Reflective mulch to repel thrips in onions

Final Report for ONE06-062

Project Type: Partnership
Funds awarded in 2006: $9,911.00
Projected End Date: 12/31/2006
Region: Northeast
State: New York
Project Leader:
Judson Reid
Cornell Vegetable Program
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Project Information


Reflective silver mulch was investigated as a potential cultural tool to reduce yield losses to onion thrips. Yields were increased at two out of three cooperating farms hosting replicated trials. A cost-benefit analysis from these farms indicates that mettalized silver mulch is of economic benefit to fresh market onions growers in New York. Thrips repellency was not consistent across sites.

Note to online readers of this report: This project is data-intensive, and best understood with tables and charts available in the hard copy. To get a full copy of the report, send e-mail to [email protected] and request materials for ONE06-062.


Onions grown on black plastic are an important commodity for Northeast fresh market vegetable farms. Onion thrips (Thrips tabaci) are an annual pest capable of significantly reducing yields. Current IPM recommendations establish a very low threshold, and “tight” spray schedule for thrips with conventional chemicals. The result is up to 12 sprays per season with field Environmental Impact Quotients ranging from 126 to 414 (high). Growers report that populations quickly rebound from chemical sprays. Resistance to commonly sprayed pyrethroid insecticides in thrips has been documented in all regions of New York by Cornell University. Organic growers using the same IPM action thresholds as conventional farmers report marginal control with OMRI approved materials.

Black plastic mulch has become popular with Northeast onion growers as a cultural form of weed control. However, silver reflective mulch has been documented to reduce species of thrips on tomatoes, pepper, gladiolus, and others. This project researched and promoted the use of reflective mulch as a viable pest control strategy to reduce the use of insecticides and increase the profitability of mulched onions for Northeast fresh market vegetable growers.

Project Objectives:

Conduct mulch trials on 3 farms growing fresh market onions.

Collect data on impact of mulch on plant height, thrips population and yield.

Promote reflective mulch through a summer field meeting, multiple publications and winter educational meetings.


Materials and methods:

Four 20’ sections of black plastic (farmer provided), 1 mil Solid Silver Embossed (Pliant), and 1 mil Reflective Mettalized Silver on Black (Pliant); as well as four 20’ bare ground sections, were established on April 12, 2006 on a Honeyoe Silt Loam at a cooperating farm in Interlaken, NY. The sections were laid with a combination mulch/drip tape layer. The treatments (3 mulches and bare-ground control) were arranged in a complete randomized block design with 4 replicates. This procedure was repeated on April 20 on a Lima Silt Loam soil at a cooperating farm in Penn Yan, NY and a Honeoye loam on approximately May 15 at a cooperating farm in Elba, NY. In the center 10’ of each block bare root transplants of variety Sweet Sandwich were set by hand at Interlaken on April 19, 3 plant rows across the plastic with an in-row spacing of 8”. In the center 10’ of each block bare root transplants of variety Mars were set by hand at Penn Yan on April 28, 4 plant rows across the plastic with an in-row spacing of 6”. Bare root transplants of variety Prince were mechanically transplanted with a home-made waterwheel at Elba on May 25, four plant rows across the plastic with an in-row spacing of 4” in 20’ blocks. A label-rate application of Pyganic (organic pyrethrum, McLaughlin Gormley King, Inc.) was made to control thrips at the Interlaken farm on June 24 and again on July 19. A pyrethroid insecticide was applied at label rate on July 7 at the Penn Yan site. No sprays were applied at the Elba site. Drip irrigation was applied as needed at each site. Plant height and thrips per plant were collected on a sample of 15 plants per block biweekly throughout the growing season culminating in yield data collected on August 10 in Penn Yan, August 11 in Interlaken and September 19 in Elba. Data were analyzed using statistical software Analysis of Variance (ANOVA) procedure, and treatment means were separated using Fishers Protected Least Significant Difference Test.

Research results and discussion:

Results from each site followed by general discussion.

Mettalized silver yielded more pounds of onion per 10’ block than any other treatment. This increase is due to a yield of more jumbo bulbs than any other treatment. Final adult thrip populations were significantly lower on mettalized silver than any other treatment. Although total thrips (adults plus nymphs) were lower on mettalized silver, it was not significantly different from other mulches. Lower thrips numbers on mettalized silver were a season long trend at this site.

Mettalized and embossed silver mulch harbored significantly fewer onion thrips per leaf than black plastic and bare ground treatments until a population surge in late July. This is likely the result of taller plants with more leaves, capable of supporting more thrips in the silver mulch blocks. Harvested yield from 20’ row was significantly greater from metallized silver mulch than all other treatments. Black and silver embossed yielded significantly more than bare-ground (control). Metallized silver also yielded significantly more 2-3” bulbs than all other treatments.

Early thrips counts indicated lower populations on mettalized silver and embossed silver than black plastic or bare-ground. However final thrips counts showed significantly more total thrips on metallized silver than bare-ground or black plastic mulch. Yields were lowest on metallized silver and highest on embossed silver, but no significant differences were detected.

The use of metallized silver mulch increased yield at 2 out of 3 of our cooperating farms when compared to black plastic, bare ground and embossed silver. In Penn Yan this may be attributed to thrips repellency and in Elba to increased plant height. At both of these locations increased yields on metallized silver were correlated with an increase in the number of larger bulbs (jumbos and larges). Results from the Interlaken site are likely confused by high bacterial disease pressure, which appeared randomly distributed in the field. Many plants died prematurely and often bulbs were too soft to be pulled from the soil.

In Elba, metallized silver had fewer onion thrips than black or bareground for the first three thrips evaluations, a period more critical for the plants to be free of insect stress. Furthermore when onion leaves dry up from thrip feeding the populations move to the nearest green plants. This would explain the difference between the first three evaluations and the final count. Similarly, the Interlaken site also showed the same trend with lowest thrips numbers in the metallized mulch up until the final sampling date. From this viewpoint, in 3 out of 3 sites, metallized mulch controlled thrips at the most critical stage of crop growth.

Temperature influence of mulch may vary with variety and planting date. For example, at Penn Yan (red variety) and Elba (late planted yellow), the black plastic may have been a hindrance to bulb development during July because conditions under the plastic were excessive. As onions increase bulb size when temperatures are cooler, metallized silver may be appropriate for longer maturing varieties, grown in the heat of summer. However, for an early maturing sweet variety, such as at the Interlaken site, the early heat provided by the black plastic may be an advantage. Clearly these interpretations demand further research on temperature trends under the various mulches.

Research conclusions:

Metallized silver did suppress adult populations of thrips at all three sites early in the season. This could lead to a decrease in pesticide applications, with perhaps a single well-timed insecticide combined with the use of reflective mulch for conventional onion growers. This would be a decrease in Environmental Impact Quotients (EIQ) from 414, under current ‘high’ Cornell recommendations, to an EIQ value of less than 30.

No sprays, OMRI approved or otherwise, were used at the Elba location (certified organic). Metallized silver had fewer adult and nymph thrips for the first three sampling dates compared to other treatments, prior to a population spike. Therefore the combination of metallized silver with an OMRI approved material, prior to the population spike could be an effective approach. However, the improved plant height and corresponding bulb size under metallized silver could provide acceptable yields to organic growers, with no sprays.

A final anecdotal observation made at both Elba and Penn Yan was a decrease in weed population on metallized silver mulch (some weed growth through transplant holes is expected). Our current speculative hypothesis on this observation is that weed seeds may have experienced advantageous germination conditions (heat and humidity) under black plastic.

Participation Summary

Education & Outreach Activities and Participation Summary

Participation Summary:

Education/outreach description:

An on-farm demonstration was held at the Interlaken farm on July 28, with data from two sites presented. Results were also presented at the Finger Lakes Produce Auction Winter Grower Meeting (January 8, 130 attendance), Western New York Fresh Market Vegetable Meeting, (January 10, 50 attendance), Family Farming Conference at the New York State Agriculture Experiment Station (January 20, 45 attendance), and the Northeast Organic Farming Association-NY Annual Meeting (January 26, 60 attendance). Presentations are scheduled for the Great Lakes Vegetable Working Group Workshop, (March 1, Leamington, Ontario), the Chautaugua Produce Auction (Panama, NY, March 22) and the November 2007 Ag Inservice (Cornell University, Ithaca, NY). Results will be featured in an upcoming edition of the ‘Vedge Edge’ newsletter, posted on the Cornell Vegetable Program website ( and other industry journals as possible.

Project Outcomes

Project outcomes:

A cost/benefit analysis reveals an increase in price for metallized silver of $0.025 per linear foot when compared to conventional black plastic (0.05 vs 0.025 per foot). At the Penn Yan site, for every 0.025 invested into metallized silver we gained 1.07 lbs. in yield. At the Elba site we gained 0.98 lbs. per linear foot of metallized silver. In both situations, if the market price of onions is greater than 0.025 per pound, this technology provides an economic return. Embossed silver can be considered a midpoint in yield increase between black plastic and metallized silver; however, it is priced similarly to metallized silver and therefore gives a lower output-to-input ratio. At our Interlaken site we lost yield on metallized silver, which caused a negative return to investment.

Assessment of Project Approach and Areas of Further Study:

Areas needing additional study

Alternative pest control will be of increasing interest to all produce farmers in the Northeast as we face escalating fuel and pesticide costs as well as consumer demand for sustainable food. Based on this Northeast SARE project, silver mulch shows promise as one of these alternatives. Future research should investigate temperature trends under different colored mulches and the possible application of metallized silver mulch for other fresh market crops.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.