Today’s post takes a closer look at and offers some suggestions for controlling one of the most widespread and detrimental species of thrips – Western Flower Thrips. Sometimes, when we strive to create the perfect growing environment for our plants we provide an optimal habitat for some nasty pests. Thrips thrive in warm environments protected from harsh outdoor conditions and their illusive nature can make them difficult to spot before the damage is done. Identifying the signs early and acting accordingly is the key to thrip control. The information below is a brief outline to aid you in doing so.
Adult WFT are less that 2 mm long. The female adult ranges in color from light yellow to dark brown. Males are slightly smaller and are light yellow in color. Both have slender cylindrical bodies and two sets of clear, nearly veinless, narrow wings that have dark, hairy, fringes. The wings also have two complete rows of setae. The larvae are tubular shaped, less than 1 mm long and are translucent white to yellow in color. They resemble the adults but do not have wings until the third instar.
The annual WFT life cycle varies depending on environmental conditions. In cold climates such as the Northern U.S. they cannot over winter outdoors. In slightly milder climates they can over winter in weed hosts, plant debris, and soil. In a greenhouse setting their life cycle is continuous and overlapping. It does however consist of these five main stages regardless of environment: egg, two larval instars, prepupa and pupa instars, and finally adult.
The population increase and time required for development of each generation is highly dependent on environmental conditions and the nutrient levels of the host plant. The following account is based on an average host and temperatures between 68 and 98 degrees F. The female can lay between 130 and 230 eggs. The eggs then hatch in 2 to 4 days. During both instars of the larval stage the larvae feed on plant material and then enter the non – feeding prepupal stage after 3 to 6 days. The pupal stage is also non – feeding and lasts for 1 to 3 days. The adult then emerges and can feed and reproduce for a subsequent 30 to 35 days.
WFT damage is caused by feeding and oviposition, as well as indirectly through virus transmission. The female’s ovipositor has two serrated blades that are used to cut through the plant epidermis and deposit eggs into the tissue. This can cause scars on the leaves and fruit of some crops and decrease their economic value. Feeding damage is caused by both the larvae and adult thrips and can occur on fruit, flowers, flower buds, leaves, and leaf buds. WFT use their modified mouthparts to puncture the plant epidermis, inject saliva, and siphon out the cell contents. The damaged plant cells collapse, resulting in distorted growth, silver patches, and scars on expanded leaves. Flower petal damage can appear as dark streaks in lighter flowers and lighter streaks in those of a darker color. Feeding on the leaf or flower buds can cause them to abort, and when they do grow they can exhibit distorted growth. This can make diagnosing thrip problems difficult because the symptoms will not be evident until the bud opens.
Western flower thrips are also notorious for being the primary vector of viruses such as tomato spotted wilt (TSWV) and impatiens necrotic spot (INSV). Adult thrips cannot acquire the virus and it is not passed on to offspring so infection occurs during the larvae phase as a result of feeding on infected plant tissue for about thirty minutes. After three to eighteen days the virus can be passed on when the insect feeds for only five to ten minutes on another host. Thrips retain the infection until death and the concentration of the virus increases as it replicates throughout the vector’s lifetime. The virus was historically common in the western and southwestern United States, but began spreading rapidly in the 1970 and ‘80s with the spread of western flower thrips.
As with any integrated pest management strategy a thorough monitoring and identification system is the first priority. Yellow and blue sticky traps attract thrips and can be used to reduce and monitor pest populations. Exclusion acts as a first defense against thrips. All plant material being brought into the greenhouse should be thoroughly inspected before hand. Production areas should be sealed tight and limited to authorized personnel. This can be achieved by the use of door seals and thrip preventive netting over all areas where external air is allowed to enter the greenhouse.
Because of the wide host range of WTF it is important to remove all weeds both inside and in close proximity to the greenhouse. These serve as safe harbor for thrips and viruses when a non – susceptible crop is being grown. If the weed removal is thorough then alternating between susceptible and non – susceptible crops can also prevent pest problems. A layer of hydrated lime thick enough to cover the soil on the greenhouse floor can prevent algae, weeds, and insects. Some growers also remove all plant material between crops for a period of seven days to allow adults to emerge from the pupal stage and then kill them with insecticides or elevated temperatures. A continuous temperature of 102 degrees F for two days will kill the adults.
Predacious minute pirate bugs, mirids, and mites have all been used to control WFT populations as well as fungi. The minute pirate bugs and the predacious mites both go into diapause under short day conditions and so have not been as affective in the winter. However, both have proven to be able to derive enough nutrients for development from WTF alone. The mites do best on plants that provide a surplus of pollen for diet supplementation in order to reach a sufficient control population faster. The time it takes for all three predator populations to establish themselves and increase to an effective control level is the main limiting factor. The rapid development and proliferation of WFT usually exceeds that of these predators in the case of most crops. Fungi such as Beauveria bassiana and Verticillium lecanii have also been used to develop registered biological controls. The spores infect the pest through contact with its cuticle and are not limited by environmental conditions or the crop being grown.
WFT are commonly controlled with endosulfan, chlorpyrifos, bendiocarb, and synthetic pyrethrinoids, but their spread has, in a way, been aided by pesticides. They have the ability to quickly develop a resistance, and when native varieties of thrips and predators are eliminated WFT face less competition and thrive in the new areas. If a chemical control is going to be used it is recommended that the pesticide class be rotated every generation (about 14 days) to help prevent pesticide resistance development. The pupa and eggs are also unaffected by sprays so it is important that the application program lasts long enough to eliminate emerging larvae and adults.