Dr. Galen Dively, Professor of Entomology, University of Maryland
Organic farms in intensively managed agricultural landscapes are usually next to or surrounded by conventional agriculture. To prevent contact with a prohibited substance applied on adjoining farms, organic certification requires defined boundaries and buffer zones, which can be as narrow as a tree line or hedgerow 25 feet wide. Although such buffers may prevent the movement of soil and pesticide drift, insect pest populations move about the landscape in a much larger spatial scale; thus, pest management practices used in conventional farming can influence pest populations and control measures applied on organic farms. Presented here are case examples of how conventional pest management practices have both positive and negative impacts on organic farming.
Regional suppression of pests. Bt corn hybrids containing the bacterium Bacillus thuringiensis (Bt) genes are now planted in >90% of the corn acreage in the mid-Atlantic region. Expressed insecticidal proteins in the plant tissue provide 100% control of the European corn borer (ECB). Because corn is the major reproductive host plant of this insect, areawide adoption of Bt corn has significantly reduced population recruitment of corn borer moths that migrate to and lay eggs in other host crops. Bt corn provides only partial suppression of the corn earworm (CEW), which is more tolerant to the expressed proteins. However, larval development is significantly delayed in Bt corn, so surviving earworms pupate later in the summer, are triggered into diapause, and stay in the soil to overwinter; thus, fewer moths emerge to infest late season crops.
With data spanning four decades and across the mid-Atlantic region, a recent study (Dively et. Al. 2018) published in the Proceedings of the National Academy of Sciences analyzed trends in ECB and CEW activity, prior to (1976 – 1995; pre-Bt corn) and since Bt corn introduction (1996 – 2016). The study shows significant regional population suppression of both pests, resulting in potential benefits to conventional and organic vegetable growers. Mean nightly captures of ECB and CEW moths declined significantly from means of 6.8 and 7.5 moths respectively during 1976-1995 to less than 2 moths per night during 1996-2016, a net decline of 72-75%. The decline in moth captures is significantly related to increasing percentage of Bt acreage planted. ECB damage in untreated sweet corn and peppers also declined as a function of Bt corn adoption. Using long-term data on ECB damage from insecticide efficacy trials, mean ECB damage in peppers in the mid-Atlantic region decreased significantly from 35% during 1980-1995 to 8% since Bt introduction in 1996, a 78% reduction. Similarly, mean sweet corn ear damage by ECB significantly declined from 50% during 1984-1995 to 15% since Bt corn introduction, a 70% reduction. Trends in control action based on blacklight trap counts of adult moths also showed a significant decline in the number of recommended insecticide sprays per crop cycle in each vegetable as a function of the percentage of Bt acreage planted. Furthermore, reported insecticide use in vegetable crops confirms the reduced usage over the past 25 years. In New Jersey, the total amount of insecticides applied in sweet corn and peppers declined by 79% and 85%, respectively. Taken altogether, the regional suppression effect of Bt corn adoption has had a positive impact on organic farmers by reducing ECB and CEW populations in their crops. Both pests have broad feeding and migratory behaviors as economically important pests on many crops, including field corn, sweet corn, popcorn, green bean, lima bean, cowpea, peppers, tomato, okra, potato, and soybean. Growers of these crops may also benefit from the regional suppression.
In addition to the regional effects of Bt corn, there are documented cases of more localized positive influences from conventional pest management practices on nearby organic farms. One case is the PRSV-resistant transgenic papaya that has been commercially grown in Hawaii since 1998 to control ringspot virus disease. Large-scale plantings of transgenic papaya provide a buffer zone to protect organic and nontransgenic papaya grown nearby. Virus-infested aphids feed on transgenic plants without causing the disease, which removes the virus from the aphids before dispersing to nontransgenic plantings. This has drastically reduced the amount of available virus inoculum and allowed growers to produce an economic crop of nontransgenic papaya. Similarly, organic farmers also could be benefiting by management practices used to control other crop diseases and possibly weeds on conventional farms. Arguably, overall weed pressure on commercial farms has been significantly suppressed by the herbicide tolerant crops, which have probably reduced the seed bank size of wind-dispersed weed species. In theory, this could reduce weed pressure on neighboring organic farms; however, to date there is no scientific evidence to document any positive impact.
Pollen contamination. As a potential negative impact, organic farmers have concerns about contamination of their crops, particularly sweet corn, due to the outcrossing of pollen from adjoining Bt corn fields. This route of genetic contamination depends on a number of factors, i.e. the distance between the Bt corn and organic crop, overlap in pollen shed, area of Bt corn around an organic farm, and the speed and direction of prevailing winds. It is important to note that corn pollen is one of the heaviest of the wind-pollinated plants, and pollen dispersal studies report that corn pollen deposition drops by 50 to 75% over distances of just 2 to 4 m from the field edge. A Greenpeace study in Germany found that Bt pollen contamination of organic corn declined to 2% and resulted in a contamination rate of 0.05-0.2% at 10 m. Pollen contamination is usually very low and easy to minimize by isolating organic crops both spatially and temporally from adjoining Bt corn. The organic farmer can plant sweet corn as an early crop to avoid high insect pressure later in the growing season or plant several weeks later than conventional farmers to avoid overlapping pollination periods; or not grow organic corn directly adjoining Bt corn. While these methods can limit the amount of pollen contamination, it is virtually impossible for organic farmers to grow an entirely pure crop. For this reason, the National Organic Program does allow a presence of GMO traits within organic crops as long as the organic farmer has met the required qualifications and regulations to reach certification. To date, there has never been an instance in which an organic farm has lost its certification because of cross-pollination with GMOs.
Pesticide drift. A more serious issue for organic farmers is the off-target contamination due to the airborne movement of pesticides from a conventional farm. Pesticide drift has always been a recurring problem, even on conventional farms. An organic farm next to a conventional farm should have a buffer zone in place to reduce the risk. Talk to your neighbors concerning spray applications and explain that you have sensitive crops and would appreciate it if they would spray when the prevailing winds are light or are moving away from your land. It should be noted that drift problems could increase now that conventional producers are experiencing Roundup-resistant weed problems and may be switching to the new 2,4-D and dicamba-resistant soybeans. Because these broadleaf weed herbicides do not harm grasses, they are also commonly used for lawn and turf management, and along roads, powerlines and railroads. Under certain weather conditions, these herbicides can vaporize days after application and drift off-site and cause damage to non-target crops. To date, there have been few documented incidences of herbicide drift on organic farms in the Mid-Atlantic region. More restricted use directions will be placed on these herbicides for 2018.
Insect resistance to Bt proteins. As mentioned above, transgenic Bt corn engineered with genes expressing insecticidal proteins are a major tool in insect pest management. With its widespread use, insect resistance is a major threat to the sustainability of the Bt technology. For all Bt corn types, the high dose requirement for resistance management is not achieved for corn earworm, which is more tolerant to the Bt proteins. In a long-term Maryland study (Dively et al. 2016), Bt expressing sweet corn hybrids were used as in-field screens to measure changes in field efficacy and Bt protein susceptibility to corn earworm. Results show significantly increased susceptibility and reduced control efficacy of Cry1Ab Bt sweet corn to corn earworms, since its commercial introduction in 1996, and significant reductions in field performance of Cry1A.105+Cry2Ab2 sweet corn, particularly during 2015-2017. Supportive data from laboratory bioassays confirm that surviving earworms collected from Bt sweet corn are now 54 times more tolerant to the Cry proteins than a susceptible laboratory strain. Together, this rapid change in field efficacy in recent years and decreased susceptibility of corn earworm to Bt sweet corn provide strong evidence of field-evolved resistance in H. zea populations to multiple Cry toxins. Many conventional farmers either have stopped growing Bt hybrids or are applying more insecticide sprays to compensate for the reduced control efficacy. Additionally, since 2002, field-evolved resistance has been reported in fall armyworm for Cry1F protein in Bt corn, and bollworm (corn earworm) for Cry1Ac protein in Bt cotton.
The rise in insect resistance to the insecticidal Bt proteins has important implications for pest management in organic farming. There are 53 different Bt products listed on the OMRI website that are certified organic and may be used as a pesticide to control caterpillars and other insect pests. Many organic farmers use these insecticide sprays to control corn earworm and fall armyworm on several important vegetable crops, especially sweet corn, tomato, okra and cotton. Products such as DiPel, Javelin, Thuricide and others contain spores and active Cry 1 and Cry2 proteins that are biologically the same as those expressed in Bt corn. Thus, since they have the same mode of action, insect resistance to Cry proteins expressed in Bt corn and cotton has ultimately reduced the control efficacy of Bt insecticide sprays. Some organic farmers are experiencing this negative impact; however, Bt resistant populations of corn earworm and fall armyworm may be localized in the mid-Atlantic region to some extent and could change from year to year depending on the source of moths carried northward on weather fronts. Currently, this only affects control effectiveness of Bt sprays for corn earworm and fall armyworm; most products still work on cabbageworms, loopers, and other caterpillars.
Insect resistance to conventional insecticides. There are several examples of insect populations developing resistance to conventional insecticides that have negative impacts on organic insecticide use. Pyrethroid insecticides are commonly used in vegetable pest management to control insect pests. Because there are many inexpensive generic products, this class of insecticides has been used extensively to control corn earworm (also known as fruitworm on tomato, headworm on grain sorghum, podworm on soybeans, and bollworm on cotton). Resistance monitoring has been conducted over the last 15 years to track the efficacy of pyrethroids against this insect. When first introduced, pyrethroids provided 95 to 98% control of corn earworm, but currently control efficacy has declined to around 50% due to resistance development. For conventional farmers, it is becoming increasingly necessary to shift to the more expensive and newer classes of insecticides. Pyrethroid insecticides are not certified organic; however, there are 23 pyrethrum products, such as Azera and PyGanic, on the OMRI list that are certified organic. These products are made naturally in contrast to the synthetic pyrethroids, but they use the same mode of action to kill insects. Thus, conventional use of pyrethroid insecticides has compromised the control efficacy of organic pyrethrum products, which affects many resistant insect pests in addition to the corn earworm.
Another example of how insect resistance development negatively impacts organic pest control involves the use of conventional insecticides that contain spinetoram (Radiant SC) or spinosad (Blackhawk, Consero, Conserve). These products have been commercially available for more than a decade, and both active ingredients have the same mode of action as 22 organically certified spinosad insecticides. One in particular is Entrust which is widely used in organic production. Unfortunately, conventional use of this insecticide class has led to resistance development in populations of several important insect pests (i.e. beet armyworm, onion thrips, western flower thrips). This means that products like Entrust are likely to have less control efficacy against these pests.
Emergence of new pests. Western bean cutworm is a new emerging pest that is increasing in population size and expanding its range from the Midwest eastward over the past decade. It has become a serious pest of sweet corn, legumes and dry beans in Ontario and parts of New York, detected in Pennsylvania with minor economic injury, but only first sightings elsewhere in the mid-Atlantic region. Most entomologists agree that the widespread adoption of Bt corn has led to this population shift. As an ear feeder, larvae are normally outcompeted in non-Bt ears by corn earworms which are highly aggressive and cannibalistic. In Bt ears, western bean cutworms are highly tolerant to the Bt proteins and able to survive due to the lack of competition with corn earworm. In effect, this shift in interspecific behavior has created a new pest problem for both conventional and organic farmers. If this pest reaches economic levels in the mid-Atlantic region, it will be very difficult to control with organic insecticides.
