Occurrence Patterns and Control Methods of Cotton Pests under Drip Irrigation

With the large-scale application of drip irrigation technology for cotton, the irrigation method has been improved. It delivers water and nutrients evenly and continuously to the vicinity of the cotton root system at scheduled times and specific locations, and the supporting cotton cultivation techniques have changed accordingly, causing corresponding changes in the ecological environment of cotton field pests and their occurrence patterns. For this reason, it is highly necessary to clearly understand the occurrence patterns of cotton pests and the technical control measures under mulched drip irrigation, so as to ensure sustained and balanced increases in cotton yield.

1. Cotton bollworm

1. Occurrence level

Compared with previous years, the occurrence in 2005 was moderate to severe, among which the 1st and 2nd generations occurred at a moderate to severe level, and the 3rd generation occurred at a mild level, which was the first time in the Xinhu reclamation area.

2. Causes of occurrence

① Related to the change in irrigation method. Temperature and humidity are the main factors affecting the growth and development of the cotton bollworm. Compared with flood irrigation, mulched drip irrigation for cotton is characterized by: reduced irrigation volume per 667 sq m, shortened irrigation cycle, increased number of irrigations, and shallower seepage depth, which changed the ecological microclimate of the cotton field.

The suitable temperature is 25-28°C and the relative humidity is 70%-80%. Mulched drip irrigation can always keep the cotton field in a relatively moist state, which is conducive to mature larvae pupating in the 5 cm soil layer of the cotton field. Field surveys of pupation of the 1st and 2nd generations showed that near the main and branch pipes, in the soil at the film edge of the border rows, there were more cotton bollworm pupae than in other places in the cotton field.

② Related to the farming system. According to the literature, the overwintering mortality rate of pupae under autumn plowing and winter irrigation is more than 60%; the overwintering mortality rate of pupae without plowing or irrigation is only 6%, a difference of 10 times. When the soil water content is 30%, the overwintering mortality rate of pupae can reach 70%-90%.

③ Related to cultivation management. Within a certain density range, increasing the number of retained seedlings per 667 sq m is an important means of achieving high yield, but dense yet uneven stands lead to inconsistent cotton management, such as water and fertilizer scheduling, chemical regulation dosage, and timing of topping, resulting in a shaded field and excessive growth, which provides very good conditions for the occurrence of the cotton bollworm.

④ Related to climate warming. In recent years, autumn and winter temperatures have been significantly higher than in previous years, spring temperatures rise quickly, the beginning of spring is 5-7 days earlier than in previous years, and the first frost period is 9 days later than in previous years and 13 days later than in 2004, which has caused relatively large changes in the occurrence of the cotton bollworm, with a large overwintering base, low mortality rate, and an earlier start of eclosion of the overwintering generation.

⑤ Related to control methods. The area of autumn and winter irrigation has decreased year by year to less than 30%, resulting in a large overwintering base and low mortality rate. Some effective agricultural control techniques have not been implemented, such as shoveling ridges and removing pupae in autumn and spring, which is the best control measure for eliminating the overwintering generation, and corn trap strips are only planted but not protected.

⑥ Related to field pest surveys. Insufficient attention has been paid to pest surveys in cotton fields, and by the time pests are found in the cotton field, it is already too late to control them.

⑦ Related to the geographical environment. The severity of occurrence of overwintering-generation adults is related to the surrounding environment of the cotton field. According to surveys, in 2005 the plots with relatively severe occurrence of overwintering-generation adults were: first, cotton fields near sand dunes; second, cotton fields on sandy land; third, cotton fields with poplar shelterbelts at the edge but without winter irrigation; and fourth, cotton fields where cotton bollworm damage was severe last year.

⑧ Related to drip irrigation for seedling emergence. Generally, sowing in cotton fields was basically completed by April 18, and drip irrigation for seedling emergence ended on April 25. During the sowing period, spring winds were strong, soil moisture escaped quickly, and soil relative humidity was low. Pupae overwinter in earthen cells at 7-8 cm, while the soil at 8-10 cm is basically a dry soil layer. Drip irrigation increased soil humidity, which was conducive to earlier pupal eclosion.

3. Occurrence patterns and damage characteristics.

① Overwintering generation. According to observations from frequency-vibration insect-killing lamps at three forecasting points, female cotton bollworm moths were trapped on May 11, and male cotton bollworm moths were trapped on May 15. Females were 3 days later than males, and the male-to-female ratio was 1:2.4. The eclosion period was May 11-30, lasting 21 days, and the peak eclosion period was May 23-28, lasting 6 days.

② 1st generation eggs: peak period May 25-30, lasting 6 days. Larvae: duration May 22-June 15, lasting 25 days. In the early stage, they mainly damaged cotton terminal buds and tender leaves, causing multi-headed cotton plants; in the later stage, they mainly damaged cotton buds, causing a large number of inner buds to drop. Pupae: duration July 17-31, lasting 15 days. Adults: eclosion duration June 7-July 10, lasting 34 days; peak period June 18-July 3, lasting 16 days; male-to-female ratio 1:1.3. From June 11-20, each company placed 10 poplar branch bundles and one insect-killing lamp to observe adult occurrence dynamics. On June 14, 70% of companies trapped adults. Starting June 21, workers began placing poplar branch bundles in large numbers, and all insect-killing lamps were turned on for trapping and killing. From June 22 to July 7, a total of 177,067 poplar branch bundles were placed, with an average of 11,804 insects trapped per day.

On June 11, tagged observations were carried out on egg laying in cotton fields. From egg to hatching of 1st-instar larvae required 2.5-3 days; the average temperature at that time was 26.5°C, with a maximum of 33.4°C. Preliminary determination showed that after eclosion of the cotton bollworm, adult mating and egg laying required 3-4 days.

③ 2nd generation eggs: peak period June 25-July 4, lasting 10 days. At 11:00 on June 25, 6-8 eggs per 100 plants were found. Larvae: duration June 28-July 19, lasting 22 days, mainly damaging cotton flower buds, causing large numbers of flower buds to drop, with a long damage period and severe damage. Pupae: duration July 17-31, lasting 15 days. Eclosion required 6-9 days. Adults: eclosion duration July 25-August 17, lasting 24 days; peak period July 27-August 7, lasting 13 days; male-to-female ratio 1:1.4.

④ 3rd generation eggs: peak period July 28-31, lasting 4 days. From July 24-31, Bt powder was sprayed during the egg-laying peak period. Observations of eggs showed that 8 eggs laid on July 26 had not hatched by July 29, while in cotton fields where Bt powder was not sprayed, eggs laid on the morning of July 27 hatched on the morning of July 28. Larvae: duration July 30-August 22, lasting 24 days; field surveys showed very little damage to cotton. Adults: eclosion duration August 25-September 11, lasting 18 days; the peak period was not obvious, and the occurrence pattern was very irregular. According to observations at the forecasting point, from August 25-September 11 and September 22-26, 27 female moths were trapped; from September 6-9 and September 24-26, a total of 10 male moths were trapped in 7 days; male-to-female ratio 1:2.7.

4. Control techniques

① Strengthen field pest survey efforts. Before the start of adult eclosion, each company should use poplar branch bundles and insect-killing lamps to monitor the adult eclosion peak period (to determine the time for placing poplar branch bundles), with special attention to egg surveys (to determine the timing of biological control). A combination of fixed-point surveys and non-fixed-point surveys, and regular surveys and irregular surveys, can be adopted to form a forecasting network centered on the forecasting station and with each company as a forecasting point, providing a reliable basis for scientific prevention.

② Agricultural control is the most economical and effective measure in cotton bollworm control. For autumn plowing and winter irrigation, winter irrigation efforts should be increased. In early spring, comprehensively carry out ridge shoveling and pupa removal, especially in heavily affected cotton fields, vegetable fields, corn fields, and shelterbelts, leaving no blind spots, so as to reduce the overwintering-generation pest population base. Sow early and manage early to fully achieve April seedlings, advance the growth process, destroy pupal cells through current intertillage, pull out field weeds, especially velvetleaf, and carry out chemical control of Elaeagnus angustifolia trees in field-edge shelterbelts. Strengthen management at all growth stages of the cotton field to achieve uniform water, uniform fertilizer, and uniform seedlings. Reasonably regulate water and fertilizer to achieve balanced and consistent cotton growth, prevent excessive growth and late maturity, and carry out timely early topping and pruning. At the same time, during the adult eclosion peak of each generation, evenly spray 150-200 g of potassium dihydrogen phosphate per 667 sq m, which helps repel egg laying, promotes egg calcification, reduces the number of eggs laid in the cotton field, and lowers the effective egg hatching rate.

③ Trap and kill control. Plant and manage corn trap strips well, adopt a large grid-style planting pattern, ensure sowing and drip irrigation for seedling emergence are synchronized with cotton, and promptly carry out chemical control on corn trap strips during the egg-laying peak period. Place poplar branch bundles and insect-killing lamps at the beginning of the adult stage; the number placed must be sufficient, and the method and location must be correct.

④ Biological control. Grasp the control period well and achieve scientific and reasonable control. Evenly spray Bt powder within 2-3 days during the egg-laying peak period, avoiding strong light, and use it in the morning and evening; it can be mixed with other insecticides; use biological agents for control before the 3rd-instar larvae. The main control target is the 1st generation cotton bollworm.

Forecast promptly, clarify the pest situation, and target the control without blind pesticide use; for chemical control of the 1st generation, use 8-10 eggs per 100 plants or 2 larvae; the 2nd generation must absolutely not be chemically controlled; improve control quality; the spraying amount must be sufficient, with the pesticide solution volume not less than 30 kg per 667 sq m. Investigate control efficacy, and promptly re-control where efficacy is poor.

⑤ Chemical control. The use of broad-spectrum insecticides is prohibited.

⑥ Manual capture of larvae and pupa control.

2. Cotton spider mites and cotton aphids

1. Red spider mites are perennial pests. Do forecasting well, observe frequently, control thoroughly when they occur in spots, and inserting markers well is key. Alternately use acaricides throughout the whole process.

2. Cotton aphids are stage-specific pests. Biological control is key; sweep the field edges and field borders.

3. Three new types of cotton field pests

1. Tetranychus truncatus. Previously, the Xinhu reclamation area was mainly dominated by Tetranychus turkestani. Tetranychus truncatus began to occur and cause damage in large numbers in cotton fields in 2005, and its damage symptoms are not obvious and difficult to detect.

2. Twin-spotted flea beetle. In the past, it mainly damaged cereal crops, and has now entered cotton fields to damage cotton. Adults are gregarious, have chewing mouthparts, and mainly feed on cotton leaves, forming leaf holes and dead spots, affecting the normal growth of cotton leaves.

3. Yellow-bellied tiger moth. Adults have black spots on the forewings and a yellow abdominal dorsum; mature larvae are earthy yellow on both the dorsal and ventral surfaces, with a dark longitudinal band on the dorsum. Larvae are mainly distributed in weed clumps around cotton fields near sand dunes, enter cotton fields to cause damage when cotton emerges, and gather as larvae on cotton cotyledons, true leaves, terminal buds, and young stems to gnaw, causing missing seedlings and broken rows. Use systemic insecticides such as monocrotophos at 1000-1500 times dilution for control.