Battlefield Shifts To Soil Pest Control
Editor’s Note: American Vegetable Grower introduces Shimat V. Joseph, an IPM entomology advisor at the University of California (UC). He joins UC’s Steve Koike, Richard Smith, and Michael Cahn as authors of the the “View From The West” column.
There was a time when foliar pests, specifically larval stages of moths, were very tough to manage. Subsequently, growers suffered crop losses as they failed to suppress worm infestations. Over the past decades, with persistent efforts, scientists have succeeded in developing novel pesticide chemistries and learned how to use these chemistries to target vulnerable life stages of these foliar pests.
Parallel research also progressed in developing crops containing the imported genes (such as the Bt gene) from unrelated organisms; the activity of such genes reduced the feeding damage from foliar pests. Therefore, these days growers have pretty effective tools which, when appropriately used, can successfully manage moth pests.
The Soil Arthropod Challenge
Soil arthropods, on the other hand, can be a challenge to keep under control. These pests either attack the seeds once planted or feed on the roots once seedlings start growing. Most of the known arthropod soil pests of economic importance are certain flies and beetles.
Adults of these pests are free-living and only the larval stages cause the feeding damage. For instance, one of the devastating pests of brassicas is cabbage maggot. The adults of cabbage maggot feed on nectar and pollen, and they lay eggs at the base of the plant. The hungry maggots that hatch from these eggs seek the roots and immediately start causing root damage. Other destructive root feeding pests, such cucumber beetles and wireworms, have a similar biology.
Not all soil pests have a free-living stage. There are pests that spend their entire lifetime in the soil. These pests also feed on roots of crops and cause retarded growth and even plant death.
An example of a soil arthropod pest of this nature is the garden symphylan. Garden symphylans are tiny, centipede-like creatures that can cause severe feeding damage on several crops including lettuce, artichoke, celery, and tomato. They tend to collect in certain spots in the field, resulting in a patchy distribution of damaged plants.
Their occurrence is unpredictable and populations vary from field-to-field and year-to-year. Garden symphylans have been detected as deep as 10 feet below the soil surface.
It is believed this pest uses the channels created by other soil invertebrates to move through the soil profile. Garden symphylans are hard to manage because very little is known about their biology and habits.
Other Pests To Consider
Other arthropods such as springtails or bulb mites could be significant pests under favorable conditions especially during early spring. Unlike garden symphylans, springtails or bulb mites do not move between the root zone and deeper soil profiles. These arthropods reproduce into huge numbers when there is plenty of food (incompletely decomposed crop residues) and soil moisture and temperature conditions favor their development and survival.
In California’s central coast, soil temperature often remains on the cooler side in winter months and rarely goes to freezing conditions. Crop residues incorporated into soil in late fall decompose at a slower rate and provide optimum conditions for these pests to rapidly multiply. When seeds are planted into fields having high populations of pests, these soil pests are more likely to invade the germinating seeds, which could result in poor germination, reduced crop stands, and eventually yield losses.
Growers and pest control advisors in the central coast of California find that soil pests are increasing in importance and are often reaching severe levels. They differ in opinion, however, about why these soil organisms are emerging as key pests.
Some think that soil pests are now a problem because the industry lost access to several older pesticides that were used previously. It is believed that older chemistries have longer residual activity than the current ones. As a result, they provided long-lasting control against soil pests. Two examples are chlorpyrifos and diazinon, which are heavily regulated due to their non-target effects and contamination of water bodies after regular use in agriculture.
Others believe that because the older chemistries were used repeatedly over the years, these materials are now gradually starting to lose their activity and are losing the battle against the pests. Regardless of the actual cause, all these changes make one wonder whether we are entering into a new chapter of pest management.
Conventionally, soil arthropod pests are managed by broadcast spray, granular or banded-spray applications. These methods of pesticide delivery might have been appropriate for older generation crop protectants. These materials are placed close to the seeds or they move in the soil to reach the target site of pest activity, providing a greater level of suppression.
Because of the unique movements of soil pests such as cabbage maggots and garden symphylans, for new generation products to be effective against them, their chemical natures must allow for some movement in the soil profile and for some long-lasting persistence. However, new reduced-risk products may lack such features, making management of such pests difficult to control and perhaps out of reach. It is time to re-evaluate our pesticide delivery strategies to find a right fit for new pesticides compatible with the biology of target soil pests.