Western Flower Thrip

Over the past 30 years, western flower thrips, Frankliniella occidentalis, has become one of the most important agricultural pests worldwide. It is arguably the most studied thrips in the world today.

The increasing importance of western flower thrips is clearly reflected by the increasing number of publications on this species relative to the proportion of publications on all Thysanoptera. There are over 5,000 species of thrips, yet western flower thrips alone has accounted for one third of the publications on all Thysanoptera in the past 30 odd years.

This increasing interest in western flower thrips is a result of its significance as an agricultural pest, which raises the question of what has enabled it to become such a pest. Its pest status can be attributed to several factors, including its reproductive potential, invasiveness, range of host crops, ability to transmit plant viruses, and insecticide resistance. All of these factors are interrelated, and all are related to the basic life cycle and life history strategy of the species. This review addresses the biological and ecological attributes

of western flower thrips that have enabled it to become a significant, difficult to manage pest. Many other species of Thripidae share these attributes of western flower thrips and therefore could emerge as significant pests.

Biology of Western Flower Thrips

The general life cycle of western flower thrips is similar to that of other species in the family Thripidae, consisting of an egg, 2 active feeding larval instars,  and the adult. Adults and larvae aggregate in flowers or other concealed areas on plants, such as developing fruits, foliage, and floral buds. This preference for residing in tightly enclosed and concealed spaces of plants is termed thigmotactic behavior. Females have a saw-like ovipositor, which they use to deposit eggs into leaves, petioles, flower bracts and petals, and developing fruit.

Sex determination in the western flower thrips is through haplodiploidy. The haploid males are produced from unfertilized eggs, whereas the diploid females are produced from fertilized eggs (arrhenotoky). Although sex ratios of adults from field samples are often biased towards 1 sex or the other, mated females do not appear to allocate the sex of their progeny. Therefore, biases found in those adult sex ratios are likely a function of differences between the sexes in their dispersal, distribution in response to host quality, and longevity.

Development is temperature and host dependent but can be quite rapid, allowing multiple generations to occur in a single cropping season. Western flower thrips does not have an obligatory developmental or reproductive diapause. Therefore, development occurs whenever temperatures exceed a minimum threshold of 8-10°C. At the most favorable temperatures of 25-30°C, egg to adult development time can be as brief as 9-13 days. The duration of the egg stage is relatively long, with hatching in 2-4 days at optimal temperatures. The first stadium is typically about half the length of the second, after which feeding stops and pupation begins.

Thrips often drop to the soil to pupate, but significant numbers can remain on host plants, especially if hosts have complex floral architecture. The first pupal instar is termed the propupa, a non-feeding stage that is followed by the pupa, another non-feeding pupal stage. Winged adults then emerge from the pupal stage in 1-3 days. Under laboratory conditions, adult lifespan is relatively long compared with immature development time. For example, at 28°C, median egg to adult development time is 12 days, whereas median longevity for females is 26 days, with some females living up to 5 weeks. The relevance of these data to actual longevity in the field is unclear, but overlapping, continuous generations are likely to occur in the field. Although determining longevity in the field is problematic with such small vagile insects, mark-recapture studies indicate that adults can survive for over 5 days following release in pepper and tomato plantings.

Western flower thrips 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,.Western flower thrips 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. Western flower thrips fits the classic definition of an r-selected species.

All studies of reproduction in western flower thrips have reported high fecundity for females. After an initial preoviposition period, a female can oviposit throughout her lifetime. With optimal temperatures and diets, females can produce up to 7 progeny per day and have average total lifetime fecundities exceeding 200 per female. This  high level of fecundity leads to high intrinsic rates of population increase, so uncontrolled populations can multiply rapidly. One of the most important aspects of western flower thrips biology is its polyphagy. This species is known to feed on over 250 different crop plants from more than 60 plant families. In addition, it occurs on many uncultivated plants. However, it is critical to distinguish between plant species that support successful reproduction and those on which adults feed but do not support successful breeding populations. found that the range of adult feeding hosts emphasizing the need to look beyond static records of plant associations to understand the ecology and population dynamics of western flower thrips. To avoid misunderstandings and misinterpretations, it is clear that the “host plant” must be applied in the proper context. Because of its polyphagous feeding and breeding behavior, western flower thrips is exposed to a broad diversity of plant allelochemicals. Therefore, it must be able to metabolizea broad range of allelochemicals, as  well as produce inducible enzymes in response to specific compounds. Unfortunately, there is little basic ecophysiology information on the response of western flower thrips to host plant chemistry. Based on pesticide resistance studies, western flower thrips has various metabolic detoxification enzyme systems that could help it to overcome secondary plant defenses. Chief among these systems are cytochrome P-450 monooxygenases, esterases, and glutathione S-transferases. Apparently, this generalist herbivore has many allelochemical- metabolizing genes to enable it to cope with the diversity of allelochemicals that it is likely to encounter.

Western Flower Thrips as a Pest

Beginning in the late 1970s, western flower thrips began to spread widely from its native range in western North America. The exact cause for its spread is uncertain but increased global trade in floricultural and horticultural products has been implicated. A highly insecticide resistant strain originated in California as a result of intensive insecticide use greenhouse crops in the 1970s and 1980s. Western flower thrips is now established throughout North America, and many countries of Europe, Asia, South America, Africa, and Australia.

Whereas human assisted movement is undoubtedly responsible for many of the introductions of western flower thrips to new geographic areas, this species is also able to spread by other means within new areas. Thrips can move long distances on wind currents. Spread is further enhanced by polyphagy and the ability of small founder populations to succeed. Several biological factors make western flower thrips an ideal invasive species to be spread by human activity. The small size and thigmotactic behavior of larvae and adults make detection difficult. In addition, because eggs are deposited within plant tissue, they are even less readily detected, and are less susceptible to fumigation than are other life stages. The polyphagous nature of western flower thrips increases the number of crops on which it may be exported from a country, and then enhances the probability of introduced individuals finding suitable hosts in new areas. The high fecundity of females makes it possible for small founder populations to become established and grow rapidly. Further, the haplodiploid sex determination leads to strong selection against deleterious alleles in the haploid males. Consequently, some small founder populations may readily adapt to new environments and be relatively resistant to the detrimental effects of inbreeding. Also, because of their potentially long adult lifespan, rapid immature development rate, and haplodiploid sex determination, unmated founder females could produce male progeny initially and survive long enough to mate with those males, thus making introduced populations as small as one potentially viable. The sheer number of crops that western flower thrips attacks is awesome. It is a significant pest of virtually all crops, including fruiting vegetables, leafy vegetables, ornamentals, tree fruits, small fruits, and cotton. The range of crops damaged by western flower thrips is simply a reflection of its inherent polyphagy. Direct crop damage results from both feeding and oviposition. In addition, high fecundity and reproduction on a broad range of hosts enables large numbers to disperse into crop fields from many sources. Consequently, attempting to manage the sources of thrips is virtually impossible. In many floral and horticultural crops, western flower thrips populations are virtually guaranteed to exceed the low to non-existent damage thresholds.

Adult and larval feeding causes considerable aesthetic damage to ornamental and fruiting crops. Because of their thigmotactic behavior, feeding damage is often inflicted on developing tissue, which then goes undetected until flowers or fruit mature. Not all crops damaged by western flower thrips are reproductive hosts for the species. Those that only serve as adult feeding hosts, for example tomato, can still be adversely affected by adult feeding. Further complicating management, western flower thrips feeding damage can be confused with damage caused by other pests or disease. Such incorrect diagnoses may result from the small size and cryptichabits of western flower thrips and the fact that damage is not immediately apparent and associated with the causal organism.Female oviposition causes another type of damage to developing fruits. Females insert eggs under plant epidermis with their saw-like ovipositor. This wounding elicits a physiological wound response in some plants that produces spotting on fruits.

Perhaps the most important problem with insecticideuse is the ability of western flower thrips to develop resistance to insecticides.

The first reported insecticide failure against western flower thrips was in 1961 and, since then, there have been numerous documented cases of resistance to most classes of insecticides around the world. The extensive resistance found in a California greenhouse populations has been implicated as a contributing factor in the worldwide spread of western flower thrips.

The polyphagous nature of western flower thrips plays a key role in its ability to develop resistance to insecticides. Because it is a pest of many crops, populations are often under constant insecticide pressure, which increases selection for resistance. Enclosed greenhouse environments also place populations under intense selection for resistance because they provide constant exposure to insecticides and limit immigration of susceptible individuals.

The haplodiploid sex determination system in western flower thrips greatly accelerates the evolution of insecticide resistance. In haplodiploid species, resistance genes

are exposed to selection from the outset in haploid males, regardless of whether resistance alleles are dominant or recessive. Thus, resistance alleles can become fixed much more rapidly than if western flower thrips were diploid. Not only can western flower thrips evolve resistance rapidly, resistance can persist over many generations in the absence of selection . Even more troubling for resistance management programs is recent evidence that resistance to certain insecticides does not come with a fitness cost to western flower thrips. Consequently, resistance could evolve faster and be maintained in populations longer, which would greatly affect the development and viability of insecticide rotation schemes and resistance management programs. As a polyphagous herbivore, western flower thrips has evolved numerous metabolic detoxification pathways to contend with diverse plant allelochemicals that it encounters. These versatile enzymatic systems predispose it to be able to metabolize many insecticides and often confer crossresistance to other insecticides Western flower thrips is clearly a formidable pest because of the range of crops it attacks throughout the world, the ever increasing amount of damage caused by its feeding, oviposition and virus transmission, and the propensity with which it develops insecticide resistance. While much has been learned about this species and how to manage it, there is a clear need to continue development of more economically and environmentally sustainable management

strategies for this devastating pest. To better manage this species, a greater understanding is needed of its biological and ecological attributes especially its biology, ecology and population dynamics outside of cropping systems. As formidable a problem as the western flower thrips has become, other thrips with similar biological andecological attributes exist and could, likewise, rapidly emerge as serious global pests. Thus, increased knowledge about western flower thrips will help to avoid or mitigate damage due to other pest thrips.