The Western flower thrips, Frankliniella occidentalis is a most economically important pest of many field- and glasshouse-grown vegetables and ornamentals.
Adults lay eggs in the parenchyma tissue and there are two larval stages (first and second instars), prepupal and pupal stages are present in the life cycle of thrips.
Adult thrips generally feed by piercing and scraping of the stem, leaf, flower and fruit tissues.
Both instars also feed on all the aerial plant parts including leaves, flowers and fruits.
Piercing and scraping of the plant tissues leads to discoloration and drying of the damaged area, in some cases, abortion of flower/leaf buds or distortion of emerging leaves, thus reducing field crop yield and aesthetic value of ornamental plants.
Thrips are also capable of transmitting tospoviruses such as tomato spotted wilt virus (TSWV) and impatiens necrotic spot virus (INSV) during feeding, thus causing a tremendous loss to agricultural and horticultural greenhouse industries.
Controlling western flower thrips is difficult because of their small size and cryptic behavior.
Western flower thrips are commonly eradicated using endosulfan, chlorpyrifos, bendiocarb, and synthetic pyrethrinoids but use of these insecticides is restricted due to their environmental pollution and human health concerns, development of resistance to pesticides and removal of some of the most effective products from the market.
Biological control agents including predacious mites (Neoseilus cucumeris and Neoseilus degenerans), predacious bugs (Orius insidiosus), entomopathogenic fungi (Beauveria bassiana, Metarhizium anisopliae) and entomopathogenic nematodes (see below) have been used as alternatives to chemical pesticides.
The entomopathogenic nematodes species including Heterorhabditis bacteriophora, H. indica, H. marelata and Steinernema abassi, S. carpocapase, and S. feltiae have been found to be effective alternatives to chemical insecticides in controlling western flower thrips.
The entomopathogenic nematodes specifically attack soil-dwelling second instar larval, prepupal and pupal stages.
Generally, Heterorhabditis species are more effective than Steinernema species nematodes in controlling western flower thrips.
The insect- parasitic nematodes such as Thripinema nicklewoodii also have a potential to use as a biological control agent against western flower thrips.
Application of entomopathgenic nematodes at the rate of 400 infective juveniles/ cm2 of soil surface can cause over 50% mortality of thrip population.
Nematodes can be easily applied in water suspension as spray applications to the surface of plant growing medium or on the plant foliage infested with western flower thrips.
Although larval stages, prepupae and pupae are susceptible to entomopathogenic nematodes, H. bacteriophora HK3 strain can cause higher mortality of larval and prepupal stages than pupal stages
How Entomopathogenic Nematodes kill Western Flower Thrips
When the infective juveniles are applied to the surface of plant growing medium or injected in the potting medium, they start searching for their hosts, in this case Western Flower Thrip larvae, prepupae and pupae.
Once a larvae, prepupae and pupae has been located, the nematode infective juveniles penetrate into the larvae, prepupae and pupae body cavity via natural openings (mouth, anus and spiracles).
Infective juveniles of Heterorhabditis also enter through the intersegmental members of the grub/pupa cuticle.
Once in the body cavity, infective juveniles release symbiotic bacteria (Xenorhabdus spp. for Steinernematidae and Photorhabdus spp. for Heterorhabditidae) from their gut in the larvae, prepupal and pupal blood.
Multiplying nematode-bacterium complex in the blood causes septicemia and kills the grub usually within 48 h after infection.
Nematodes feed on multiplying bacteria, mature into adults, reproduce and then emerge as infective juveniles from the cadaver to seek new larvae, prepupae and pupae in the potting medium/soil.