Biology and Behavior of Western Flower Thrips: Comparison
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Please note this is a comparison between Version 2 by Sirius Huang and Version 1 by Daniel Rodriguez.

The western flower thrips (WFT), Frankliniella occidentalis Pergande, is a significant agricultural pest that challenges crop production. WFT are tiny insects that feed on various plants, causing damage that affects crop yield and quality. Effective control strategies are necessary to mitigate their impact.

  • Frankliniella occidentalis Pergande
  • management
  • biological control

1. Introduction

Western flower thrips (WFT), Frankliniella occidentalis Pergande (Thysanoptera: Thripidae), are small, winged insects that belong to the Thripidae family [1]. These pests are native to western North America but have spread to various parts of the world due to human activity, including the global trade of ornamental plants, and since WFT are highly adaptable they have become a significant agricultural pest, causing damage to a wide range of crops [2]. The WFT adult measures only about 1.2 to 1.5 mm in length, making them barely visible to the naked eye. Their slender bodies with fringed wings allow them to fly short distances and they are typically yellowish-brown in color, but may appear darker or lighter depending on environmental factors and life stage [3]. In addition, one of the notable characteristics of WFT is their piercing-sucking mouthparts [4]. Feeding by WFT can result in several plant problems, threatening crop health [5].

2. Biology and Behavior of Western Flower Thrips

The WFT individuals possess distinct physical characteristics that contribute to their identification. Understanding these features is essential for accurately identifying and distinguishing them from other thrips species. WFT are tiny insects, measuring approximately 1.2 to 1.5 mm in length as adults [1]. Their diminutive size makes them barely visible to the naked eye. The WFT’s body is elongated and slender, with a distinctive cigar-like shape. It appears narrow and cylindrical, allowing the thrips to maneuver through plant tissues easily. Their coloration can vary, but they typically exhibit a yellowish-brown or pale-brown hue, but the intensity of the color may vary depending on factors such as age, environmental conditions, and life stage [8][6]. Adult WFT have two pairs of fringed wings, usually longer than their bodies. These wings are narrow and extend beyond the length of the abdomen, and the fringed edges of the wings help distinguish them from other thrips species [9][7]. In addition, the WFT antennae are relatively long and segmented. They are noticeably thicker at the base and taper to a slender point; therefore, the length and shape of the antennae can aid in differentiating WFT from similar-looking insects [10][8]. On the other hand, WFT possess relatively short legs compared to the length of their bodies, with each leg terminating in arolia. These specialized structures serve as cushioned pads that enable thrips to adhere to plant surfaces effectively [11][9]. WFT have piercing-sucking mouthparts adapted for piercing plant tissues and extracting plant sap. The mouthparts enable them to feed on the sap of various crops and ornamental plants [4]. These physical characteristics can serve as general identifiers; however, accurate WFT identification may require a hand lens or microscope due to their small size. Furthermore, distinguishing features may vary slightly among individual thrips specimens and populations, often called pest complex, i.e., biotypes and subspecies of F. occidentalis [12][10].

2.1. Life Cycle and Reproductive Behavior of WFT

The life cycle and reproductive behavior of WFT play a crucial role in their population dynamics and ability to rapidly infest crops. WFT have a rapid life cycle, with females capable of laying up to 60 eggs during their lifespan of about 30 days [13][11]. The eggs are laid within plant tissues, and upon hatching, the larvae emerge and undergo two instars. Subsequently, they progress through the propupal and pupal stages before reaching adulthood. This quick reproductive cycle allows the thrips population to grow exponentially under favorable conditions [14][12].
The WFT life cycle begins with the oviposition. Female thrips insert their eggs into plant tissues, such as leaves, stems, or flowers [15][13]. The eggs are usually laid individually or in small groups. They are elongated, translucent, and extremely small, making them difficult to observe without magnification. After an incubation period of a few days, the eggs hatch into larvae. The larvae go through two stages, commonly called first and second instar [16][14]. During these stages, the larvae resemble smaller versions of the adults but lack wings. The WFT larvae typically display a pale yellow or white coloration, complemented by red eyes. Distinguishing between first and second-instar larvae can be achieved by examining the position and number of abdominal setae. Vierbergen et al. [17][15] identified specific characteristics of the WFT second-instar larvae, such as the presence of well-developed posteromarginal teeth measuring 5–7 μm in length in the abdominal tergite IX. These teeth are at least twice as long as the width of the D1 setae. Additionally, plaques found in the abdominal tergites II–V either lack microtrichiae or, if present, do not exceed 1 μm in length [17][15]. Following the larval stages, individuals enter the propupal stage and then progress to the pupal stage. During these stages, individuals cease feeding, and although they are not actively mobile, they may exhibit some limited movement if disturbed. Both pupal stages exhibit a cream or white coloration. Prepupae display short wing buds and forward-protruding antennae from the head. In pupae, the wing buds are more developed, often extending over halfway along the abdomen, while the antennae curve backward over the head [18][16]. During pupation, the individuals undergo metamorphosis, transforming into adult thrips. The pupal stage is generally short, lasting a few days [19][17]. After emerging from the pupal stage, WFT reach adulthood. Adult thrips are sexually mature and capable of reproduction. They possess two pairs of fringed wings, elongated bodies, and distinct coloration, typically yellowish-brown or pale brown [9][7].
WFT’s reproductive strategy enables them to multiply rapidly under favorable conditions. After reaching adulthood, WFT males and females engage in mating behavior. Male aggregations are a key aspect of the WFT mating behavior, usually observed on flowers. Within these aggregations, males emit an aggregation pheromone that attracts both males and females [20][18]. Females are observed to fly towards the aggregations, mate with a male, and then leave [21][19]. While males often engage in fighting behavior within the aggregations, the clear advantage of this behavior remains unclear, as females that arrive in the aggregations seem to mate with the first male they encounter. Indeed, experienced males are strongly inclined to mate with virgin females while actively avoiding mating with previously mated females [22][20]. Female thrips are highly fecund and can lay a significant number of eggs during their lifespan (ca. 45 days). For instance, on average, a single female thrips can lay up to 60–80 eggs [23][21]. The eggs are deposited into plant tissues using the female’s ovipositor. Various factors, including temperature, host plant quality, availability of suitable oviposition sites, and population density, influence the WFT reproductive rate [24,25][22][23]. Under optimal conditions, the population growth rate can be exponential, increasing thrips populations rapidly [1,2][1][2]. WFT are known for their ability to have a haplodiploid reproductive system, allowing for both sexual reproduction and arrhenotokous parthenogenesis. In sexual reproduction, fertilized eggs develop into females, while unfertilized eggs give rise to males [25][23]. This reproductive strategy allows females to produce offspring, potentially increasing their population growth rate. However, WFT females typically engage in monandry, mating with a single male, and they can re-mate five days after the initial mating event. Contrarily, males exhibit polygyny, having the capability to mate with multiple females [26][24]. This pattern of mating behavior results in a general bias towards females in WFT populations [27][25]. Understanding these aspects of their biology is essential for developing effective control strategies and implementing appropriate management practices.

2.2. Feeding Habits and Damage Caused by Thrips to Crops

WFT are notorious for their feeding habits and the damage they can cause to a wide range of crops [28][26]. Their piercing-sucking mouthparts can penetrate plant tissues and extract sap by a distinctive feeding strategy called “punch and suck” [29][27]. This feeding strategy involves the piercing of individual epidermal and mesophyll cells using their stylets, followed by the extraction of cell contents through sucking, and exhibiting sustained ingestion from unidentified plant tissues, potentially xylem sap [29[27][28],30], or the contents of previously ruptured cells [31][29]. Moreover, WFT engage in cell rupture feeding, as prolonged feeding by thrips leads to significant cell damage, including the destruction of cell walls, and they still feed on the released cell contents. This feeding behavior causes stippling/silvering, scarring, and discoloration of plant tissues. This damage appears as brown or bronze patches, streaks, or blotches on leaves, stems, and fruits [32][30]. WFT feeding on developing flowers can also lead to distorted or deformed blooms. Infested flowers may fail to open correctly or display abnormal shapes, and feeding on fruits can cause deformities, such as scars, pitting, or malformations, rendering them unmarketable or reducing their quality [33][31]. Severe thrips infestations can stunt plant growth and development, and consequently, the continual sap extraction weakens the plants, resulting in reduced vigor, decreased yields, and overall compromised plant health and fitness. In addition to causing direct feeding damage, WFT are significant vectors of plant viruses, e.g., the primary vector of tomato spotted wilt virus (TSWV) [34][32]. They can acquire the tospovirus from viruliferous plants within a well-defined interval, specifically during the first and early second larval instar stages. This period typically begins approximately 160 h after oviposition. During this phase, the insect undergoes ontogenetic changes characterized by the fusion of salivary gland cells with the mid-gut and visceral muscle cells. In late second instar larvae, the intimate contact between the salivary glands and visceral muscles is lost, reducing the movement of the virus within the salivary glands. In the metamorphosis process, the salivary glands separate from the mid-gut, effectively suppressing the virus’s movement into the salivary gland. Nevertheless, the individuals have already acquired the infection, and the virus continues to accumulate in the Malpighian tubules [35][33]. Upon reaching the adult stage, infected F. occidentalis can transmit the tospovirus to healthy plants during feeding events since, although the movement of the virus within the salivary glands is restricted in adults, the transmission can still occur due to the virus’s presence and activity in other anatomical structures [35][33]. This intricate pattern of virus acquisition, accumulation, and transmission highlights the role of WFT as vectors in the epidemiology of tospovirus diseases. Understanding these dynamics is crucial for devising effective management strategies to mitigate the transmission of tospoviruses by WFT and reduce the associated crop losses [36][34]. And finally, WFT have a broad host range and can affect numerous crops, including vegetables (e.g., tomatoes, peppers, cucumbers), fruits (e.g., strawberries, grapes), and various ornamental plants. Their ability to infest various crops makes them a significant concern for growers across different agricultural sectors [28][26].

References

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