Western flower thrips, Frankliniella occidentalis, first arose as an important invasive pest of many crops during the 1970s–1980s. The tremendous growth in international agricultural trade that developed then fostered the invasiveness of western flower thrips. We examine current knowledge regarding the biology of western flower thrips, with an emphasis on characteristics that contribute to its invasiveness and pest status. Efforts to control this pest and the tospoviruses that it vectors with intensive insecticide applications have been unsuccessful and have created significant problems because of the development of resistance to numerous insecticides and associated outbreaks of secondary pests. We synthesize information on effective integrated management approaches for western flower thrips that have developed through research on its biology, behavior, and ecology. We further highlight emerging topics regarding the species status of western flower thrips, as well as its genetics, biology, and ecology that facilitate its use as a model study organism and will guide development of appropriate management practices.

Integrated Pest Management (IPM) is endorsed as the future standard for crop protection worldwide. This holistic concept integrates preventative and curative measures amongst which host plant resistance (HPR) plays an essential role. Up to now HPR has been a somewhat under-utilized tool in pest management due to widespread use of pesticides and technological hindrance. Thrips are key pests in agriculture and horticulture worldwide. Here we provide an overview on the current status of research on constitutive and induced HPR including thrips-host relationships and thrips as virus vectors. We stress modulation of plant defense responses by abiotic and biotic elicitors to increase HPR and provide an outlook on the increasing potential of HPR inspired by the fast advancement of -omics techniques.

Ultraviolet radiation (UV) is an important modulator of plant defenses against biotic stresses. We have recently described that different supplemental UV exposure times and irradiance intensities enhanced tomato (Solanum lycopersicum) resistance to Western flower thrips (Frankliniella occidentalis). UV increased jasmonic acid-isoleucine (JA-Ile) and salicylic acid (SA) levels, as well as the expression of JA- and SA-responsive genes, before thrips herbivory. Here we report how UV affects tomato defense responses upon thrips infestation, and resistance to pathogens that are susceptible to the activation of SA-associated defenses. Our experiments reveal that, at 7 days after thrips infestation, UV did not enhance the levels of jasmonates, auxin or abscisic acid. UV also did not affect the expression of JA-responsive genes in the cultivar Moneymaker, the jasmonate deficient mutant def-1, the type-VI trichome deficient mutant od-2, or their wild-type Castlemart. However, UV strongly activated SA-associated defense responses in def-1 after thrips infestation. Further bioassays showed that UV increased def-1 resistance to the hemi-biotrophic bacterial pathogen Pseudomonas syringae pv. tomato DC3000, which is susceptible to SA-mediated defenses. Our results suggest that UV might enhance tomato resistance to this pathogen in the JA deficient genotype through the activation of SA defenses.

Understanding the mechanisms involved in host plant resistance opens the way for improved resistance breeding programs by using the traits involved as markers. Pest management is a major problem in cultivation of ornamentals. Gladiolus (Gladiolus hybridus L.) is an economically important ornamental in the Netherlands. Gladiolus is especially sensitive to attack by western flower thrips (Frankliniella occidentalis (Pergande) (Thysanoptera:Thripidae)). The objective of this study was, therefore, to investigate morphological and chemical markers for resistance breeding to western flower thrips in Gladiolus varieties. We measured thrips damage of 14 Gladiolus varieties in a whole-plant thrips bioassay and related this to morphological traits with a focus on papillae density. Moreover, we studied chemical host plant resistance to using an eco-metabolomic approach comparing the ¹H NMR profiles of thrips resistant and susceptible varieties representing a broad range of papillae densities. Thrips damage varied strongly among varieties: the most susceptible variety showed 130 times more damage than the most resistant one. Varieties with low thrips damage had shorter mesophylls and epidermal cells, as well as a higher density of epicuticular papillae. All three traits related to thrips damage were highly correlated with each other. We observed a number of metabolites related to resistance against thrips: two unidentified triterpenoid saponins and the amino acids alanine and threonine. All these compounds were highly correlated amongst each other as well as to the density of papillae. These correlations suggest that papillae are involved in resistance to thrips by producing and/or storing compounds causing thrips resistance. Although it is not possible to distinguish the individual effects of morphological and chemical traits statistically, our results show that papillae density is an easy marker in Gladiolus-breeding programs targeted at increased resistance to thrips.

Auxins are commonly used for commercial propagation of chrysanthemums by stem cuttings. Recent studies imply that these root-promoting hormones also affect plant defense responses. The underlying motive of this study stems from the serendipitous observation that water dipping of auxin-coated cuttings beneficially affected thrips herbivory. Therefore, the primary objective of this investigation was to explore the role of indole-3-butyric acid (IBA) in relation to herbivore susceptibility in chrysanthemum. We observed contrasting findings concerning the physical presence of IBA and it’s role in promoting susceptibility of cuttings to thrips, which may in part be explained by the phenotypical variations of cuttings generated from mother plants. Nonetheless, we repeatedly demonstrated considerable protection, in some experiments up to 37%, against thrips and leaf miner upon water dipping of IBA-coated cuttings. Assessment of polyphenol oxidase activity (PPO), 14 days after dipping treatment, suggests that neither direct induction nor priming of plant defenses are involved. Future experiments aimed at understanding the early signaling events may help to explain the underlying mechanisms involved in conferring herbivore protection. We propose a dual role for auxins in early integrated pest management strategies to maximize plant development and minimize herbivory through feasible, cost-effective water dipping treatments.

Editorial: Plant Responses to Phytophagous Mites/Thrips and Search for Resistance