Biology

Thrips are tiny insects with fringed wings that can range in size from 0.5mm to up to 14mm [1], although most are only about 1mm in length with slender bodies. Thrips are a diverse group of hemimetabolous insects that have a range of diets consisting of both plant and animal prey, depending on the species in question. These tiny insects feed on

Frankliniella tritici (Fitch); the eastern flower thrips. Photo from University of Florida Entomology.

prey items with rasping-sucking mouthparts that resemble a soda straw. Although thrips possess two mandibles, or jaw-like structures, the right mandible is greatly reduced and vestigial [2]. The left mandible is predominant, and is used to pierce the cell wall of plant tissues [3]. Some species inject an enzyme into food tissues that is used for extra-oral digestion from tubes called stylets in their mandibles [4]. Their bodies are elongate and slender. Some species have fringed wings, making them capable of flight.

Life Cycle

Thrips undergo incomplete development in which they begin their lives as an egg, which may take several weeks to develop, but can hatch in as little as a day [1]. Thrips have two immature stages, in which the insect is wingless.

After the second immature stage, thrips enter a dormant stage in which they are mostly immobile and do not feed [1].

All described thrips species also utilize a form of asexual reproduction called parthenogenesis in which an adult female can lay an unfertilized egg to develop into a genetically identical female offspring [1]. Sexual reproduction seldom occurs in all species of thrips.

Thrips in your rose garden

Thrips are difficult to spot on a rose blossom, as they are generally located at the base of the petals [6]. If you think your rose plants may be impacted by thrips, there are a few steps you can use to monitor for these insects:

1. remove a physically healthy blossom from your plant
2. peel back the first layer of petals, checking the base of the blossom for tiny brown to orange insects approximately 1/16 inch long
3. check for browning areas, which can be especially visible on lighter colored blossoms
4. blow gently into the blossom and pay close attention for any small movements within

More overt physical damage of rose plants by thrips can also be present. This damage is generally characterized by deformed buds that may only partially open, or even fall from the plant before full bloom. Thrips feeding on rose petals may also result in a flecking pattern on leaves near the affected buds.

Thrips damage on roses is often more apparent on lighter-colored blossoms. Photo from Clemson University Entomology Extension.

Control

The physical control of thrips in your rose garden, or any agricultural setting for that matter,  is often quite difficult. Below are a few precautionary steps that you might take to prevent the further spread of Thrips infestation in your rose garden:

• remove damaged buds and blooms and dispose of them well away from the planting area
• keep weeds and grass trimmed back and away from the base of your rose plants
• in the case of extensive bloom damage, a periodic spray of insecticide prior to thrips entering the rose blooms can be used to suppress Thrips infestations.

Economic Impact

Many thrips species are considered agriculture crop pests due to herbivory of flowers, fruits, leaves and stems of plant commodities of economic importance. Many species are also known to be the only insect vectors of plant Tospoviruses [1], which can cause

Scanning electron micrograph of a thrips nymph feeding on a leaf midvein. Photo author: Murray Gans, Scanning Electron Microscope Blog.

spotting and wilting of the fruit and leaves, reducing market value of the crop in question. The western flower thrips, Frankliniella occidentalis (Pergande) is considered a widespread plant pest worldwide, and is the primary vector of plant Tospoviruses [1]. In a two-year North Carolina State University study on the effect of thrips on peanut plants in Virginia, of symptomatic plants tested for Tomato Spotted Wilt Virus (TSWV, a type of plant Tospovirus), more than 98% of plants tested positive for infection [5].

Management Strategy

Due to the small size of thrips and the confined space of their natural habitat (the inside parts of flowers and tight spaces of plants), control strategies are often limited to chemical pesticides, such as aldicarb and phorate [5]. Some biological pesticides, such as Beauveria bassiana, have been tested as well, but with limited efficacy. Biological control agents are also used, including species of parasitoid wasps, but these insect control agents must be small enough to fit into confined spaces to exhibit active predation. Often, outbreaks of thrips within a host plant species are left unchecked until a point of economic threshold is reached [1], in which insect damage negatively affects profit margins of the commodity in question.

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References

[6] J.M. Scott, “Rose Insects & Related Pests. HGIC Horticulture Extension Department, Clemson University. 2009.