Thirps
Thrips are tiny winged insects between 1- 4 mm in length that are amongst the most troublesome pests within a greenhouse. Species commonly encountered include Thrips tabaci (onion thrips), Heliothrips haemorrhoidalis (greenhouse thrips), Hercinothrips femoralis (banded greenhouse thrips) and Frankliniella occidentalis (western flower thrips). The latter only appeared in North America within the last 15 years and has become a serious pest because of its resistance to most pesticides.
Western flower thrips (WFT), Frankliniella occidentalis, are tiny insects about 1mm long that feed on flower and leaf tissues by biting into cells and sucking out the contents. When the cells die, conspicuous silver scarring is seen often with small black dots that are excreta. Young thrips larvae frequently feed within developing flower buds and stem tips resulting in severe deformation of flowers and foliage. WFT carry many viruses including tomato spotted wilt virus (a broad spectrum virus attacking many plants). Plants that become infected with viruses must be destroyed. The mature thrips has two pairs of fringed wings and varies in color from yellow, orange tan, reddish brown to black.
Thrips are capable of both sexual and asexual reproduction, a feat that adds to the difficulty in controlling them. During asexual reproduction an adult female undergoes facultative parthenogenesis producing only male offspring (Brodsgaard, 1989; Immaraju et al, 1992). During sexual reproduction mainly female offspring are produced. Adult females make a small hole within plant tissue with their saw-like ovipositor and deposit their eggs. The eggs develop within the tissue forming larvae. The larvae emerge from the leaf tissue, feed for 8-10 days and undergo three more changes in development before pupating. Depending upon the species the pupa either remains on the leaf or falls to the ground. Within 7 to 10 days an adult appears and flies to the upper portion of the plant to feed and lay eggs repeating the cycle.
Aphids
Green peach aphids (Myzus persicae) are small (3mm or less) soft-bodied insects that feed on plants by piercing plant tissue and sucking out the sap. Tissue may become deformed after infestation due to hormonal imbalance by the aphids and their secretion of a sugary substance (honeydew) that becomes black with fungal growth. Winged aphids may transmit viruses.
In greenhouses, asexual reproduction is commonly encountered. Adult females become parthenogenic and give birth to live young that are genetic clones. These in turn give birth to more live young in as little as 7-10 days. In certain instances each individual can produce between 40 to 100 offspring. Within a few days under optimal conditions, an aphid colony can become established. When the aphid colony becomes too populated, winged females develop that migrate to other plants and areas. Sexual reproduction sometimes occurs during the autumn. Male and female forms appear and mate. Eggs are produced that over winter and develop into nymphs in the spring.
Spider Mites
Two spotted spider mites (Tetranychus urticae) are common greenhouse pests that often appear during hot dry conditions. Adults are minute, oval shaped, lightly colored mites with two dark spots on their backs. Spider mites bite into the leaf tissue and suck out the cellular contents resulting in a needle like speckled appearance. At high densities, plants are covered in webs and if left untreated will die.
Reproduction can be either sexual or parthenogenic with only males produced. Adult females lay eggs on the undersides of leaves in a webbing. Nymphs emerge and resemble adults in form. Five stages of development occur from egg to adult and all stages feed on plants. The length of the life cycle is greatly dependent on temperature. At 21°C it is 14 days. At 30°C adult females can lay 100 eggs within 1 week and live for 30 days, producing theoretically 13,000,000 offspring.
Fungus Gnats & Shore Flies
Fungus gnats (Bradysia impatiens) are small (2 - 5 mm long) black flies found on wet soil. They are distinguished from shore flies by their long segmented antennae, long legs and a prominent Y-shaped vein on their wings. Their larvae are thin white threadlike maggots with black heads and are found in the soil. They feed on decaying organic matter, roots and the bases of young plants and cuttings. Damage can be considerable for young plants and diseases from fungal, bacterial or viral pathogens can be spread.
Females can lay up to 200 eggs that hatch into larvae within 4 - 6 days. Maggots remain active for approximately two weeks and then pupate in the soil 4-6 days at which time the adult emerges.
Shore flies (Scatella stagnalis) are strong flyers in contrast to fungus gnats. They are larger, have short antennae, red eyes and dark wings with five clear spots. Their larvae are up to 6 mm in length and are yellowish brown in color with no identifiable head. The life cycle is similar to that of the fungus gnat. Both adults and larvae eat algae on the soil surface and surrounding areas. Although they do not damage plants directly, they can be vectors for soil pathogens. Control is dependent on reduction of wetness and subsequent elimination of algae.
Whitefly
Throughout North America, there are three species of whitefly that are major greenhouse pests. They include, greenhouse whitefly, Trialeurodes vaporariorum, Sweet Potato Whitefly, Bemisia tabaci, and silverleaf whitefly, Bemisia argentifolii, that until recently was also known as the "B" strain of sweet potato whitefly (Flint and Parrella, 1995). The latter two species have become resistant to most pesticides and have caused severe damage to both field and greenhouse crops.
The adult greenhouse whitefly, Trialeurodes vaporariorum, is approximately 1 mm in length with an average lifespan of 20 to 30 days. It has a yellow body with white wings that lay in a flat plane against its body. Whitefly females lay between 200-400 eggs on the undersides of leaves. Each egg is attached on a small stalk. The eggs hatch within 2 to 8 days forming oval larvae that resemble scales. These in turn molt during three more instar stages. In the final stage of metamorphosis, the pupa is formed from which the adult emerges after about 5 days. Both adults and young feed by inserting a stylus into the phloem and sucking sap, damaging tissues. A sticky substance, "honeydew" is secreted that secondarily becomes infected with a black mold. Infected plants show symptoms of yellowing, wilting and stunted growth. If not caught in time, entire plantings may be decimated.
In practice, it is difficult to differentiate between B. argentifolii and B. tabaci. By examining pupae, one can however, distinguish between T. vaporariorum and Bemisia spp. The pupa of T. vaporariorum is oval shaped and has a ring of wax filaments around the edge of its body. The pupa of Bemisia is pointed at one end and has no wax filaments. In side view, the Bemisia pupa looks like a squished football; the edges are tapered. The pupa of T. vaporariorum in side view resembles a hockey puck; there is a cylindrical "wall" between the parallel upper and lower surfaces. It is important to identify which species of whitefly is present to ensure successful control.
Mealybugs
In the citrus mealybug, Planococcus citri, males are rarely seen. Females are oval shaped, about 3-4 mm in length and 2-3 mm wide. Their body is pinkish with a white waxy covering and they are mostly immobile. Males are smaller than females and have two pairs of wings on their back. Their sole purpose is to fertilize the female and they do not feed. Female nymph and adults feed on all plant parts, sucking sap that stunts or deforms growth. Leaves, flowers and fruit may abort. Honeydew is secreted that subsequently becomes infected with black sooty mold.Longtailed mealybug, Pseudococcus longispinus, differs from citrus mealybug by the presence of long filaments at the end of its body.
Scales
Scales are divided into two groups, the armored and the soft shell.
Armored scales secrete a shell of wax and cast skins that protects the underlying soft body and may be removed. The armored scales have large mouth parts that invade mesophyll enabling them to suck out rich cell contents. The food obtained is used so efficiently that no honeydew is produced. Examples of scales of this type are oystershell scale (Lepidosaphes beckii), California red scale (Aonidiella aurantii), and San Jose scale (Aspidiotus perniciosus).
Soft shelled scales have no covering shell even though their skin becomes sclerotized with age. It cannot be removed without killing the insect. Soft shelled scales feed from phloem and produce copious amounts of honeydew that subsequently becomes infected with black sooty mold. Examples of this type of scale include, brown soft scale (Coccus hesperidum), hemispherical scale (Saissetia coffeae) and black scale (Saissetia oleae).
The female brown soft scale (Coccus hesperidum) keeps its eggs within its body till giving birth to live crawlers. The young mobile crawlers are yellowish with a mottled rounded shell. They begin to feed immediately after birth and molt twice. Older second stage nymphs are sessile. Males are rare and undergo four stages of metamorphosis before emerging as winged adults.
Adult female California red scale (Aonidiella aurantii) have a round cover underneath which they lay 100 -150 eggs during their life time. Newly hatched crawlers are mobile for only a short period before settling and molting to the adult sessile form. Within the second instar differences are seen between male and female nymphs. Males develop an elongated form, while female continue in a circular pattern. Adult males are winged and live for only 6 hours during which time they mate.
Complexities of the Sytem
The above protocols address individual pests. What happens when there is more than one pest present? The more pests present, the more complex the system becomes, the more balancing is required. The use of a chemical when two or more pests are present has to be carefully decided. When choosing a pesticide to knock back a pest population increase, look at what effect it has on the biocontrol agents present.
For example, if green peach aphids and western flower thrips are both present, and the aphid population booms, spraying with Pirliss (an aphicide) would not be a good choice. It will kill the biocontrols of the aphids (Aphidius and Aphidoletes) and one of those of the thrips (Orius). The residue left will prevent their reintroduction for one to two weeks. In that time period, the aphid population may or may not increase depending upon resistance but the Thrips population will definitely increase. The biocontrol agent that targeted the adult phase of the life cycle will be gone and although Pirliss is not lethal to the other thrips controls (A. cucumeris and H. miles) some mortality will occur resulting in decreased predation. Two pesticides that could be used to knock back the aphid population and not leave lethal residues would be insecticidal soap or Enstar. The soap would have an immediate effect on the aphids and also their controls to some extent (Aphidius that are in the mummy stage would be protected but adults would be killed as would all stages of Aphidoletes). The Enstar would not affect either biocontrol but would have a slower effect on the aphid population because it is an insect growth regulator; it prevents maturation.