While the organic vegetable market can be lucrative, successfully growing a healthy crop using organic methods can feel impossible at times. Multiple issues can arise, caused by either biotic (insect pests and diseases) or abiotic (temperatures, light and moisture) stressors.

Organic insecticides can have limited efficacy, are expensive and require repeated applications. Fortunately, mesotunnels have emerged as a tool for vegetable growers to provide strong insect and pathogen management options, extend the growing season and improve production quality for specialty crops.

Mesotunnels are medium-sized tunnels between 36 and 43 inches tall and are considered nearly permanent structures on the farm. To build them, bent electrical conduits are covered with ProtekNet material featuring mesh sizes small enough to form an effective barrier to insect pests (and the diseases they may transmit), therefore lowering the overall damage to the crop grown.

A bonus: Pesticides can also be sprayed through the nylon-mesh fabric.

Looking at pest management using insect netting in organic Chinese cabbage production systems specifically are graduate student Smriti Chaudhary and Professor Ajay Nair of the Iowa State Department of Horticulture (with collaborators at the University of Kentucky and Cornell University).

Chaudhary and Nair noted that mesotunnels also help in altering the microclimate of the crop, improving temperature and moisture regulation and reducing possible damage from high winds.

Their study, which they shared at the most recent Great Lakes Expo, had a goal of evaluating the effectiveness of mesotunnels for pest management and season extension in organic Chinese cabbage production.

Their experiment was conducted on certified organic land at the Iowa State Horticulture Research Station in Story County. The Chinese cabbage cultivar ‘Minuet’ was transplanted there using a water wheel on black plastic mulch.

The experiment included five treatments: 85 gram ProtekNet; 85 g ProtekNet and OMRI-listed insecticide sprays; a 0.55 oz. row cover; just OMRI insecticides; and the control (with no netting and no insecticides).

Iowa State Professor Mark Gleason, who is also studying the efficacy of these tunnels, says these mesotunnels fit a sweet spot between high and low tunnels. Photo courtesy of Mark Gleason

The mesotunnels were installed on 20-foot-long and 30-inch-wide beds the day the cabbages were transplanted. The tunnels were constructed using electrical conduit hoops approximately 3.5 feet tall and covered with the 85 g ProtekNet.

Low tunnels were built using wire hoops and covered with the 0.55 oz. row covers. Sandbags were used to secure the edges of the netting in both systems.

The researchers conducted weekly visual surveys for insect pests, focusing on major Brassica pests like aphids, loopers, caterpillars, harlequin bugs and flea beetles. Sticky traps were deployed for additional pest monitoring. Pesticides were applied on the respective treatments when pest populations reached economic threshold levels.

Data loggers also monitored variations in light intensity, air and soil temperature and relative humidity under the tunnels and in the control plots throughout the growing season.

At harvest, cabbages were categorized as either marketable or non-marketable and graded according to USDA standards. Non-marketable cabbages were classified based on specific defects: insect damage, rotting, puffiness, loose heads and small size. Additionally, the average length and width of cabbage heads were measured to evaluate head size across treatments.

When it came to marketable vs. non-marketable yield, the cabbage grown under ProtekNet alone fared the best, followed by ProtekNet + spray and then row covers. However, there were more non-marketable heads in the latter two treatments, while the former had more marketable than non-marketable heads.

The worst problem across all treatments was insect damage, but again, ProtekNet performed the best, followed by ProtekNet + spray. Rotting tended to be the second worst issue in both treatments, but it was seen in low percentages in both.

Likewise, in all treatments the biggest pest troublemaker was the flea beetle but ProtekNet and ProtekNet + spray kept their damage to a minimum. Row covers and OMRI insecticides performed similarly in dealing with insect abundance as well – not poorly, just not as well as the ProtekNet options.

Chaudhary and Nair concluded that ProtekNet mesotunnels significantly reduced non-marketable cabbage yield and significantly reduced insect abundance and flea beetle damage.

If organic growers are looking for more control options, mesotunnels may work well for them.

by Courtney Llewellyn