Are other pests causing weed problems?

Are there times when you might consider controlling weeds with an insecticide, nematicide, or rodenticide instead of an herbicide? Yes, says Carl Bell.

Bell, a longtime Imperial County farm advisor now stationed in San Diego as a University of California regional advisor on invasive plants, talked about the interaction of other pests with weeds during the conference of the California Weed Science Society at San Jose recently.

Pest herbivores — insects, diseases, nematodes, or vertebrates — affect the balance of plants in agriculture as well as the general ecology.

Intuition, if not personal experience, Bell said, suggests when a crop is damaged, competing vegetation will fill the vacuum. “We all know this, but finding data, studies, literature to verify it is pretty hard to do.”

Yet some studies do exist on the subject. One was done in the 1970s by UC weed experts Robert Norris and Carl Schoner on the interaction between alfalfa, alfalfa weevil, and weeds.

A Midwestern study measured an increase in alfalfa yield by use of an insecticide against the alfalfa weevil, with nothing done to control weeds. “If you turn that around, by not controlling insects, you have more weeds,” Bell said.

Egyptian weevils

In yet another study on variegated cutworms in alfalfa, crop yield was found to slip readily as worm counts went up, allowing weed populations to arc upward.

California growers may recall that once Egyptian alfalfa weevils strip buds and leaves from regrowth alfalfa beneath windrows, the following spring strips of grassy weeds show up where the windrows had been.

It's a matter of the competition being shifted to benefit the grass at a critical time, in this case early in the crop's development.

Research in Canada as early as the 1920s and 1930s surveyed the phenomenon. Observations of plots of equal numbers of oat and barley plants showed that oats, in the absence of a pest, typically out-competes barley. But when oat-feeding nematodes were introduced, the competition ratio shifted, and barley out-yielded the oats.

Bell said a trial on nematodes in alfalfa he did in the Low Desert in the mid-1980s is another good illustration. He set out to learn if alfalfa yield in February would predict yield in July.

Indeed, he found that a strong stand in the winter was strong the following summer. But he also found the health of a stand is an indicator of weeds. Areas weak in the winter had more grassy weeds in the summer, or simply, the lower the alfalfa yield, the more weeds.

He also attributed the typical decline of desert alfalfa stands to root damage over time by diseases, facilitated by over-irrigation. With damaged roots, plants don't fare well in the heat.

Conceding that he may not have convinced all his colleagues, Bell said he is “personally convinced that grass in alfalfa is almost solely a function of how strong the alfalfa stand is.”

Vertebrate herbivores such as cattle or meadow mice are selective in which plants they eat, and their preferences influence the mix of plant species afterward.

This, he added, calls to mind the traditional western rangeland antagonism fired by different grazing habits of cattle and sheep, even though the livestock are not considered pests.

Meadow mouse

A common pest in the Imperial Valley and Coachella Valley, he said, is the meadow mouse, which feeds selectively on emerging melon seedlings. Grass sprouts where the seedlings are destroyed. Grass also emerges where gophers destroy alfalfa roots and disturb the soil. Despite anecdotal material, few good “proofs from the ground” are in the research literature.

In wildlands, he said, native insects, diseases, nematodes, and herbivores, large and small, prefer native, not introduced, plants. When native plants are somehow weakened, the balance tips and non-natives flourish. The same appears true for a weakened crop when a weed is present.

All this seems obvious to the layman, and evidence can be found. Nevertheless, Bell reiterated that the subject is relatively under-researched, and he encouraged weed scientists to assign graduate students to investigate it.

e-mail: [email protected]

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