High temperature and drought, insect-resistant cotton also need to guard against cotton bollworm
Insect-resistant cotton is introduced into Bt insect-resistant gene of conventional cotton varieties and selected from successive generations through successive years of orientation selection. Under normal conditions, the insect-resistant genes can be well expressed and have strong resistance to insects, but they are resistant to insects. Insect cotton is not insectless cotton, but it still needs to be controlled according to local conditions, otherwise it will also cause harm. First, national standards for insect-resistant cotton. The State has the following provisions on whether it is insect-resistant cotton and resistance grading. First, it is a standard for toxic protein content. The second and third true leaves of cotton plants at the top of the cotton field are randomly tested for their toxic protein content, and more than 200 are insect-resistant cotton, and 400 or more. For the high resistance; Second, the second generation of cotton boll beetles damage reduction rate standard, in the absence of the control of the case of insect resistant cotton than conventional cotton bud bell damage reduction rate of 50% to 60% for the anti-grade, 60% ~ 80 % is a high resistance level; third is the third generation of indoor bollworm test mortality rate standards, collecting the second true leaf of the top of the cotton plant for insecticide test, larvae corrected mortality rate of 40% to 60% for the resistance, death 60% ~ 80% were anti-grade and more than 80% death was high resistance. The fourth is the standard of the rate of field resistance to insects, taking random sampling in the field, and the rate of insect-resistant strains exceeding 90% is confirmed as insect-resistant cotton, and 95% or higher is reaching high-resistance level standards. It can be seen that insect resistance is relative. Second, insect-resistant cotton insect performance is not good for a reason. In general, the insect-resistant genes of insect-resistant cotton can be fully expressed and have a strong resistance to the cotton bollworm. However, due to the influence of external environmental conditions, sometimes the insect-resistant gene can not be fully expressed, so that the insect-resistant ability is weakened or lost. The main causes are as follows: First, the variety of varieties, because the breeding of insect-resistant cotton to adopt hybridization, breeding and other methods of breeding, so the general offspring have the phenomenon of separation, plus cotton has 3% to 5% of the natural string flowers phenomenon , coupled with the impact of high temperature, drought and other factors, it is easy to make the field appear not resistant to insects. The second is the degradation of varieties. After selection and cultivation of insect-resistant cotton varieties, if they are not selected and purified and rejuvenated after successive years, degradation will occur and the insect-resistant ability will decline. The third is the influence of climate and cultivation conditions. In the normal climate and cultivation conditions, the insect-resistant genes can be well expressed. However, when high temperature, drought, or due to nutrient imbalance occur, the insect-resistant genes are poorly expressed or silenced. This reduces or loses insect resistance. Third, the anti-insect ability of different organs of cotton plants is different. The order of resistance to insects in the organs of the cotton plants identified by the scientific research unit is: the tender tip is larger than the young leaves, the young leaves are larger than the old leaves, the leaves are larger than the buds, and the buds are larger than the flowers. Cotton buds and flowers have poor insect resistance. Therefore, if eggs are produced on buds and flowers, the cotton bollworm will survive easily and cause damage to some cotton plants. Fourth, it is easy to produce resistant cotton bollworm. Due to the long-term planting and promotion of the insect-resistant cotton, it is susceptible to the natural selection rule, and it is easy to produce resistant H. armigera, and the insect-resistant cotton is resistant to the H. armigera reduced or lost the resistance to insects. In summary, insect-resistant cotton is not insectless cotton, and it is still necessary to carry out field inspections during the occurrence of cotton bollworms. Once prevention and control indicators are reached, prevention and control must be stepped up. Otherwise, it will cause varying degrees of harm if missed the critical period of prevention and control. For example, isolated mutants that are not resistant to insects in cotton fields can be removed to prevent cotton bollworms growing on this cotton plant from transferring to neighboring cotton plants. The key period of prevention and control of the bollworm is three days after the peak of egg laying, and the larva hatching period; the second generation of cotton bollworm prevention and control index, more than 20 eggs per 100 eggs, more than 100 third-instar larvae must be immediately carried out. Prevention and control, general control 1 to 2 times, prevention and control three times a year; three generations of Helicoverpa armigera more than 50 eggs, more than 10 third-instar larvae will immediately prevent and control, general control 2 to 3 times.
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