During the past seven years, over a dozen oak field trials have been established at the Sierra Foothill Research and Extension Center (SFREC). While we have discovered numerous obstacles to successful seedling establishment, including weed competition, acorn depredation, root damage, and herbivory from livestock, deer, rabbits, and voles, one of the most persistent and difficult problems has been damage from grasshoppers. This damage consists of defoliation, and in extreme cases, bark removal and girding.
The number of grasshoppers and degree of damage to oak seedlings varies greatly from year to year as population levels rise and fall. The cycle begins as eggs laid the previous fall hatch in the spring. Heavy rainfall years tend to promote the development and survival of large numbers since the grasshoppers thrive in the abundant grass produced in uncultivated areas. Some years it’s truly amazing how many there are as unprotected oak seedlings may have dozens feeding on them at the same time, and every step in grassy areas can cause scores to take flight. Population levels also typically vary greatly from location to location throughout the state. Even within the Field station, we have observed tremendous differences in the number of grasshoppers present within a couple of hundred yards.
Unfortunately this past year has been one of the worst grasshopper years we have observed since 1987, in spite of the fact that it has been a below-normal rainfall year. It began in late May when we started noticing large numbers of small grasshoppers (Melanoplus devastator) on the ground. By mid-June they seemed to be everywhere, though they did not seem particularly interested in the oaks. By late June, however, feeding on oak (and other) foliage began in earnest. During mid-July the numbers of grasshoppers, and the damage observed, seemed greatest. Typically, small seedlings would be completely defoliated and then have all their bark eaten off. Larger seedlings would be defoliated and then some of the smaller branches would be stripped, though the bark on larger branches would generally not be damaged. By mid-August, however, populations again dropped off and damage declined.
To date, we have not found a foolproof way to protect oak seedlings from grasshoppers. In our earlier trials we covered seedlings with cylinders of aluminum screen. This was reasonably effective, except that it was difficult to seal the cylinders sufficiently at their base so that grasshoppers could not occasionally crawl underneath. Once they were inside, the cages seemed quite effective at keeping them in, instead of out, and we would often find bare seedlings with a dead (but quite fat) grasshopper next to them. Another problem with this method was that as the seedlings grew, it was necessary to open the cylinders, which exposed the foliage and often attracted grasshoppers.
During the past couple of years, we have increasingly been using tree shelters (plastic tubes which act like mini-greenhouses) to protect seedlings. These devices have not only proven extremely effective in protecting plants from a variety of damaging agents (including grasshoppers), but have also promoted increased height growth (see Oaks ‘n Folks, March, 1993). However, this past summer we found that seedlings which had grown up and out of the top of 4 foot shelters were being seriously damaged by grasshoppers. Of more than 100 such seedlings in one plot, most were completely defoliated. The grasshoppers would begin feeding on the exposed foliage above the tubes, and then move down inside to eat what was left. Interestingly, those seedlings which had not yet grown above the tops of the shelters were rarely damaged.
Fortunately, oaks have evolved mechanisms for withstanding damage by grasshoppers. Most seedlings which are defoliated in mid-summer will refoliate normally the following spring. Even when plants are stripped and girdled, they will often sprout from their base the following year. While such damage can set them back considerably, it is probably not lethal unless repeated year after year.
Another fairly effective method of protecting seedlings and minimizing grasshopper damage is to eliminate favorable grasshopper habitat in the vicinity of the seedlings. Grasshoppers thrive in tall, dense vegetation. Keeping the ground fairly bare through mowing, disking, or the use of herbicides seems to create conditions that the grasshoppers don’t like, and they apparently move out into untreated areas. We have noticed substantial reductions in grasshopper numbers and damage to seedlings in plots that have only a 10-foot bare area between them and untreated areas.
In extreme cases, when the protective measures described above have not proved successful, we have used poisonous bait (bran treated with carbaryl) around seedlings to provide relatively short term protection. This is initially very effective in eliminating grasshoppers in the immediate vicinity, but may have to be repeated if there is a large reservoir of grasshoppers which continue to move into the area. Commonly, a line of this bait is placed on the perimeter of the plot so that any grasshoppers moving in will have to pass through it, and in the process ingest some of the poison. Needless to say, caution must be used and the label strictly adhered to with any poisonous baits to insure non-target organisms aren’t affected, and pets and children aren’t exposed.
It is clear from our experiences at the SFREC that completely eliminating grasshoppers and grasshopper damage in oak restoration plantings is not likely to occur. However, we feel that in most cases we can reduce damage to acceptable levels, even in outbreak years, through habitat modification and protection of individual plants.
Doug McCreary, Natural Resources Specialist
Jerry Tecklin, Staff Research Associate, Univ of California Cooperative Extension
prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc