Insect-Resistant Crops

Insect-resistant crops were developed to contain genes from Bacillus thuringiensis (Bt) that encode for proteins toxic to insects. Bt is a soil bacterium, which produces crystalline proteins (referred to as Cry proteins) that are toxic, to select insect orders such as lepidoptera, diptera and coleopera (Swadener, 1994). When insects ingest the protein produced by Bt, the toxin binds to specific receptors on the mid gut epithelial cells. As a result, the cell membrane develops pores, which affects the insects' ability to regulate osmotic pressure. The function of insects' digestive system is thus disrupted, leading to starvation and eventual death.

The Cry proteins have been used as insecticides since 1961, mainly in organic crop production. The commercial Bt products are available in powder formulation, containing a mixture of dried spores and toxin crystals. They represent about 1% of the total agrochemical market across the world.

There are four insect-resistant Bt crops that have been approved for commercial production in the United States to date. They are field corn, cotton, potato and sweet corn. Field corn, cotton and potato were commercialised in 1996 while sweet corn was commercially introduced in 1998.

Insect-resistant field corn was developed to express Cry1A(b) and Cry1A(c) proteins that confer protection against European corn borer (ECB), south-western corn borer, fall armyworm, corn earworms and stalk borers. Insect-resistant Bt corn has no activity on other corn pests such as aphids, spider mites, cut worms and soil insects such as rootworms, wireworms and seed corn maggots. European corn borer is the primary target pest for Bt corn. Since its introduction, adoption of Bt corn steadily increased until 1999 (26%) but remained flat at 19% in 2000 (Table 61.2). Low corn prices and low pest pressure in the past two growing seasons were suspected to be the reasons for the slight decline in adoption rate in 2000
Insect-resistant cotton acreage steadily increased since its introduction and was planted on about 28% of the total cotton acreage in 2000 (Table 61.2). Bt cotton adoption rate is higher in certain states in the United States such as Alabama, where losses from bollworm/budworm infestations are huge. Target pests for Bt cotton are tobacco budworm, cotton bollworm and pink bollworm. While cotton bollworm and tobacco budworm infest south-east and mid-south production areas, pink bollworm is prevalent in western states of the United States.

The Colorado potato beetle-resistant potatoes developed to express Cry proteins (CryIIIA) from Bacillus thuringiensis var tenebrionis were marketed as NewLeafpotatoes in 1996. NewLeafPlus with resistance to the potato leafroll virus; and NewLeafY with additional resistance to potato virus Y were developed by stacking the virus-resistance traits with Bt and were introduced in 1999. Combined adoption of all three types of Bt potatoes has been limited and planted acreage never exceeded greater than 4% in the United States (Table 61.2). The low adoption is attributed to marketing concerns.

Insect-resistant Bt sweet corn [Cry1A(b)] tolerant to lepidopteran pests such as corn earworm and fall armyworm was commercialised in 1998. Adoption of Bt corn has been very low thus far due to the reluctance of fresh corn marketers' to purchase biotechnology-derived produce.

Use of insecticides has been the most commonly used method for insect control since 1930s in the United States. Insecticides have routinely been used in an integrated approach along with cultural practices such as crop rotation, tillage, and insect-resistant crop varieties. Major limitations to the use of insecticides are resurgence of primary and secondary pests and development of insect resistance to insecticides.

Resurgence occurs when insects normally killed by insecticides return in larger numbers. When insecticides remove target insects and their natural enemies, opportunity exists for the temporarily removed pests to reproduce before their natural enemies return. Spider mites, for example, caused havoc when DDT and other insecticides killed their predators.

The problem of insect resistance to conventional insecticides is already a serious issue, estimated to contribute about 25% of the pest control expenditure in the United States. Insect-resistance results in diminished utility of insecticides and places tremendous selection pressure on few products, which could further aggravate the problem. Resistance is the result of selection, where few insects in the population with genes of specific resistance mechanisms survive the insecticide sprays and multiply, thereby increasing the proportion of resistant insects in the population. The Colorado potato beetle is considered to be the most resistant pest in North America since it has developed resistance to every group of insecticides that growers have used against it. Other gaps in insect management using conventional tactics specific to each crop for target Bt pests are discussed below. Biotechnology-derived insect-resistant crops bridge the gaps in conventional insect management tactics with no need for chemical sprays to control target pests.

Corn

European corn borer damage results in poor ear development, broken stalks and broken ears, and ultimately yield losses due to larval feeding on kernels, leaves and conductive tissue. Its feeding on stalks and kernels increases the incidence of secondary infestations of stalk-rot fungi and mycotoxin-producing fungi, respectively. European corn borers also carry spores of secondary pathogens such as ear rot fungi from the leaves to the developing kernels and thereby increase the incidence of kernel rot and symptomless infections.

European corn borer is a difficult pest to control for two reasons. First, European corn borer levels are difficult to predict and vary greatly from year to year. As a result, growers are usually reluctant to incur costs on scouting to determine the feasibility and profitability of insecticide applications. Second, ECB control is complicated due to the feeding and survival behaviour of the insect. Corn borer larvae feed in leaf whorls after hatching and eventually move into the stalks to pupate inside the stem burrows thereby avoiding insecticide applications. Insecticides need to be applied during the two to three days period between egg hatching and their burrowing in the stems. Thus, carefully timed insecticide applications are the key for the successful control of ECB.

European corn borer control with the conventional insect-tolerant varieties and available insecticide options is only marginal to good. Consequent yield losses from ECB have been as high as 300 million bushels per year accounting to monetary losses of up to one billion dollars in the United States (Mason et al., 1996).

Cotton

Chemical control costs for cotton bollworm, tobacco budworm and pink bollworm amount to about 60% to 70% of the total pesticide costs to US cotton growers. Cotton insect management is very intensive; more than 90% of the entire cotton acreage is treated with insecticides and use of about ten insecticide applications was not uncommon in one season. More insecticides are applied to cotton than in any other crop in the United States (Gianessi and Marcelli, 1997). A limitation to cotton insect management using conventional insecticides has been the development of resistance in insects to pyrethroids, organophosphates and carbamates.

Potato

Roughly two-thirds of the total potato insecticides are targeted to control two insect pests, Colorado potato beetle (CPB) and green peach aphids. Colorado potato beetle is the most devastating and is particularly difficult to control as it has developed resistance to a broad range of insecticides such as arsenicals, organochlorines, organophosphates, carbamates and synthetic pyrethroids. Sprays of Bt are not widely used due to lack of effectiveness on early instars, lack of residual activity and stringent requirements on application timing (Whalon and Ferro, 1998).

The green peach aphid serves as a vector in transmitting potato leaf roll virus (PLRV). Potato leaf roll virus causes net necrosis in tubers, thereby lowering the marketable value of the crop. Since no chemical control options are available for virus control, the only way to limit virus infestations is to control insect vectors that transmit the virus.

Sweet Corn

Similar to ECB, the internal feeding habit of corn earworm and fall armyworm on sweet corn makes them only susceptible to pesticide applications during a narrow window of time when they migrate to newly developing ears. Once the larvae enter the ears, they are virtually impervious to chemical sprays. Most sweet corn growers consider 2% to be the maximum tolerable damage level for fall armyworm or corn earworm. As a result, it is not uncommon for sweet corn growers in states such as Florida to make up to 12 insecticide applications throughout silking stage to control these pests. Insect-resistant sweet corn varieties and biological methods have been used to no avail.

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