Evaluation of Insect Repellents and Barriers as Methods to Control Cucumber Mosaic Virus of Bell Peppers
Joe Nunez, UCCE-Kern
Bell peppers and chili peppers in Kern County have been afflicted by cucumber mosaic virus (CMV) for the past several years. Some fields have had over 50% yield reduction due to CMV. There is no pattern as to when it or how severe the infection will be. However earlier in the season that CMV appears, the more severe the yield loss will be.
CMV is a cucumovirus that is vectored by several different species of aphids, but most efficiently by Aphis gossypii and Myzus persicae, the cotton aphid and the green peach aphid respectively. It is transmitted in a non-persistent manner, meaning the aphid vector acquires the virus after only a few minutes of feeding on an infected plant and that it can transmit the virus for a few hours afterward.
Even though the plants are being treated with a systemic insecticide from the time they are young seedlings, fields are still being infected with CMV. The reason for this is most likely because once an aphid lands on a plant surface it immediately begins to probe the plant to see if it is a suitable host plant. Once this probing begins the virus is transmitted to the plant. Even if the aphid is killed by the insecticide, it is not killed quickly enough to prevent the vectoring of the virus. Although treating pepper fields with imidacloprid does reduce the buildup of aphids in field, it does not prevent viruses from being introduced to a field.
Two trials were conducted in spring of 2008 with bell peppers to determine if CMV can be controlled by insect repellents, reflective mulch, and insect barriers. The insect repellents are composed of botanical oils that are commercially available from California Organic Fertilizers(California Organic Fertilizers, Inc.,7600 N. Ingram Ave., Suite 121, Fresno, CA 93711). The botanical oils tested were:1) 40% citronella oil; 2) 25% citronella oil, 25% clove oil, and 5% of geranium oil; 3) 20% clove oil and 10% rosemary oil; 4) 5% garlic oil; and 5) 3% citronella oil and 0.5% garlic oil. Other treatments included a floating row cover and silver reflective mulch. The floating row cover used was AG 15 by Agribon and the reflective silver mulch was Brite-N'up Silver Mulch, both obtained from Peaceful Valley Farm Supply (P.O. Box 2209, 125 Clydesdale Court, Grass Valley, CA 95945).
The floating cover was the lightest weight available but the weave was tight enough to prevent aphids from passing through. Reflective mulches have been shown by others to repel aphids and thus reducing plant virus infections. A second trial was conducted without the botanical oils, instead looking at only the floating row cover and reflective mulch.
The trials were evaluated for aphid counts on a weekly basis by placing yellow sticky cards just above the plant canopy. The impact of CMV was determined by harvesting the bell peppers over the course of several weeks. The floating row covers were removed one week before harvest of the peppers. At harvest the peppers were graded, number in each grade counted, and weighed for yield.
Aphid counts were significantly reduced by the floating row cover and silver reflective mulch as compared to the control (figures 1 and 2, tables 1 and 2). The line graph for the floating row cover is difficult to see because it follows the zero line so closely. The aphid counts were not different for any of the botanical oils compared to the control. At harvest, the floating row cover and silver reflective mulch had significant yield increase over the control plots (figures 3 and 4). The botanical oils plots yielded the same as the control plots. The use of silver reflective mulch and floating row covers can reduce the incidence of CMV in peppers.
From this first yearís experiments it appears that keeping aphids from landing onto the pepper plants at all is the key to reducing the incidence of CMV. Although the botanical oils didnít achieve this goal, the use of reflective mulch and the floating row cover did an excellent job of this. The floating row cover did have reduced yields in one plot compared to the control and less than the reflective in the other. It may be that the floating row cover shaded the plants too much and preventing the plants form reaching their potential. Overall the results show the yields can be significantly higher with the use of reflective mulch or floating row cover.
Figure 1. Weekly aphid counts for Trial 1
Figure 2. Weekly aphid counts for Trial 2
Table 1. Aphid counts of botanical oils, reflective mulch and floating row cover in trial 1.
Table 2. Aphid counts of reflective mulch, floating row cover in trial 2.
Figure 3. Yield Total for field 1
Figure 4. Yield Total for field 2
Table 3. Harvest data for field experiment 1.
Table 4. Harvest data for field experiment 2.