Not another Thrip and fall case!!
By: Michael J. Green
(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.
http://thrips.ifas.ufl.edu/thripsweb/thripsbiology.htm
http://www.cbwinfo.com/Biological/PlantPath/Insects/Thrips.html
http://animals.jrank.org/pages/2433/Thrips-Thysanoptera-DIET.html
http://www.fullbooks.com/Cactus-Culture-For-Amateurs1.html
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.
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7429.html
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pni74291.html#GREENHOUSE
Black
adult and clear nymph Damage produced by
|
|
Heliothrips haemorrhoidalis Heliothrips haemorrhoidalis
note liquid feces on abdomen tips. feeding on wood fern
Photo by Jack Kelly Clark. Photo by Mike
Merchant,
Texas Cooperative Extension Service
|
|
Photo by Cheryle
O'Donnell)
Photo by Cheryle O'Donnell)
