Not another Thrip and fall case!!
(and for the grammatically correct, both the singular and the plural are known as thrips) Thrips are yet another insect that has both a good (predatory) side and an evil (pest) side. There are more than 5,000 described species worldwide, divided into two suborders (Terebrantia and Tubulifera). Certain thrips species from the family Aeolothripidae (Predatory Thrips) are beneficial predators that feed only on mites and other insects . Beneficial species include Leptothrips mali (black hunter thrips) and Scolothrips sexmaculatus (the sixspotted thrips). Thrips are tiny insects, only a few mm in length at the most. Thrips may or may not have wings. When wings are present, they are narrow with few or no veins and fringed with long hairs. Thrips have asymmetrical mouthparts (no right mandible) suitable for piercing and sucking. Antennae of thrips have four to nine segments and are relatively short.
For cactus and succulent hobbyists there are several pest thrips species to deal with. Scopaeothrips bicolor (prickly pear cactus thrips) damages the buds of the prickly pear cactus resulting in deformed or aborted fruits. Frankliniella occidentalis is native to the desert southwest of the United States and can survive from sea level to sub-alpine altitudes. Also known as the Western flower thrips, it is of increasing concern to greenhouse growers and hobbyists alike. They seem to eat indiscriminately by rasping the plant surface. Western flower thrips have spread throughout the worldwide horticulture industry on plugs, cuttings and small plants. Frankliniella occidentalis does feed on agaves, yuccas and “holiday” cactus (Schlumbergera species). Heliothrips haemorrhoidalis, or greenhouse thrips, occurs in the United States outdoors in central and southern Florida and southern California. It is found in greenhouses throughout the United States. Thrips palmi has been found on ornamental Pachypodiums, particularly Pachypodium lameri. Thrips attack Phyllocactus, Rhipsalis, and Epiphyllum, especially when the plants are grown in less shade, or in a higher temperature, than is good for them. Most species feed on plants, though some feed on fungus spores or are predaceous on other small arthropods. Plant-feeding thrips are generally found on soft living plant tissue, though some larval stages may be spent on soil. Plant-feeders often cause damage to leaves and flower petals. Contradictory opinions exist as to actual plant damage.
Thrips have a relatively uncomplicated life cycle. Thrips reproduce by mating, with males developing from unfertilized eggs. Thrips are haplodiploid, that is males have half the number of chromosomes (the haploid number) that are found in the females (the diploid number). The males develop from unfertilized eggs. Males usually are smaller than females. It begins when adult females lay eggs in the leaves of host plants. These are preferably older plants. T. palmi clutches average 50 eggs and those of F. occidentalis are about 200 eggs in size. The eggs of flower thrips and leaf-feeding species are inserted into plant tissue by means of a serrated ovipositor (that is a specialized egg-laying structure). In warm weather, life cycles take 21 days or less. The larvae feed in groups on the leaves (T. palmi) or flowers (F. occidentalis) of the host plant. They pass through four larval stages (instars) with the first two being most active. The third and fourth instars lead to pupation and the appearance of the adults. The adults feed on younger plants causing physical damage and also introduce viruses during their feeding. The length of the life cycle is very dependent upon temperatures. They eat pollen as well as new flower and leaf tissues. They will occasionally prey on mites.
The importance of thrips as pollinators is often overlooked by botanists. Thrips pollinate a wide range of flowers. Western flower thrips fly up to one hundred yards at a time when their food plants are disturbed. Their distribution across oceans, however, is a result of hitchhiking on plants sold around the world. Males will compete with other males for territory on a leaf, but only when the population density of thrips is low.
The feeding apparatus of thrips is unique. Only one mandible is present and another mouth structure forms a stylet or tube through which food is drawn. Larvae and adults use a similar punch and suck feeding technique. The single mandible punches a hole in the plant surface through which the stylet is inserted.
Thrips induce a range of symptoms in plant tissue by their feeding. On small fruits, feeding results in deformity. Some species cause similar damage to leaves. Silvering is common, due to air entering cells from which the contents have been removed, and on fruits this leads to scarring and corky tissue development. Very large populations of thrips can induce premature flower loss, and can reduce available pollen below critical levels.
As a side (strange) note, there has been an interest in the use of insects as anti-crop (aka biological) weapons for some years. The appearance of thrips on the Biological and Toxin Weapons Convention (BTWC) lists of potential biological agents was unexpected, and apparently due to a single, unfounded accusation: An incident in Cuba in the late 1990's. Cuba under Fidel Castro has accused the United States of being behind the outbreak of disease affecting crops on more than one occasion. In October 1996, a US registered aircraft flying over Cuba was seen to be releasing what looked like smoke. The flight plan taking the plane over Cuban territory had been approved by the Cuban government. Shortly after the flight was observed, there was a serious outbreak of Thrips palmi on farms under its flight path. Cuba claimed that the thrips had been spread by the flight and demanded a review of the incident under Article V of the BTWC. The US denial of the charges and its explanation of the observations of the Cubans were convincing to almost all of the members of the committee investigating the claim. However, the incident seems to have brought these pests to the attention of the arms control community. UC Davis held a three day seminar on thrips as biological weapons in October 2007.
Control of thrips progresses through the usual cultural, mechanical, biological and pesticide pattern. Proper growing conditions (i.e. those minimizing thrips) would include keeping seedlings under an insect excluding mesh dome, removing plant debris, or using bright yellow sticky traps. Blue sticky traps are most effective for capturing western flower thrips, but thrips are harder to discern on this darker background. it is wise to avoid planting susceptible plants next to these areas or to control nearby
weeds that are alternate hosts of certain thrips. In small gardens, thrips can be knocked off plants with a spray of water. Vigorous plants normally outgrow thrips damage; keep plants well irrigated, but avoid excessive applications of nitrogen fertilizer, which may
promote higher populations of thrips. Remove and dispose of old, spent flowers. Reflective mulch or mesh confuses and repels certain flying insects searching for plants, apparently because reflected ultraviolet light interferes with the insects’ ability to locate
plants. Most uses of reflective mulch have been against winged aphids, but
infestation of young plants by other pests including leafhoppers, thrips, and whiteflies has also been prevented or delayed. It is most effective during early growth when plants are small; as plants grow larger, it is less effective, and other methods may be
needed. Reflective mulches cease to repel insects when the plant canopy covers more than about 60% of the soil surface.
Although certain predators and parasites of thrips are produced commercially and can be purchased through the mail, little or no research has been conducted on the
effectiveness of releasing thrips predators or parasites in landscapes and gardens. Releasing purchased natural enemies in most situations is unlikely to provide satisfactory pest control. Conserving naturally occurring populations of beneficials by controlling dust and avoiding persistent pesticides is the most important way to encourage biological control of pest thrips. Biological control becomes easier later in the spring, and populations of flower thrips are low in the summer and fall. Natural enemies include the minute pirate bugs (true bugs of the order Hemiptera, family Anthocoridae, genus Orius) and entomopathogenic ( that is, insect pathogenic) nematodes that are specialized parasites of thrips (order Tylenchida, family Allantonematidae, genus Thripinema). The importance of other natural enemies of thrips is not well understood. Examples include the bigeyed bugs (order Heteroptera, family Lygaeidae, genus Geocoris) and predatory thrips (order Thysanoptera, family Aeolothripidae, genera Aeolothrips and Franklinothrips). UC Davis recommends the following beneficial enemies of thrips:
Although thrips damage to leaves is unsightly, thrips activity does not
usually warrant the use of insecticide sprays. No pesticide provides complete control
of thrips. In comparison with other insects, most thrips are difficult to control effectively with insecticides. Reasons include thrips’ tiny size, great mobility, hidden feeding behavior, and protected egg and pupal stages. Improper timing of application, failure
to treat the proper plant parts, and inadequate spray coverage are also common mistakes and can be more important in influencing the effectiveness of treatment than choosing which pesticide to apply. Before using a pesticide, learn more about your
specific plant situation and the biology of your pest species. Often you will learn chemical control cannot be effective until the next season when new plant growth develops. If insecticides are used, they will only be partially effective and must be combined with appropriate cultural practices and conservation of natural enemies. Greenhouse thrips is an exception; because it is sluggish and feeds in groups on
exposed plant parts, thoroughly applying most any insecticide will kill this species.
Narrow-range oil (Sunspray, Volck), azadirachtin (Safer BioNeem), neem oil
(Green Light Garden Safe), pyrethrins combined with piperonyl butoxide
(Garden Safe Brand Multi-purpose Garden Insect Killer, Spectracide Garden
Insect Killer), and (at least for greenhouse thrips) insecticidal soaps (Safer), can be somewhat effective for temporary reduction of thrips populations if applied when thrips are present and damage first appears. These materials have the benefit of allowing
at least a portion of the natural enemy populations to survive because they do
not leave toxic residues. Sprays must be applied to thoroughly cover susceptible plant tissue, such as new leaf growth and buds. Repeat applications (usually 5 to 10 days apart, depending on temperature) are usually required because these insecticides
only kill newly hatched thrips and recently emerged adults. Other insecticides for thrips (Conserve, Green Light Lawn & Garden Spray Spinosad, Monterey Garden Insect Spray) include spinosad, which is derived from beneficial microbes and has low to moderate adverse impact on thrips natural enemies. Spinosad should be applied no more than once or twice a year, and can be more effective against thrips than the previously listed insecticides.
For ornamental NON-food plants, a licensed pesticide applicator can use
the systemic organophosphate insecticide acephate (Orthene), but acephate
can be highly toxic to natural enemies and it commonly causes spider mites
to become abundant and damage plants within a few weeks after its application. Another systemic insecticide, imidacloprid (Bayer Advanced Garden Tree & Shrub Insect Control, Merit) provides some suppression of foliage-feeding thrips only, but it is also toxic to some natural enemies of thrips. Avoid the use of organophosphate insecticides (e.g., malathion), carbamates (carbaryl), or pyrethroids (e.g., cyfluthrin, fluvalinate, and permethrin) because all these materials are highly toxic to natural enemies, can cause dramatic increases in spider mite populations, and are not particularly effective against most thrips.
Crespi, B.J. 1993. Sex ratio selection in Thysanoptera. Evolution and Diversity of Sex Ratio in Insects and Mites (eds. D.L. Wrensch and M. Ebbert), pp. 214-234. Chapman and Hall.
11. Loomans, A.J.M., T. Murai, and I.D. Greene. 1997. Interactions with hymenopterous parasites and parasitic nematodes. Thrips as Crop Pests (ed. T. Lewis), pp. 355-397. CAB International, Wallingford, UK.
Heliothrips haemorrhoidalis Heliothrips haemorrhoidalis
note liquid feces on abdomen tips. feeding on wood fern
Texas Cooperative Extension Service
Photo by Cheryle O'Donnell)
Photo by Cheryle O'Donnell)
Photo by Jack Kelly Clark.
Photo by Jack Kelly Clark.
Photo by Jack Kelly Clark.
Photo by Jack Kelly Clark.