Thrips, Frankliniella spp.

See Note on 2007 outbreak of thrips in brambles

I. Introduction: Thrips belong to the order Thysanoptera, and the species discussed here in the family Thripidae.  They use their asymmetrical mouthparts to rasp plant cells, especially in flowers and young, developing fruiting structures. The most common species are the flower thrips, Frankliniella tritici (Fitch), and the western flower thrips, F. occidentalis (Pergande).  The latter species is more damaging, has been expanding it geographic range, and may be more common.  WFT infestations appear to be somewhat local and often occur in the proximity of greenhouses with a history of high incidence of WFT.

II. Injury: Thrips feed on plant cells in flowers and young fruit.  This injury causes fruit to be abnormally shaped later. 
Caneberries and strawberries are very susceptible.  In caneberries, individual drupelets may be killed.  In strawberries, achenes are killed.  Adjacent parts of berries do not grow, causing an apparent crowding of the achenes, termed "apical seediness".  Thrips are also important vectors of many plant viruses.  Thrips populations in brambles therefore pose two, and potentially three, problems.  The first relates to feeding injury.  Feeding by thrips can injure floral parts and also drupelets after berries are formed.  Feeding in fruit may cause individual drupelets to be white.  The second problem arises when high populations persist until harvest (not usually the case), and active thrips found in the harvested fruit may be a concern with buyers.  The potential third problem relates to virus transmission (see below).

III. Hosts:  The list of host plants includes many species, across many plant families, and including both wild and cultivated species. 
FT and WFT both have an extremely wide host range including weeds, floricultural crops, field crops, and tree and small fruit crops.  WFT adults are slender, about 6/100 inch (1.5 mm) long, yellowish, and hold their fringed wings over their backs. Differentiation from FT requires microscopic examination.  Larvae are smaller and wingless, but otherwise resemble adults. Thrips generally move quickly to shelter when disturbed.

IV. Biology: The life cycle of thrips is complex.  After the egg stage, there are two feeding instars called larvae.  Following these larval stages there are two non-feeding stages, called the prepupa and pupa.  These are followed by the adult stage, also a feeding stage.
Little is known about WFT biology in the eastern states. Research conducted in 1992 at Penn State showed that WFT adults emerged from soil overwintering sites in May after most stone fruit bloom was complete. The WFT population built up on weeds, especially clover, in and around orchards throughout the season. Density of adult WFT in water traps peaked in July through September in stone fruit orchards in south-central Pennsylvania in 1992. The generation time of this species requires about 250 DD50.  In areas with cold winters, thrips may overwinter as pupa in earthen cells, but in warm areas may survive as active forms all year.

Plant factors in addition to the crop plant can have a profound impact on thrips numbers, since they can breed in such a wide variety of hosts.  Climatic factors can have a major impact on thrips populations, and may be at play here.  Rains can wash off more than 90% of thrips and kill those trapped in the soil, so populations may be higher after sustained periods without rainfall.  This spring has seen many areas with dry weather, and this may have led to higher population densities of thrips.  A recent text on thrips (Lewis 1997) raised the possibility of an effect of climate change, but this has not been examined closely, and it would be impossible to show this based on an outbreak in a single year.

Virus transmission:  Thrips transmit a couple of related viral diseases.  Though these viruses have been found in Rubus, their significance in caneberries is unclear.  Tomato spotted wilt virus is acquired during larval feeding by several species of Frankliniella.  Impatiens necrotic spot virus was formerly considered a strain of the previous virus.  INSV is also transmitted by several species of Frankliniella, and has been isolated recently in RubusThese are major viruses in some cropping systems, though their significance in brambles is unclear.  Thrips have been examined in the potential transmission of some other viruses, and are apparently NOT involved in transmission of raspberry bushy dwarf idaeovirus.

Biological control:  There are natural enemies that may suppress numbers of thrips.  The degree of control provided in field settings is controversial, though biological control of thrips in greenhouses is growing in use.  Predatory bugs like the minute pirate bug, Orius, feed on thrips, as do some predatory mites.  Some species of thrips are in fact predatory, and feed on various small prey, like plant-feeding thrips and spider mites.  Lacewing larvae, lady beetles and syrphid flies also feed on thrips.  Certain solitary wasps have been known to capture thrips with which to provision their young.  Wide ranges in parasitization of thrips have been reported, with some studies showing as low as 2% or as high as 70% parasitization of larvae, or 0.5-51% parasitization by egg parasites.

Chemical control:  There are several chemical alternatives.  SpinTor (spinosad; Restricted Entry Interval 4 hours, Preharvest Interval 1 day) is registered for thrips control on some crops, including caneberries.  It is quite safe for humans.  Aza-Direct (azadirachtin; REI 4 hours, PHI 0 days) is also recommended.  This is also environmentally selective, and is OMRI-certified.  Malathion (REI 12 hours, PHI 1 day) is a more conventional organophosphate, and is less effective for this use than the other materials.  Pyrellin has been recommended for thrips on caneberries, and is a blend of pyrethrum and rotenone.  Although these are both botanical insecticides, Pyrellin is not OMRI-certified because of synthetic inert ingredients.  At any rate, it is going out of use because EPA has announced its intention of canceling all uses of rotenone except as a fish poison (piscicide).  Some pesticide labels recommend an adjuvant to improve efficacy toward thrips.

The PHI value is of critical importance in caneberries.  Thrips are most prevalent during bloom, but there is broad overlap between blossoming and fruit development, including harvest.  Not only is proximity to harvest a concern, but bee hazard is an issue as well.  Malathion is highly toxic to bees.  SpinTor is moderately toxic (do not apply to blossoms if bees will forage within 3 hours).  Aza-Direct and Pyrellin are relatively non-toxic to honey bees.

V. Monitoring: Thrips may be monitored by shaking blossoms over a white piece of paper.  There are no thresholds established.

Lewis, T. (ed.) 1997.  Thrips as Crop Pests.  CAB International, NY.  740 p.

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