Mulch Repels Pests

Mulch Repels Pests


As reported in American Vegetable Grower just over two years ago, Nunez had embarked on a three-year study to see if pepper plants could be protected from cucumber mosaic virus with mulch. In his part of the country — the south end of the San Joaquin Valley — the virus has proved tough to pin down since it first appeared about seven years ago. There is no pattern as to when it appears or how severe the infection will be, says Nunez. Some fields have had minor infections, and some fields have had more than 50% yield reductions, but he’s also seen fields that had to be disked under.                                                                                     

“Planting” Soil
As California Cooperative Extension Farm Adviser Joe Nunez’s trials demonstrate, reflective mulch appears to be the best way to repel aphids. The only problem is that for growers who don’t already use plastic mulch, it represents an additional expense. Also, with plastic mulch, there is the added headache of having to dispose of old mulch when it begins to deteriorate.
But because it’s not the plastic itself that repels the virus-vectoring aphids, but the reflective quality, Nunez decided to paint the soil around the plants with the kaolin clay product, Surround (Tessenderlo Kerley). It worked. “I was really surprised how well it worked,” he says. “Also, (unlike mulch) it doesn’t have to be removed. It’s worth looking at on a commercial scale.”
Nunez believes the clay is worth considering because of cost alone. The cost per acre for the various mulches was $178 for black, $296 for green and $370 for silver. But the Surround would be $50 if applied twice at 25 pounds per acre each time. That’s just the material cost, and the labor and tractor work would be more for the mulch than a ground spray rig for the kaolin clay. “I hope to look into the clay more,” he concludes.

Cucumber mosaic virus (CMV) also doesn’t affect all peppers the same way. While it can be quite serious on the long bell peppers, it doesn’t seem to be much of a problem at all on other bells and chili peppers. Because of the way it only hits certain types, many growers felt CMV might be coming in on transplants. But after checking hundreds of transplants, which all proved clean, Nunez became convinced that CMV was being vectored in the field by aphids. It can be vectored by several species of aphids, but most efficiently by the cotton aphid, Aphis gossyppi, and the green peach aphid, Myzus persicae.

What’s particularly problematic is that killing the aphids doesn’t necessarily prevent CMV. Even though the plants are treated with a systemic insecticide from the time they are young seedlings, fields are still being infected. That’s because once an aphid lands on a plant it immediately begins to probe to see if the plant is a suitable host, says Nunez. Once the probing begins, the virus is transmitted. The insecticide may do in the aphid, but not quickly enough. “Killing them doesn’t do any good,” he says. “You’ve got to keep them from landing on the plants in the first place. That’s where I got the idea to repel them.”

Silver Pays Off
Nunez set up a trial evaluating several types and colors of mulch. The biggest surprise? “The growth differences with some of the mulches,” he said. “The red and the green two years in a row increased both the size of the plants and the yields. I’d read that other people have found tomato yields increased, but I doubted it. I didn’t think color would have any impact. I saw it last year, and I kind of doubted it. But this year I weighed the plants and roots as well, and they were also bigger.”

However, Nunez emphasizes that reflective silver mulch would be superior to red and green mulch if there was a lot of aphid and virus pressure. It doesn’t matter how large the yields are if the plants are badly infected with CMV, because the crop wouldn’t be marketable. “In the first year of the study there was high virus and aphid pressure and the reflective mulch kept aphids off. Last year, there were hardly any aphids, even in the control. This year there were lots of aphids, but not much virus. If there were high virus pressure this year, the red and green wouldn’t have done so well. The silver mulch would have done much better.”