Is There a Future for Granular Soil Insecticides? Is a 20% Target Enough?
 |
Michael E. Gray, Professor and Extension Coordinator Department of Crop Sciences Tel: (217) 333-6652 Email: megray@uiuc.edu |
Since the early 1960s, granular soil insecticides applied at-planting have served as the primary control tactic for corn rootworms and other secondary soil insects such as wireworms and white grubs. Historically, Illinois’ farmers have relied extensively on these products for corn rootworm control, treating nearly 90% of non-rotated corn acres each spring. Prior to the evolution of the variant western corn rootworm, farmers across the state typically treated fewer than 15% of their first-year corn acres. Following the aftermath of the widespread damage caused by the variant western corn rootworm in 1995, farmers have come to expect potential root injury in both rotated and continuous corn acres. Crop production inputs targeted at soil insect pests are now largely regarded by many farmers as insurance premiums they have elected to purchase. This management scenario has called into question the IPM paradigm, championed by land grant entomologists for many decades.
Of increasing interest to farmers are Bt corn hybrids, plants that have been
genetically-engineered to prevent economic losses against a few key insect pests
such as the European corn borer, some other lepidopteran pests (e.g., black cutworms,
western bean cutworm), and corn rootworms. Not all Bt products are the same and
farmers are encouraged to carefully review product literature before a purchase
is made. We have learned that Bt hybrid performance can vary with respect to
corn rootworm protection even with the same event. In most instances, transgenic
hybrids have provided effective control against the two primary corn insect threats,
hence the growing popularity of stacked Bt hybrids. However, Bt hybrids do not
express insecticidal proteins that are effective against the wider array of secondary
soil insects. Consequently, seed manufacturers continue to treat Bt seed with
insecticidal seed treatments. The two most common products, clothianidin (Poncho®)
and thiamethoxam (Cruiser®), are neonicotinoid insecticides. Each has systemic
properties and the active ingredient is absorbed by the root system. These insecticides
are now in great use across the U.S. Corn Belt. Resistance to neonicotinoid insecticides
has developed previously in other insect pests such as the destructive Colorado
potato beetle. After only three field seasons, Colorado potato beetle resistance
to imidacloprid (a nicotinoid) was detected. No resistance management plan for
the neonicotinoids has been advocated or developed, and none is anticipated.
It seems almost certain that because of the ubiquitous use of these products
on Bt seed, insecticide resistance is inevitable. Product performance of the
neonicotinoids has been inconsistent against secondary insects. Against this
backdrop, do the granular soil insecticides have a promising future? Will the
private sector continue to invest in the production of these products and the
human resources necessary to market and deliver these insecticides to farmers?
Will a 20% market be attractive for some manufacturers?
Escalating use of transgenic Bt corn hybrids is anticipated as more acres are
devoted to corn production to meet the increasing demand for ethanol. Favorable
commodity prices, introgression of Bt events into elite corn germplasm, convenience
and flexibility of planting stacked hybrids, larger farms, newer bulk planter
designs, producer concerns over human health and safety during calibration and
application, and perceived yield benefits of Bt hybrids compared with their non-Bt
counterparts (even those non-Bt hybrids treated with soil insecticides), are
among the many factors that will contribute to a continuing decline in the use
of granular soil insecticides. However, granular soil insecticides can play an
important role in the protection of corn planted into refuges. Thus far, insecticidal
seed treatments have not offered consistent root protection in refuges when corn
rootworm densities are high. If resistance to Bt emerges at some point in the
future, the use of soil insecticides would again increase, assuming an infrastructure
remains in place to implement this change. Ironically, resistance to granular
soil insecticides has not developed despite their use for decades. Producers
have unwittingly employed a refuge strategy each spring by banding these products;
thus, exposing only a portion of the corn rootworm population each year to the
chosen toxin. Granular soil insecticides now seemingly have a new and important
role to play – as a refuge protector. In this role, they serve to prolong
the potential usefulness of Bt events by delaying or preventing resistance to
these new transgenic tools.