Dr. Claudio C. Pasian and Dr. Richard K. Lindquist
A very useful, often overlooked tool for early detection and management of pest populations in greenhouses are the so called "yellow sticky cards." While all greenhouse growers know of their existence, some still do not use them because they do not know how to handle the information these traps provide. The following fact sheet starts out with general information that will help growers take advantage of this easy to use tool. Next, the fact sheet gives an example of data collected from the cards, what it means, and how to understand it. We encourage owners/managers to use this information when training workers to become scouts.
Sticky traps will catch pests and beneficials. Certain sticky traps will attract and catch pest insects such as winged aphids, whiteflies, thrips, leafminers, fungus gnats, and shore flies. Beneficial insects such as the whitefly parasitoid, Encarsia formosa, will also be caught at times. There will be many other species as well, but most of them are of no concern to greenhouse growers. The primary task is to recognize the problem insects.
Sticky trap color. Traps that reflect certain wavelengths of yellow or blue are most often used. White or red traps are also effective for some insects. Most studies show that blue traps are better at capturing western flower thrips and shore flies, so if these insects are the only problems, go ahead and use blue traps. However, we suggest that yellow traps be used in a monitoring program that will include whiteflies and fungus gnats.
Number of traps needed. The number of traps needed depends on the main target pest. For example, for western flower thrips, you can get a fairly good idea of activity with one trap per 10,000 square feet. However, for a reasonably accurate picture of whitefly activity, you may need one trap per 1,000 square feet. For leafminers, the number of traps required is somewhere in between the above figures. You need to determine your main pest(s) and then check with your county or state extension specialist.
Mode of sticky trap deployment. The basic suggestion has been to hang or place traps vertically, at or slightly below the tops of crops. This works well for most species, but horizontal traps will be more effective in trapping silverleaf whiteflies early in a poinsettia crop. Also, fungus gnat and shore fly trapping is much more effective on horizontal traps--at least until the crop canopy fills in. These traps may be placed face up on bench or potting mix surfaces. Hang or place vertical traps facing the same direction(s), such as east-west. Place traps where they will be most efficient. For western flower thrips, place traps facing all four directions among the most favored plants for thrips infestation, not the most virus-susceptible crop. Place traps around crop perimeters as well as within the crop. In addition to traps, certain petunia cultivars will detect thrips injury and whether the thrips are carrying the viruses causing Impatiens Necrotic Spot Virus (INSV) and Tomato Spotted Wilt Virus (TSWV) before most other crops show symptoms.
Learning pest I.D. Pictorial guides, experience, and attending workshops and grower meetings are helpful and necessary. This may not be easy at first, because many pests are quite small. Get some kind of magnification equipment--a hand lens is a good start. With experience you will become acquainted with the major pest groups and will be able to easily identify them. If you are not sure, send samples to your nearest pest diagnostic clinic. Be certain to wrap the sticky traps in clear plastic wrap before sending.
Sticky traps as a control method. Traps can control some pests or at least slow the rate of increase of pests such as whiteflies, if you have sufficient trap surface area in the crop and begin before pest numbers get out of hand. Some growers use strips of sticky yellow tape, from 4 to 12 inches wide, strung among plants to accomplish this. There is very little information of a scientific nature here, except for some studies on greenhouse whiteflies on greenhouse tomatoes in Canada. However, a number of growers have observed whitefly numbers were lowered by using yellow tape. Always use sticky traps combined with other techniques. Do not rely on this method alone.
No silver bullets. Sticky traps alone cannot adequately detect all of the most serious pests. Traps will only capture winged aphids, which often do not appear until numbers are very high. Whiteflies start out in scattered areas within a crop, and traps may not be placed in these areas. The numbers of thrips caught on traps may not be related to numbers on plants. Further, the thrips on the traps may not even be pests of the crop(s) being produced. Spider mites and caterpillars (or moths) are not generally caught on traps. Moral: use sticky traps along with regular (at least weekly) plant inspections, and get suspected pests identified.
Sticky traps examination. If you want a picture of population trends, examine traps weekly. If you want to see a "snapshot" of pest activity at the moment, place traps in the greenhouse for a few hours once or twice each week. The important matter is to be consistent. Be careful about deploying traps just prior to using some pyrethroid insecticides. These insecticides can cause an increase in insect activity, leading to misleading conclusions about insecticide effectiveness and pest populations. Insect count can be facilitated by using sticky traps with a background grid. The grid becomes especially useful when there are many insects on the card.
Counting the Pests. Depending on your objectives, you can count all of the pests on each trap, those on a one-inch vertical strip on each trap, or place pest numbers in categories, depending on estimated numbers. For example, categories could be none, few, some, too many. You will have to establish these levels depending on the pest crop and season. The numbers of western flower thrips that can be tolerated will be higher if INSV is not a problem. Do what you can accomplish consistently.
Insect counts (a hypothetical example). Periodically (once a week would be ideal), the sticky cards should be inspected and the stuck insect-pests counted. Numbers should be written down in forms such as the one shown in Table 1. This form is used to collect information from one card.
|Table 1. Insect counts for "Card 1" located in section "A-1." Observations were made during a one-month period.|
|STICKY CARD INSECT COUNT|
|Card number: 1|
|Date of |
|Spray||Insects present on cards|
In this hypothetical example, we are dealing with card "1" located in section "A-1." The card was placed on the date of 1-1-98. On that date, all numbers are zero because the first observation takes place when the card is first placed. One week later, the scout counted 2 fungus gnats and 3 thrips. The other two cards in section A-1 gave similar results: nothing to worry about. On 1-15-98, the scout detects 32 fungus gnats and 4 thrips. The scout considered that the fungus gnats level was too high and ordered a control treatment in section A-1 (the treatment is indicated in Table 1 by a check mark). The 1-23-98 inspection indicated that there were 32 fungus gnats and 6 thrips. The lack of increase in the number of fungus gnats on the card may be interpreted as a successful treatment. The number of thrips grew slightly because the product used has no effect on this pest. That day, the scout decided to replace that card because it was too complicated to count 38 insects (32 + 6) in addition to other nonpest insects. The scout changed the card on 1-25-98. The following inspection yielded only two thrips. The scout concluded that the fungus gnat problem had been controlled and the thrips population was low and did not warrant treatment.
Having a form or table for each card has a disadvantage: the scout has to carry around a lot of forms during each inspection. The advantage is that trends are seen immediately without having to plot the results. An alternative form is shown in Table 2. Insect counts presented in Table 1 (sticky card 1 in section A-1) are shown in Table 2 along with insect counts from the other two cards in that section. Note that trends are not clearly visible in this form.
|Table 2. Insect counts for the three cards located in section "A-1." Observations were made during a one-month period.|
|YELLOW STICKY CARD INSECT COUNT|
|Number of insects present on card|
These two forms are just examples that should allow growers to start their scouting process. They are not meant to be THE insect count forms. Each scout should redesign the forms according to his/her preferences.
In order to be successful, the scouting program should be the responsibility of one employee. The best formula for failure is when the owner or head grower takes this responsibility on his/her own: there is always something more urgent to do...
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Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
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