Insect and mite pests during the dog days of summer
Monday, 02 August 2010 11:04
The hot, dry summer has resulted in outbreaks of western flower thrips, spider mites, potato leafhoppers, and tarnished plant bugs have also been troublesome. Smaller, black melon aphids also tend to be more prevalent in the summer and fall months.
Thrips
Thrips are pollen feeders, so as more plants are flowering, their populations tend to increase. In the summer months, as grasses or nearby hay fields are mowed, thrips often enter greenhouses thru vents and open roofs. (It is often helpful to close the roofs if nearby areas are being mowed in the evening.) They also may migrate to yellow flowering weeds such as galinsoga, so good weed control always helps.
Here’s an example of how temperature speeds up their development:
Temperature: Egg to Adult Development Times for Western Flower Thrips
(On chrysanthemum leaves, 45-55% Relative humidity, Robb 1989)
59F 39 days
69F 26 days
77F 12.9 days
81F 10.2 days
80F 9.3 days
90F 10.7 days
With the increased development time, and emigration of thrips, it is very difficult to keep thrips levels at those maintained in the spring before plants bloom. Sticky card counts vary depending upon the how attractive the plants are in the greenhouse to thrips, whether the plants are flowering, placement of cards, what stage of thrips is present, and migration of thrips into greenhouses. Growers also need to relate sticky card counts to crop damage. For example, high numbers of thrips may be present on the sticky cards in open roof greenhouses, but limited plant damage to garden mums may be observed.
Working grower action thresholds of 10 per card may be appropriate only during spring production. Higher action thresholds may be more appropriate in the summer months for garden mum and poinsettias. (Woody plants such as roses and poinsettias are not hosts of impatiens necrotic spot virus).
If using chemical sprays, spray intervals need to be shortened to every 3-4 days. For more information see, Western Flower Thrips- Management on Greenhouse Grown Crops
Nematode application frequency needs to be increased in hot summer weather conditions. It's also important to apply enough moisture with the nematodes. Sprays may dry out too rapidly in the high, dry environment. One grower says he is now applying 500 gallons instead of 200 gallons of spray solution to the same sq footage. As with any biological control measure, they are most effectively used preventively in conjunction with good cultural practices for thrips control (sanitation, rigorous weed controls, etc).
For more information see the article: Nematodes in Pest Management.
Fact Sheet: Field Grown Cut Flowers: Tarnished Plant Bugs
Leanne Pundt, University of Connecticut
