The Strawberry Thrip, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) is often recognized as a serious pest of strawberries worldwide. It’s commonly known as “Western Flower Thrips”. It’s very highly polyphagous pest with at least 250 plant species from more than 65 families being listed as hosts.
Strawberry thrip is a major constraint in the production of appropriate quantity and quality fruits of strawberry for domestic and export markets. Strawberry thrips are most widespread, polyphagous and destructive pest with a wide range of host plants viz. Strawberry, Alocasia, Raphanus spp., Alfa alfa, Apple, Apricot, Peaches, Plums and Nectarines, Roses, Chrysanthemum, Carnation, Sweet pea, Gladiolus, Gerbera, Tomato, Capsicum, Cucumber, Melon, Grapes, Cotton and many more.
- F. occidentalis is naturally abundant in temperate region in many wild flowers throughout western North America from Southern California into Canada, Netherlands and spread outwards across Europe and Israel. After being well established in Europe, it has spread around Australia, Colombia, India, China and almost all countries across Asia, Africa and Europe. No mass migration has been reported yet but the migration is through human assisted transport of horticultural materials and it disperse naturally in the introduced area. They can also move long distances on wind currents. Spread is further enhanced by polyphagy and the ability of small founder population to find suitable hosts in new areas and succeed to survive. Thigmotactic behavior of thrips also aids in their transport via horticultural materials as they escape inspection and other sterilization treatment like fumigation.
Strawberry thrips are mostly prevalent in tropical to warm temperate regions, within the temperature range of 10-35°C (optimum 25-30°C) with relative humidity in the range of 70-85%. Development is temperature dependent. At most favourable temperature of 25-30°C egg to adult development can be as brief as 9-13 days. They are mostly active during spring (mid to late October) and Autumn (March – May). However, more number of adults were recorded in May with about 10-20% crop loss; also total crop loss in some areas. In the extreme condition of summer and winter, thrips are less active and are usually found on alternative hosts such as weeds.
- F. occidentalis is remarkably versatile and opportunistic. Even the pest strains, which are presumably inbred reproduce successfully in a wide range of temperature and humidity under experimental conditions. However, they may not survive cold winters outdoors. The incidence and appearance of pests is higher in second year of cultivation compared to first year due to over wintering of pests from first year.
Morphology and Life cycle of Strawberry Thrips:
Strawberry thrips complete their life cycle in 4 stages:
The duration of egg stage is relatively long. Eggs hatch in about 4 days at 27°C but takes 13 days at 15°C. Eggs are opaque, reniform (kidney shaped) and about 200 micrometer long; inserted into the epidermis and mesophyll layer of host plant. Eggs may be laid in leaves, flower structures or fruit.
There are two larval instars, which are spindle-shaped, and are creamy white to yellow in color. The first and second instars can be differentiated by examination of the number and placement of small setae on the abdomen. These setal patterns differ between the sexes for each instar, which allows the sexes to be differentiated. Larvae are mobile, but they tend to reside in concealed places on plants, such as within flowers or developing leaves or under the calyx of fruits.
Larvae begin feeding soon after emergence and moult within 3 days at 27°C (7 days at 15°C). Second instar larvae are very active, often seeking concealed sites for feeding, and they develop to the propupal stage in about 3 days at 27°C or 12 days at 15°C. When attacked by predators, larvae produce an anal droplet containing an alarm pheromone, which signals conspecifics to disperse. At the end of second larval stage, larvae normally drop to the ground to seek a pupation site. Most commonly it is in the surface layer of dead leaves beneath a plant, rather than in the soil, or even on the plant itself. The proportion of individuals dropping to the ground depends on plant architecture.
There are two pupal instars, neither of which feeds. Although capable of movement, neither pupal stage moves about actively, unless disturbed. Depending on host plant architecture, strawberry thrips may drop to the ground to pupate. The first pupal stage called propupa matures rapidly (1 day at 27°C ; 4 days at 15°C) and has short wing buds and the antennae protrude forward from the head whereas the pupal stage usually takes more than a week before the adult is ready to emerge and has wing buds extending more than half-way along the abdomen, and the antennae curve back over the head. Both pupal stages are usually white to cream colored.
The adult is slender with narrow, fringed wings, usually less than 2mm long, female being larger than male. A newly emerged female is relatively quiescent during her first 24 hours, but soon becomes active, particularly at higher temperature. Their life span is about 5 weeks. However, females may live about 40 days under laboratory conditions, but can survive as long as 90 days. Males typically live only half as long as females. Females have spindle shaped abdomen and vary in color from yellow to dark brown being darker at low temperature and yellowish at higher temperature. Female of the invasive pest strain is typically brownish yellow with dark brown markings medially on the abdomen. The adult male is smaller than the female, with narrower abdomen and is usually yellowish white. Adults are macropterous (i.e. they have fully developed wings). In spring and in Montane areas, the dark form predominates and it seems likely that this dark form is better able to survive low temperature, but males are rarely dark. With a good quality 20X hand lens, it is possible to see the long setae on the pronotum of adults that are typical of this species, but the structural details by which the genus and species are recognized cannot be seen without a microscope.
Nature of Damage:
The adults and larvae of Strawberry thrips (WFT) aggregate in flowers or other concealed areas on plants, such as developing fruits, foliage and floral buds so they escape inspection and fumigation. This preference for residing in tightly enclosed and concealed spaces of plants is called thigmotactic behavior. Females have a saw-like ovipositor which they use to deposit eggs into leaves, petioles, flower bracts, petals and developing fruits. WFT feed by piercing plant cells with their mouthparts and sucking out the contents.
Adults and larvae feed in a similar manner, so both stages contribute to plant damage. Individuals tend to feed in localized areas, which results in silvered or necrotic patches on foliage, flowers and fruits. Feeding within developing buds leads to deformation of leaves and flowers. WFT also feeds on pollen, which can stimulate oviposition, reduce larval development time and increase female fecundity. Although primarily phytophagous, adults and larvae will prey on spider mite eggs also.
It is a significant pest of virtually all crops including fruits, vegetables, ornamentals and cotton. High fecundity and reproduction on a broad range of hosts enables quick colonization and dispersal. Adult and larval feeding causes considerable aesthetic damage to ornamental and fruiting crops. Extensive feeding can also result in flowers and fruitlet abortion, which is a direct yield loss. Because of their thigmotactic behavior, feeding damage is often inflicted on developing tissue, which then goes undetected until flowers or fruits mature.
Female oviposition causes another type of damage as well. Female insert eggs under plant epidermis with their ovipositor; eliciting a physiological wound response in some plants that produces spotting on fruits, which can lead to downgrading of quality in tomatoes, grapes and apples. In addition to the direct damage it can cause, this species is an efficient vector of Tomato Spotted Wilt Virus and other Topso Viruses.
Strawberry thrips cause flowers and fruitlet abortion, petal browning, necrotic flecking and distorted, bronzed fruit. Feeding on the flowers can cause withering of anthers and stigmas, resulting in non-uniform fertilization which produces malformed fruit. Feeding on fruit may result in cracking, bronzing and reduction in fresh weight. Larval and adult feeding on the fruit surface causes net-like russetting, which reduces shelf-life and fruit appearance. On older fruit, adult and larval feeding causes russetting around the seed (achene). A symptom called cat- facing can also occur due to thrips feeding, though cat-facing may also be caused by poor pollination during cool conditions.
For management of Strawberry thrips, following steps can be followed:
Two methods are used to monitor thrips:
- White, yellow or blue sticky traps will attract and trap adults.
- Direct counts in blossoms of fruit crops.
Various efforts are underway to develop cucumber and pepper varieties resistant to WFT.
Many methods of control exist for thrips including cultural, biological and chemical.
Cultural control includes using UV-reflective mulch and reducing the amount of nitrogen applied to the crop.
Several predatory mites, especially the Amblyseius spp., Phytoseiid neoseiulus and the Ascid hypoaspis have successfully been used to control WFT and are reared commercially. Minute pirate bugs (Hemiptera) like Orius spp. are also in common. Entomopathogenic fungi, such as Metarhizium anisopliae, Beauveria bassiana and other species of Beauveria are also found effective against WFT. Other antagonists assayed against WFT include Ceranisus spp., Geocoris spp., Hypoaspis spp., Macrolophus spp. and nematodes like Steinernema feltiae. Lacewings and ladybird beetles are also the natural predators of WFT.
WFT are difficult to manage with pesticides because they tend to occupy narrow crevices in plant parts, and to feed deep within flowers and buds where they are sheltered from chemicals. Flower thrips may also be protected from systemic insecticides as there may be a lack of uptake into floral tissues. Thrips population also display a great ability to build up resistance to insecticides. Resistance of F. occidentalis has been reported to insecticides in several chemical groups including the organophosphates, organochlorines, carbamates, neonicotinoids, pyrethroids, avermectins, spinosyns and the phenylprazoles. When using chemical control, it is necessary to rotate insecticide groups to prevent a build up of resistance in thrips population. The application of broad spectrum insecticides may upset the control by natural enemies. Biological control programs that integrate the use of insecticides compatible with the conservation of natural population of minute pirate bugs have been implemented for fruiting vegetables in Florida.