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Greenhouses that produce vegetables or ornamentals provide an excellent environment for various insect pests. These pests are generally very difficult to control once they enter the greenhouse, partly because of the physical conditions within the greenhouse, absence of natural enemies, and the lack of insecticides registered for use in greenhouses, especially vegetable houses (refer to FS639, " Insecticides for Greenhouse Vegetables," for labeled insecticides). Further, once these insect pests invade a house, they may rapidly spread to nearby, uninfested greenhouses, being introduced on clothing, equipment or by flying directly through an opening into the house. Screening on ventilation inlets and building entrances will prevent most, if not all, vegetable insect pests from flying into the house. Selection of the proper screen for a greenhouse depends on the size of the insect to be excluded. Dr. J. Bethke, University of California, Riverside, has found that the following common insect pests of greenhouses can be excluded using screen with the hole sizes (or smaller) shown in Table 1.

TABLE 1.

   

Screen hole size

Insect pests microns inches mesh
leafminers 640 0.025 40
whiteflies 462 0.018 52
aphids* 340 0.013 78
flower thrips 192 0.0075 132

* Although aphids are larger than whiteflies, finer mesh screens may be needed because of differences in wing placement against the body.

However, screens create resistance which reduces airflow, and the smaller the hole size, the greater the resistance. Screens placed in front of ventilation inlets reduce the amount of air passing through the inlet, and may prevent ventilation fans from operating efficiently. Thus it is important to use the proper size and mesh screen.

To determine the area of screening needed for a specific screen mesh (hole size), follow the steps listed below:

  1. Calculate the volume of the greenhouse (example: a 100' x 100' house with a 10' eave height = 100,000 cubic feet). If a thermal screen is used, multiply the calculated house area by 7.5' eave height instead of 10'.
  2. Select the mesh size to screen out the specific insect pest from Table 1 (example: 52 mesh).
  3. From Table 2, determine the approach velocity (example 52 mesh = 264)
  4. Divide Step 1 by Step 3 to get required area of screen (example: 75,000 264 = 284 square feet).
  5. Divide the required area of screen calculated in Step 4 by the house length to obtain the screen area required per foot of building (example: 284 100 = 2.8 sq ft needed per foot, or almost 3 ft 2 per ft). Note that the smaller the approach velocity figure, the tighter the screen, and thus the greater the area of screen required for adequate airflow.

TABLE 2.

Screen or screen size Approach velocity (ft/min)*
Chicopee 32 mesh 336
60 mesh, stainless steel 303
Chicopee 52 mesh 264
Nylon screen, 68 mesh 253
Filter Fly Bar new 211
Woven fabric, random 192
Econet T 110

*velocities for an allowed pressure drop of 0.03" of water.

Selecting a material with a finer screen mesh will require a larger inlet area depending upon the approach velocities listed in Table 2. These figures show that Econet T (approach velocity 110), which is recommended for thrips exclusion because of the fine mesh, allows only 1/3 the air to pass through, and thus requires 3 times the area, as does Chicopee 32 (approach velocity 336) under the same conditions.

Screens will slowly foul and become clogged with dirt and their resistance to airflow will increase. The use of a manometer to measure static pressure will help to determine when screens are becoming clogged. Periodic cleaning of the screens will reduce resistance drop.

Insect sticky traps placed throughout the house (refer to FS638, " Sticky Board Traps for Greenhouses ") will trap insects present in the house. Frequent monitoring will give an estimate of the populations and the effectiveness of the screening.

Further information concerning screening for insect pests of greenhouses can be obtained from the Department of Bioresource Engineering, Cook College, Rutgers University, New Brunswick, New Jersey, 08903.

Information in this reference appears with the understanding that no discrimination is intended and no endorsement by RUTGERS COOPERATIVE EXTENSION is implied.  

RUTGERS COOPERATIVE EXTENSION
N.J. AGRICULTURAL EXPERIMENT STATION
RUTGERS, THE STATE UNIVERSITY OF NEW JERSEY
NEW BRUNSWICK

Distributed in cooperation with U.S. Department of Agriculture in furtherance of the Acts of Congress of May 8 and June 30, 1914. Cooperative Extension work in agriculture, home economics, and 4-H. Zane R. Helsel, director of Extension. Rutgers Cooperative Extension provides information and educational services to all people without regard to sex, race, color, national origin, disability or handicap, or age. Rutgers Cooperative Extension is an Equal Opportunity Employer. 750-9402

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