Greenhouse Pests Insect & Mite Pests. David J. Shetlar, Ph.D. The “BugDoc”. The Ohio State University, OARDC & OSU Extension Columbus, OH. © November, 2003, D.J. Shetlar, all rights reserved. Greenhouse & Interiorscape Pest Management.
Insect & Mite Pests
David J. Shetlar, Ph.D.
The Ohio State University,
OARDC & OSU Extension
© November, 2003, D.J. Shetlar, all rights reserved
Managing insect and mite pests in greenhouses should be a simple task! If you believe in laws of thermodynamics (i.e., matter can be neither created nor destroyed), the biological corollary should be: living organisms must come from living organisms! In short, we know that pests can not spring forth from “primordial much” which was a common thought during the Dark Ages!
In short, if you start with a clean greenhouse and clean plants, then pests should not be a problem!
Pest Management (cont’d)
Best site on Internet: The Whitefly Knowledgebase
Notes: The greenhouse whitefly has been the traditional whitefly found in greenhouses. In the mid 1980s, another whitefly which was slightly smaller, slimmer and had a more yellowish body became common in Florida and the Carolinas. This new whitefly was a major problem on certain greenhouse vegetables and flowers such as poinsettias, gerbera daises, and hibiscus. This whitefly was eventually identified as the sweetpotato whitefly – Strain B, but this pest was subsequently described in 1994 as the silverleaf whitefly. At present, the greenhouse and silverleaf whiteflies are the most common pests in greenhouses, but the sweetpotato and bandedwinged whiteflies are occasionally encountered.
It is important to identify each species since they have varying susceptibilities to biological and chemical controls.
Nymph IV (=“Pupa”)
Whitefly Life Cycles
At 70ºF, the greenhouse whitefly life cycle takes: 6-10 days for egg hatch, 3-4 days as a nymph I, 4-5 days as nymph II, 4-5 days as nymph III, 6-10 days for the pupa. Adults can live for 30 to 40 days.
Greenhouse whitefly pupa, note raised upper surface and long, glass-like spines.
Greenhouse whitefly adults hold the wings flatter over the body and they are more white in base color.
Greenhouse whiteflies: note wings held wider and more flat on body, and pupae with long spines.
Silverleaf & sweetpotato whiteflies: note wings held closer to sides of bodies, and pupae have no spines.
Other whitefly species can be found, especially in southern greenhouses. Fortunately, most are not significant problems in production settings.
Palm whitefly adult and “pupae.” Many whiteflies have such characteristic waxy fringes.
A giant whitefly and its nymphs plus an old pupal exoskeleton.
Note rings of eggs that indicate that the females were undisturbed for a considerable time!
Encarsia formosa, a commonly used parasitoid. Parasitized pupae turn black. Can you find the two adults?
Scheduling Short Residual, Contact Insecticides for Control of Greenhouse Whitefly
(assuming “pupae” are resistant)
Eggs=3 Nymphs=7 Pupae=7 Adult Preovi=1
Day: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Eggs=11 Nymphs=17 Pupae=12 Adult Preovi=2
Day: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
ALTERNATE CHEMICAL CONTROLS!
Whiteflies are notorious for developing resistance to pesticides. Their high reproductive potential and short generation time make them excellent at developing resistance. While there is considerable debate on resistance management, do not use the same pesticide again if satisfactory control was not achieved with a recent application. It is often recommended that managers alternate chemical groups on a regular basis if whiteflies need continual control. Without cultural and biological controls, chemical controls will eventually fail.
Mealybugs & Scales
Sites on Internet:
Longtailed mealybugs – crawlers, nymphs, and adults.
All stages on chrysanthemum.
Ovisacs and crawlers.
Other mealybug species often occur in lath-house ornamentals in southern states.
Pink hibiscus mealybugs being tended to by fire ants.
Hemispherical scales – all stages! Can you tell which ones are parasitized?
Oleander scales are irregularly round in shape. Males are smaller and more oval in shape.
Cactus scales are sexually dimorphic, females being rounded and males being slender, elongate forms.
Fern scale females are generally brown and oystershell shaped. Males are white with a medial ridge running down the waxy cover.
Boisduval females are irregularly rounded and males are elongate, white and with a medial ridge in the waxy covering. Settled nymphs produce a patch of elongate waxy threads as they form their covers.
Twospotted spider mites have typical spider mite life cycles. They have egg, six-legged larval, eight-legged nymph I & II, and adult stages. Between every molt, the mites settle down and don’t move for a day or more, depending on the temperature.
Typical twospotted spider mite population. The eggs are completely spherical. Most of these are adults. The male in the middle is “guarding” a female nymph II that is in the process of molting into an adult.
Larva (above); egg, nymph I & II in molt rest, cast skins (right); nymph II female and male guarding (below).
When monitoring for spider mites, the tell-tale stippling is easily spotted. The mites will be on the underside!
If left uncontrolled, twospotted spider mites will completely encase the foliage in their fine silk. Note that the mites use the webbing as a walkway. Plants this heavily infested should be thrown away!
If left uncontrolled, spider mites can eventually kill their host plants. When this is about to occur, the mites often cluster at the tops of the plants in order to balloon to other plants. This is one such ballooning cluster.
Remember that an unmated female that is blown to a new plant can start an entire colony. When unmated, she can only produce eggs that will develop into males. However, the female can live long enough to mate with one of her sons and then she can begin to produce eggs that will develop into females!
Twospotted Spider Mite
Twospotted Spider Mite
Greenhouse managers often talk about “red mites” or “red spiders” when they are referring to spider mites. This is unfortunate, since there is no such species such as a “red mite”!
Twospotted spider mites take on a red or orange hue when chilled in fall conditions (above)
Southern red mites and European red mites have conspicuous spines and a reddish color (right).
Phalaenopsis Mite is a relatively common mite pest of orchids. The mites are very flat and reddish-orange in color. Their feeding often causes white pitting of the orchid surface.
Unless the orchid is rare, disposal is the simple solution. Otherwise repeated applications of soaps or oils often work.
Cyclamen mites are small mites that often concentrate their feeding around leaf and flower buds. This causes stunting and distortion. Often heavy infestations cause the new leaves to die, making them look like they have died from Botrytus fungus.
The little white spot at the end of the arrow is a mature cyclamen mite!
The flower, greenhouse, and banded greenhouse thrips have been the most common thrips, traditionally, in greenhouses. These thrips cause damage to various greenhouse crops (floral, foliage and vegetable). Their damage was usually noted to be foliar discoloration (blanching of leaf tissues or streaking of flower petals) and damage to floral parts necessary for seed production.
In the 1980s, the western flower thrips (WFT) increased in importance when it was discovered that this thrips transmitted Tomato Spotted Wilt Virus (TSWV). Subsequent to this discovery, WFT was also found to be a vector of Impatiens Necrotic Spot Virus (INSV). Since these viruses can infect a wide range of greenhouse plants and infected plants can not be treated but have to be destroyed, considerable research has been undertaken to develop more intensive management programs for control of thrips.
The gladiolus thrips and some similar species are common pests of greenhouse grown glads and other floral crops.
Thrips feed by rasping into cells and removing their contents. This causes “blanching” of the feeding area. Thrips also produce small “tar spot” type of excrement.
Adult thrips have bladelike wings with a long hair fringe.
Western flower thrips nymphs feeding on leaf. Note typical “blanched” areas on leaf and tarspots.
Greenhouse thrips adults are black, but the nymphs are light colored. Note the greasy tarspots on this leaf.
(using gladiolus thrips)
Entomologists that work with thrips often call the first two nymphal instars “larvae,” the non-active third instar nymph a “prepupa,” and the fourth instar nymph a “pupa.”
Most thrips have four nymphal instars, but the third and fourth instars do not feed and are usually sluggish (= the third instar) or immobile (= the fourth instar). Because of this inactivity during the process of transforming from a non-winged nymph to the fully winged adults, many thrips experts call these instars the “prepupa” and “pupal” stages, respectively.
Depending on the species of thrips, the “prepupae” and “pupae” usually find protected places on host plants to undergo their molting processes, or they may even drop from the plant and undergo these instars in the soil or potting mixes.
Most thrips insert their eggs into plant tissues. Therefore, when managing thrips, one has to keep in mind that the eggs, “prepupae,” and “pupae” are usually in protected areas that will not be reached by pesticides. This usually require adoption of spray schedules (e.g., spraying every few days for a set period of time) in order to knock out hatching nymphs and newly emerging adults.
First instar nymph (upper left), second instar (above) and adult (left).
In viral disease transmission, only the feeding nymphs (“larvae”) can pick up the viral bodies from infected plants. The resulting infected adults can then transmit the virus to other plants and since they are highly mobile, spread of the virus can be very rapid.
In the past, several types of leafminers (mainly dipterous and lepidopterous) were occasional pests in greenhouse crops. In the late 1970s and early 1980s, dipterous leafminers in the Agromyzidae became significant and wide spread pests of greenhouse floricultural and vegetable crops.
At first, L. trifolii, was identified as the main pest and this was often called the “chrysanthemum” leafminer because this was the crop most affected. Soon, it was learned that a very similar fly, L. sativae, was also involved. Both flies are yellow with black markings and you have to closely inspect the head markings to differentiate between the two.
The vegetable and pea leafminers are significant pests of greenhouse vegetable production.
These species have adult females that produce the typical “pin holes” which are called “stings” in the greenhouse industry. These are places where the female flies have punctured the leaf tissues to expose the juices so that the fly can lap them up.
The larvae usually make winding mines that gradually widen as the larvae mature.
Agromyzid leafminer adults are about the size of a common fruit fly. Most are yellow with black markings, but some can be almost entirely black.
The serpentine, American serpentine, and vegetable leafminers are mainly yellow with black markings like the fly above.
The pea leafminer is mainly black with some yellow markings like the fly to the right.
Extensive pinhole damage to young Schefflera leaf. In greenhouses, these spots are commonly called “stings” or “stipples.”
Fungus gnat adults have long legs and antennae. They often “dance” around on leaves and flowers, rapidly flying and running over the surfaces. Adults cause no damage.
Fungus gnat larvae are nearly transparent, except for the white fat bodies visible through the exoskeleton. They have distinctive black head capsules. The larvae may feed on root hairs and roots of plants.
Shore fly adults and larva feeding on blue-green algae growing in a watering mat.
Shore flies are about the size of fruit flies and they have the same features – short antennae and legs. The larvae feed on blue-green algae and the adults appear to feed on similar materials.
These flies become numerous where potting mixes are kept constantly moist, especially where capillary mats are used (fiber mats that hold water under greenhouse pots).
Neither the adults or larvae cause any plant problems, but the adults produces numerous “fly specks” which are fecal and regurgitation spots. These appear as tiny white to black, round spots on pots and plant foliage.
Probably the best way to manage these flies is to conserve water so that blue-green algal growth is kept to a minimum.