Greenhouse Vegetable Seedling: Protocol for Managing Thrips and the Tomato Spotted Wilt Virus

Agdex#:	257/620
Publication Date:	February 1990
Order#:	90-054
Last Reviewed:	February 1990
History:
Written by:	R.E. Pitblado - Ridgetown College of Agricultural Technology/University of Guelph; W.R. Allen, D.W.A. Hunt and J.L. Shipp/Agriculture Canada

The Tomato Spotted Wilt Virus was observed extensively throughout Ontario in 1989 wherever field tomatoes and peppers were grown. The virus and its thrips vector were imported into Ontario with transplants. The virus-thrips complex, if not managed properly can cause serious losses in fruit quality and yield. Consequently, the Ontario vegetable seedling industry must guard against this complex. The industry must also assure their grower customers that an effective control strategy is being followed. The Ontario Seedling Marketing Board recommends the following strategy:

The principle management practices are exclusion of the complex through various cultural and management practices, and disease and insect monitoring with appropriate responses:

Provide a complete break in cropping for at least one month before vegetable seedlings emerge. This break includes the entire greenhouse complex.

• An effective break requires that no plant material is present in the greenhouses, including weeds and cull piles, thus causing rapid starvation of thrips.
• An effective break includes temperatures that do not fall below 18º C during the day, thus allowing thrips to complete their life cycle. This stipulation is not necessary if houses have been plant- and weed-free for several months.
• Based on seedling emergence in late March, the houses should be free of plant material no later than late February.

Commence monitoring for thrips when the first seeding of vegetables is complete. Hang blue, sticky insect-boards close to the seedling canopy and at 15 m (49 ft.) intervals the length of the bay.

• Place at least two trays of potted petunia at the front and rear of each bay. At least one additional tray of petunias should be located mid-way along each bay, near the walkway. Petunia can be grown in 10 cm (4 in.) plastic pots with 6 to 8 pots per tray. Petunia should be grown on the premises from seed sown 7 to 8 weeks prior to the first seeding date for vegetables, in order to have suitably sized petunia to place near the seedlings when they emerge. The attraction of thrips to petunia can be enhanced if insect boards without adhesive are placed upright in one or more of the petunia pots. Petunia can be cut back to maintain a reasonable height and to stimulate new leaf growth. Alternatively, plants can be replaced after about one month.

Only plants grown from seed may be produced in greenhouses used for vegetable seedling production. Early production of ornamental crops in these greenhouses is permissible if grown from seed.

Greenhouses for vegetable seedling production should not be in close proximity to houses used for flower production. Situations not meeting this stipulation must be evaluated on a case-by-case basis.

Maintain weed-free greenhouses throughout the seedling production period.

Restrict visitors to prevent the introduction of thrips - post no admission signs at entrances. Anyone who has visited other greenhouses the same day should not be permitted entrance, unless thrips-free coveralls are used.

Avoid introducing bedding plants onto your premises for decorative purposes since many producers of this material have thrips and/or TSWV.

Immediately apply appropriate insecticides if thrips are found on plants or sticky boards, or if feeding scars or virus lesions develop on indicator plants. Insecticides of choice are Malathion alternating with a combination of Thiodan + Cymbush (contact your Horticultural Crops Advisor for pest control updates). The initial 3 sprays should be applied 4 to 5 days apart, and thereafter as recommended.

• If thrips are found in only one part of the greenhouse, it may be possible to slow dissemination by hanging overlapping plastic sheets so as to form a temporary barrier or curtain.

When infected vegetable seedlings are detected, remove them immediately, bag them, and store the closed bags out-of-doors until they can be disposed of.