Stuart R. Reitz

Biology and Ecology of the Western Flower Thrips (Thysanoptera: Thripidae): The Making of a Pest

ABSTRACT In the past 30 years, western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), has become one of the most important agricultural pests worldwide. Certain biological attributes of this insect predispose it to be a direct pest across a wide range of crops. In addition to the direct damage it can cause, this species is an efficient vector of Tomato spotted wilt virus and other Tospoviruses. This review addresses questions regarding the biological and ecological attributes of western flower thrips that have enabled it to become a significant pest and make it so difficult to manage. These important life history traits include western flower thrips polyphagy and a tendency to reside and feed in concealed areas of flowers and fruits. Consequently, large populations can develop and disperse into a wide range of crops. The larvae and adults feed in a similar manner and can share the same host plant resources. The relatively short generation time and haplodiploid sex determination also contribute to the pest status of this species. These life history traits interact in complex ways to make western flower thrips one of the most significant and difficult to manage pests in the world.

Integrated Management Tactics for Frankliniella Thrips (Thysanoptera: Thripidae) in Field-Grown Pepper

In a 2-yr study, the impacts of different plastic soil mulches, insecticides, and predator releases on Frankliniella thrips and their natural enemies were investigated in field-grown peppers. Ultraviolet light (UV)-reflective mulch significantly reduced early season abundance of adult thrips compared with standard black plastic mulch. This difference diminished as the growing seasons progressed. Late season abundance of thrips larvae was higher in UV reflective mulch compared with black mulch plots. The abundance of the predator Orius insidiosus (Say) was significantly lower in UV-reflective mulch compared with black mulch treatments. Infection of plants with tomato spotted wilt virus, a pathogen vectored by Frankliniella occidentalis (Pergande), was <6%. In the year with the higher disease incidence (2000), UV-reflective mulch plots had significantly less disease (1.9%) compared with black mulch plots (4.4%). Yield was significantly higher in UV-reflective mulch (24,529 kg/ha) compared with black mulch (15,315 kg/ha) during this year. Effects of insecticides varied with species of thrips. Spinosad reduced abundance of F. occidentalis, but not Frankliniella tritici. In contrast, esfenvalerate and acephate reduced numbers of F. tritici and Frankliniella bispinosa, but resulted in higher populations of F. occidentalis. Spinosad was the least disruptive insecticide to populations of O. insidiosus. Releases of O. insidiosus and Geocoris punctipes (Say) reduced populations of thrips immediately after releases; naturally occurring predators probably provided late season control of thrips. Our results suggest that UV-reflective mulch, combined with early season applications of spinosad, can effectively reduce abundance of thrips in field-grown pepper.

Western flower thrips resistance to insecticides: detection, mechanisms and management strategies

Insecticide resistance continues to be one of the most important issues facing agricultural production. The challenges in insecticide resistance and its management are exemplified by the situation with the western flower thrips Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae). This highly invasive pest has a great propensity for developing insecticide resistance because of its biological attributes, and cases of resistance to most classes of insecticides used for its management have been detected. To combat insecticide resistance in the western flower thrips, several insecticide resistance management (IRM) programs have been developed around the world, and these are discussed. Successful programs rely on non-insecticidal tactics, such as biological and cultural controls and host plant resistance, to reduce population pressures, rotations among insecticides of different mode of action classes to conserve insecticide efficacy, resistance monitoring, sampling to determine the need for insecticide applications and education to assure proper implementation. More judicious insecticide use is possible with the development of well-founded economic thresholds for more cropping systems. While growers will continue to rely on insecticides as part of western-flower-thrips- and thrips-transmitted virus management, more effective management of these pests will be achieved by considering their management in the context of overall integrated pest management, with IRM being a key component of those comprehensive programs.

SEASONAL AND WITHIN PLANT DISTRIBUTION OF FRANKLINIELLA THRIPS (THYSANOPTERA: THRIPIDAE) IN NORTH FLORIDA TOMATOES

Abstract Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), the western flower thrips, is the primary insect pest of tomatoes and other vegetable crops in northern Florida and the rest of the southeastern USA. However, it is not the only flower thrips present in the region nor is it always the most abundant species. To determine the seasonal and within plant distribution of these various Frankliniella species, experimental tomato plants, grown under different nitrogen fertilization regimes, were sampled during the fall and spring growing seasons. Contrary to expectations, different levels of nitrogen fertilization did not affect the abundance of thrips species. Thrips were much more abundant in the spring than in the fall. In the spring F. occidentalis was the most abundant species, while in the fall F. tritici (Fitch) was the most abundant species. In both the fall and spring, significantly more adults occurred in flowers in the upper part of the plant canopy than in flowers in the lower part of the plant canopy. The sex ratio tended to be female biased, but with a greater percentage of males occurring in the upper canopy flowers. In contrast, significantly more immature thrips occurred in the lower part of the plant canopy than in flowers in the upper part of the plant canopy. Differences in seasonal patterns and within plant distribution should be considered in developing sampling protocols and management plans for thrips.

Within-Plant Distribution of Frankliniella species (Thysanoptera: Thripidae) and Orius insidiosus (Heteroptera: Anthocoridae) in Field Pepper

Abstract We evaluated the within-plant distribution of Frankliniella spp. and the predator Orius insidiosus (Say) in pepper (Capsicum anuum L.), over a range of field conditions, and we conducted behavioral experiments to examine the time budgets of F. occidentalis (Pergande) and F. tritici (Fitch) females on pepper plant parts. In the field experiments Frankliniella species composition varied by season and location. Still, all populations of thrips and O. insidiosus in untreated and insecticide-treated pepper were highly concentrated in the flowers, with 82–99% of individuals of each taxa found in flowers. This preference for flowers was corroborated by laboratory-choice experiments. Adult females of F. occidentalis and F. tritici showed a strong preference for pepper flowers over leaves and buds. In laboratory observations, females of F. occidentalis spent 3.6× as much time on flowers as on all other plant parts, and females of F. tritici spent over 6.3× as much time on flowers as on all other plant parts. Therefore, the concentration of these thrips in flowers appears to be behaviorally based and not an artifact of insecticide applications or sampling. Using estimates of populations from flowers of field pepper is sufficient for understanding the local dynamics of Frankliniella spp. and the predator O. insidiosus, and for estimating the benefits of biological control in scouting programs based on predator to prey ratios.

A natural M RNA reassortant arising from two species of plant- and insect-infecting bunyaviruses and comparison of its sequence and biological properties to parental species

Reassortment allows multicomponent viruses to exchange genome segments, a process well-documented in the vertebrate- and arthropod-infecting members of the family Bunyaviridae but not between distinct species of the plant- and insect-infecting members of the genus Tospovirus. Genome sequence comparisons of a virus causing severe tospovirus-like symptoms in Florida tomato with Groundnut ringspot virus (GRSV) and Tomato chlorotic spot virus (TCSV) demonstrated that reassortment has occurred, with the large (L) and small (S) RNAs coming from GRSV and the medium (M) RNA coming from TCSV (i.e. L(G)M(T)S(G)). Neither parental genotype is known to occur in the U.S. suggesting that L(G)M(T)S(G) was introduced as a reassortant. L(G)M(T)S(G) was transmitted by western flower thrips (Frankliniella occidentalis [Pergande]), and was not able to overcome the Sw5 resistance gene of tomato. Our demonstration of reassortment between GRSV and TCSV suggests caution in defining species within the family Bunyaviridae based on their ability to reassort.

Competitive exclusion of a worldwide invasive pest by a native. Quantifying competition between two phytophagous insects on two host plant species

1. High competitive ability is believed to be an important characteristic of invasive species. Many animal studies have compared the competitive ability of invasive species with a native species that is being displaced, but few have looked at systems where an invasive species has failed to establish itself. These types of studies are important to determine if competition is relevant not only to invading species but also to the biotic resistance of a community. 2. The thrips species F. occidentalis is a highly invasive pest that has spread from its original range (the western states of the USA) to a worldwide distribution. Despite this, F. occidentalis is largely absent or occurs in low numbers in the eastern states of the USA, where the native F. tritici dominates. It is possible that F. tritici is competitively excluding F. occidentalis from this region. 3. Larval competition between these two thrips species was tested on two known plant hosts, Capsicum annuum (a crop plant), and Raphanus raphanistrum (an invasive weed), using a response surface design with number of larvae surviving as the response variable. The response surface design allowed competition models to be fit to data using maximum likelihood estimation, thus generating quantitative values for interspecific competition. 4. On both plant hosts, the native F. tritici did not experience significant interspecific competition from the invasive F. occidentalis. In contrast, F. occidentalis did experience significant interspecific competition from F. tritici. Competition from F. tritici larvae on F. occidentalis larvae was estimated to be 1.72 times (on C. annuum) and 1.76 times (on R. raphanistrum) the effect of intraspecific competition. The invasive F. occidentalis appears to be competitively excluded by the native F. tritici. 5. This study confirms the importance of competition in the biotic resistance of a community and is one of the few animal studies to not only test for competition in an apparently resistant ecosystem but also to quantify the level of interspecific competition between two animal species.

Annual Cycles of Frankliniella spp. (Thysanoptera: Thripidae) Thrips Abundance on North Florida Uncultivated Reproductive Hosts: Predicting Possible Sources of Pest Outbreaks

Abstract Frankliniella spp. (Thysanoptera: Thripidae) thrips damage a variety of crops, feed on a broad range of hosts, and often migrate into cropping systems from adjacent vegetation. To determine potential sources of Frankliniella spp. thrips on crops, annual cycles of abundance of Frankliniella occidentalis (Pergande), Frankliniella fusca (Hinds), Frankliniella bispinosa (Morgan), and Frankliniella tritici (Fitch) were evaluated on seven common, uncultivated reproductive hosts. These hosts included Raphanus raphanistrum L., Rubus trivialis Michx., Rubus cuneifolius Pursh., Vicia sativa L., Trifolium repens L., Solidago canadensis L. and Chenopodium ambrosioides L. Thrips were collected from R. cuneifolius, and T. repens in the spring, R. raphanistrum in the summer, and C. ambrosioides and S. canadensis in the fall. The most common Frankliniella species on every plant species was F. tritici, and a fifth species, Pseudothrips inequalis (Beach), was collected in the fall on C. ambrosioides and S. canadensis. All thrips species were highly aggregated in the flowers or flower racemes, rather than leaves or fruit, and they were generally only collected from flowering plants. R. raphanistrum supported large populations, and they may be an important link for thrips between spring and fall. In addition, it may be an essentially enemy free host, as only one O. insidiosus, an important thrips predator, was collected from this host. S. canadensis also supported large thrips populations in the fall, and it may be a source of thrips migrating into crops the following spring. Controlling thrips on these hosts in their respective seasons may limit the number migrating into cropping systems.

Predation by Orius insidiosus (Heteroptera: Anthocoridae) on Life Stages and Species of Frankliniella Flower Thrips (Thysanoptera: Thripidae) in Pepper Flowers

Abstract We compared predation of Orius insidiosus (Say) on adult and second instars of Frankliniella occidentalis (Pergande) and on adults of F. occidentalis and F. tritici (Fitch) in arenas with pepper flowers. Also, we compared the dispersal of these thrips in the presence and absence of the predator. For each experiment, two densities of thrips (10 and 20 total thrips) and two time exposures (10 and 34 h) were tested. Second instars were more likely to move from the flower where they were released than were F. occidentalis adults. F. tritici dispersed more than F. occidentalis. The presence of the predator enhanced movement by thrips from flowers in which they were released. Despite differences in prey movement, O. insidiosus successfully preyed on all types of prey that were offered. However, O. insidiosus appeared to deal differently with each type of prey. Predation of both larvae and adults was most likely to occur inside flowers. In trials with adults and second instars of F. occidentalis, larvae were significantly more vulnerable to predation than were adults. F. tritici, the more active species, may have been vulnerable to predation because of higher rates of encounter with the predator; however, O. insidiosus may have had greater attack success against the less active F. occidentalis. The ability of O. insidiosus to prey successfully on different life stages and species of thrips in complex environments indicates that it is an efficient predator of thrips and an important biological control agent.

Differential predation by the generalist predator Orius insidiosus on congeneric species of thrips that vary in size and behavior

We investigated interactions between the generalist predator Orius insidiosus (Say) (Heteroptera: Anthocoridae) and two species of thrips prey, Frankliniella bispinosa (Morgan) and Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), and interspecific differences in morphology and behavior between these prey species that could contribute to differences in predation by O. insidiosus. Frankliniella occidentalis is significantly larger than F. bispinosa. Frankliniella bispinosa has greater mobility compared with F. occidentalis. When O. insidiosus was offered either F. bispinosa or F. occidentalis as prey in single species trials, there were no significant differences in the number of prey captured. However, O. insidiosus had significantly more encounters with F. bispinosa than with F. occidentalis. In arenas with equal numbers of both species, O. insidiosus encountered and captured F. occidentalis more than F. bispinosa. In large arenas with two pepper plants (Capsicum annuum L.), O. insidiosus preyed on more F. occidentalis than on F. bispinosa. These results indicate that O. insidiosus can prey on both thrips species, but that it preferentially captures F. occidentalis. The greater locomotion and movement of F. bispinosa, perhaps combined with its smaller size, allow it to evade predation by O. insidiosus better than F. occidentalis. Consequently, the observed preference of O. insidiosus for F. occidentalis is not exclusively a function of active selection by the predator but also could arise from inherent differences among prey. We propose this differential predation as a mechanism contributing to observed differences in the temporal dynamics of these species in pepper fields.