The European corn borer, Ostrinia nubilalis (Hbn.), is the most important pest in North America that severely affects the yield and quality of corn. The commercialization of transgenic Bt corn (hereinafter referred to as Bt corn) provides a new way to control corn borer damage. I. Breeding Plants of Bt Corn Genetic researchers insert the selected foreign DNA into the DNA of maize plants. In order to maintain important agronomic traits such as yield, high yield, and disease resistance, seed companies have selected elite hybrids for the transformation of Bt corn. The gene vector introduced into corn consists of three basic components: (1) Protein genes. That is, a modified Bt gene that expresses in corn and produces crystal protein, and the first generation of Bt corn hybrids currently used in the U.S. and Canada expresses a gene in Cryl Ab, Cryl Ac, or Cry9C. . In the near future, transgenic hybrids may produce other crystal proteins or other sources of protein. (2) promoter. It is used to control the expression site and expression level of crystal protein in the plant body. Some promoters restrict the expression of crystal protein in specific parts of the plant such as photosynthetic green tissues and pollen, such as PEPC, and some promoters can be expressed in the whole plant. Crystal protein, such as CaMV/35S. (3) Marker genes. The presence of the marker gene allows the seed company to identify whether the foreign gene has been successfully introduced. Commonly used marker genes include herbicide resistance genes such as Round Up and Liberty R and antibody genes. The gene vector is inserted into maize by different plant transformation techniques and successfully transformed, referred to as a transformant, and the transformant separates the gene vector and Bt DNA into the site of the maize DNA. Differences in entry points can affect Bt protein expression and may affect other plant functions. Therefore, seed companies carefully review the transformants to ensure that the production of Bt protein has no adverse effects on agronomic traits. After Bt corn was approved by the United States Environmental Protection Agency (EPA) in 1995, by September 1997, the United States Environmental Protection Agency (EPA) had registered four different transformants for commercial use. These were Novartis Seed and Mycogen. Seed company’s 176, Northrup King/Novartis Seed’s BT11, Monsanto’s MON810 and DEKALB’s DBT 418. 176 are respectively registered by Novartis and Mycogen as “Knock Out†and “Nature Gard,†and BT11 and MON810 The trademarks are "Yield Gard" and DBT418 is "Bt-Xtra". Recently, Agr Evo registered the CBH351 transformant as "Stralink". The proteins expressed by 176, BT11 and MON810 transformants were all Cryl Ab, the protein expressed by DBT418 was Cryl Ac, while the protein expressed by CBH351 was Cry9c. Understanding these transformants and how they affect expression is crucial for selecting corn hybrids. II. Insecticidal effect of Bt corn and its impact on natural enemies 1 Controlling effect of European corn borer Compared with the current comprehensive control technology, Bt corn has obvious potential to improve the control of European corn borer. Chemical pesticides are applied at the right time. Under the circumstances, the control effects on the first and second generation corn borer larvae were 60% to 95% and 40% to 80%, respectively. The control effect of Bt corn on the heart-leaf generation corn borer was above 99%, while the control effect on the second-generation corn borer larvae varied from variety to variety. This is related to the type of Bt gene between Bt corn varieties and the site of expression in maize plants. For example, under a large number of artificial inoculation conditions, the efficacy of BT11 and MON810 and 176 on the control of the heart leaf stage corn borer is as high as 96%. Although the Bt genes expressed in these three cultivars were all Cryl Abs, the control effect on the second generation pods in the panicle stage was significantly different. The control effect of 176 was only 75%. The reason was that both BT11 and MON810 could produce crystal proteins. While 176 only produced toxic proteins in the plant's green tissues and pollen, some of the larvae feeding on filaments and grains drilled later on stems and cobs. While artificial inoculation of 4 instar European corn borer larvae at different stages of development of different Bt corn plants, BT11 and MON810 and CBH351 can effectively control corn borer damage, 176 can effectively control the older corn borer larvae in the vegetative growth stage, but in the pumping of male into reproductive growth After the stage, the number of borehole tunnels increased, and the control effect of DBT418 on the older larvae was not ideal in the vegetative and reproductive stages. Another field trial showed that 2 to 4 years old European corn borer feeding Bt corn CBH351 all died within 4 days. Even if the artificial inoculation is excessive, the average length of each tunnel is only 0.09 to 0.14 cm, and the control It is 30 and 23cm respectively. 2 Bt corn's control of other pests and diseases in corn According to laboratory and field trials in Iowa and Kansas, currently applied Bt corns cut roots on aphids, corn leafhoppers, ground tigers, and underground pests. Leafhoppers, golden needles, cockroaches, melon seed flies, and corn cockroaches are not effective. Studies at the University of Kentucky have shown that other pest control effects on corn vary among Bt corn transformants. Bt maize expressing Cryl Ab and Cryl Ac genes is ineffective against the tiger, but preliminary field trials have shown that the Crysin gene CBH351Bt is expressed. Corn has a good control of the ground tiger. BT11 and MON810 can reduce the damage of corn earworm by 50% to 70%, and the control effect on autumn armyworm can reach 50% to 70%, while 176 has a poor effect. At the same time, the BT11 and MON810 particles are expressed on India's gluten. Good control, 176 is invalid. The control effect of different Bt corn on the southwest corn borer is obviously different. As Bt corn reduces the invasion of pathogenic bacteria, it can reduce the occurrence of corn stalk and ear rot. Three consecutive years of research showed that after artificial inoculation of corn mash, compared with non-BT corn, Cryl was expressed. The incidence, severity, and grain rot of maize ear rot caused by Fusarium spp. on Ab's BT corn dropped by 58% to 68%, 54% to 96%, and 17% to 38%, respectively, while also reducing yellowing. Aspergillus contamination of corn kernels. 3 Effects of BT corn on beneficial insects and natural enemies of pests A large number of studies have shown that BT crystal protein has extremely high selectivity for killing lepidopteran larvae. BT corn has no adverse effects on beneficial insects including bees, ladybugs, grasshopper larvae, spiders, carnivores, and parasitoids, such as those feeding on predatory natural enemies of the ladybugs and small dark-colored pods that contain the Bt toxin protein pollen. There was no significant effect on growth and predation ability. The predation rate and parasitism rate of European corn borer eggs and larvae on Bt corn and the number of predatory natural enemies were not significantly different from those on non-Bt corn. In the laboratory, European corn borer feeding on Bt and non-Bt corn was used to feed common grasshopper larvae. There was no significant difference in mortality. However, the development time of the former was longer than that of the latter, indicating that Bt reduced grasshoppers. The fitness of the larvae may be due to the combined effects of Bt and nutritional deficiencies caused by morbid prey. Nevertheless, the indirect effects of Bt corn on European corn borer natural enemies exist, and the number of predatory, parasitic natural enemies and pathogens may decrease with the decrease in the population of corn borer. III. Prediction of resistance to Bt corn in European corn borer and its remediation 1. Possibility of developing resistance It is known that insects have the ability to rapidly develop resistance to certain pesticides. Resistance is due in part to the high concentration of pesticides. Frequent application under conditions. More than 500 species of insects and mites have become resistant to insecticides and acaricides. Therefore, European corn borer has the potential to develop resistance to Bt crystal proteins. At present, laboratory populations of 15 different insects have developed resistance to Bt proteins. These insects include Indian pupa, Heliothis virescens, Spodoptera exigua, pink bollworm, and potato beetle. In the United States, Hawaiian and Florida populations of Plutella xylostella have been highly resistant to Bt insecticides. There are many factors that influence the development of resistance. For European corn borer, these factors include: the area of ​​Bt corn planting, the higher mortality rate of corn borer during the growing season, and the occurrence of two or more generations per year. Laboratory studies in Minnesota, Kansas, and Delaware have convinced that European corn borer can produce moderate resistance to Bt insecticides or Bt crystal proteins, killing 50% of the population's required toxic protein in these resistant strains More than 30 to 60 times higher than the susceptible strains, these moderately resistant levels of European corn borer were produced in relatively small laboratory populations after treatment with Bt protein for 7 to 9 generations. The corn quail was continuously fed on artificial diets with different concentrations of purified Cryl Ab protein. The results showed that corn gluten produced a certain degree of tolerance to low concentrations of Cryl Ab in artificial diets, but close to the actual expression of Cryl in Bt corn. The population of corn borer feeding on the artificial diet with Ab concentration could not be preserved, and the different groups of corn borers selected after 13 generations of inoculation did not survive after inoculation on Bt corn expressing Cryl Ab in the greenhouse. Although these findings confirm that the European corn borer has genetic potential for resistance to Bt, laboratory studies do not demonstrate that resistance can develop under field conditions. Under field conditions, Bt corn and European corn borer are in an environment completely different from European corn borer that feeds Bt insecticide in the laboratory. Despite this, the threat of European corn borer resistance to Bt corn does exist. 2 Prevention and management of resistance to Bt maize In the face of the potential threat that European corn borer may develop resistance to Bt maize, it is necessary to develop a governance plan to avoid or delay the emergence of resistance risk. Resistance governance is a key factor in pest management practices. The US Environmental Protection Agency has conditionally registered Bt corn commercialization, and companies that need to sell Bt corn seeds developed and implemented a resistance management plan in 2001. Current Bt corn resistance governance is based on two complementary principles, high doses and shelters. Bt corn designed by plant geneticists can produce very high levels of Bt crystal protein, much higher than the amount of Bt insecticides used to control corn borer. The purpose is to kill all non-resistance genes (rr) and there is a resistance gene. (rs) of the European corn borer larvae, this hypothesis is based on the goal that Bt corn hybrids have reached this high dose. If a high-dose target is not reached, corn borer larvae with a resistance gene can survive to adults and copulate with adults with other resistance genes. Most of these combinations of offspring may have resistance to Bt corn. Sex, accumulated through generations, the frequency of resistance is increasing, and Bt maize continuously expresses toxic proteins, providing an ideal environmental condition for resistance production. The second principle of the resistance management plan is the application of shelters and is currently the only effective resistance management measures. The purpose of the shelter is to provide non-Bt corn or Bt pesticides to the European corn borer, which may be related to the potential from nearby Bt corn. Resistant corn cocoon mating. The goal of a refuge is to provide an excess of sensitive adults to each resistant adult. Shelters can be non-Bt hosts for any European corn borer, including non-Bt corn, potato, sweet corn, cotton, or field weed hosts. Bt biological pesticide granules or foliar sprays should not be used in shelters. Only in the outbreak of European corn borer, non-Bt corn in shelters should be treated with pesticides to minimize economic losses while still providing A sufficient amount of sensitive corn borer. As for the size of shelters, based on the knowledge of European corn borer biology and drug resistance and computer simulation models, the area of ​​shelters should be close to 20-30%. At the same time, the field layout of Bt and non-Bt corn is also very important, and the corn stubble in the shelter must be feathered simultaneously with the resistant corn borer, and it should be within the range that can cope with the resistant corn stubble. Therefore, each farm should have one or more shelters in adjacent Bt corn fields. The actual number of shelters varies by region, farm, and corn production system, but the target of controlling the larval population exposed to Bt protein at 20-30% is consistent. In corn crops mixed with soybean and corn rotation areas, the main shelter is non-Bt corn, so 20-30% of corn planting area should be non-Bt corn. In areas where maize is used as a pest control area for corn borer, the area of ​​shelter should be increased to 40% to compensate for larval mortality. Smaller sanctuaries are feasible in areas where maize planting area is small and where conversion hosts associated with European corn borer populations do not contain Bt protein, and the reduction in the size of such sanctuaries is based on conversions from the transformed host and maize. Hypothesis of corn borer contemporaneous. When the proportion of corn borer populations originating from local non-Bt hosts is unknown, planting 20-30% non-Bt corn shelters is the easiest and safest measure to delay resistance.
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