in late spring at bud-break. Problems associated with phytotoxicity are less likely to occur when applications are made in early November through February, which is when most plants are completely dormant. In order to avoid phytotoxicity, always ensure the spray solution is continually agitated. Also, never apply dormant oils when temperatures are ≤32ºF. Dormant oils should be applied to deciduous plants when the ambient air temperature will stay above freezing for at least 24 hours. Evergreens, in general, are more susceptible to damage than deciduous plants, so it is best to apply dormant oils when temperatures remain above 40ºF over a 24-hour period although there is no quantitative evidence suggesting that applications made at <40°F will damage dormant fruit trees. In addition, general recommendations are that dormant oils should never be applied to plants that are stressed since stressed plants are more susceptible to phytotoxicity. For example, lack of moisture, extreme temperatures, and sudden drastic changes in the ambient air temperatures after spraying, prolonged windy conditions, and disease or insect infestations may predispose plants to phytotoxicity. However, there is no direct evidence indicating that dormant oils are harmful to stressed trees.

There is a general misconception that insect and mite pest populations are unable to develop resistance to dormant oils. However, this is not true. For instance, a Christmas tree plantation of Scots pine (Pinus sylvestris) trees was sprayed with dormant oils for more than 10 years in succession to “control” pine needle scale. Eventually, the scale population became more and more difficult to “control.” Why? Well, what was discovered was that the scale covers actually increased in thickness; making it difficult for the dormant oil to penetrate the outer covering and kill the eggs.

Preventative dormant oil applications may avoid dealing with abundant insect and/or mite pest populations during the season. Therefore, inputs from insecticide and/or miticide applications may be reduced, thus preserving the natural enemies of mites and scales, including; predators and parasitoids that may naturally regulate populations of these pests.