ACTION THRESHOLD LEVELS OF HELICOVERPA ARMIGERA (HBN.) ON COTTON AT DIFFERENT STAGES OF PLANT GROWTH

The African bollworm, Helicoverpa armigera (Hbn.) (Lepidoptera: Noctuidae), is the most important cotton pest in all the cotton growing areas of the Sudan and most of the protection measures are directed for its management. The previous action threshold level for chemical control interference was increased in 1993 from 10 larvae and/or eggs /100 plants to 30 eggs or 10 larvae/100 plants and no spray to be conducted before flowering advances. This level was being subject to revision since 1999. Studies conducted at the Gezira Research Farm during 2001/2002, 2002/2003, 2003/2004, 2004/2005 and 2012/2013 were carried out on this regard at growth stages of cotton plants, i.e. Pre-flowering, flowering and bolls formation stages and the consequence on yield. Barac. (67) B cultivar, Gossypium hirsutum, was used. The action threshold levels were 10, 15, 20 eggs and/or larvae/100 plants and 30 eggs or 10 larvae/100 plants, in addition to untreated and Helicoverpa infestation free as control treatments. The results of damage on fruiting bodies (squares, flowers and bolls) and seed cotton yield showed that there were no significant differences between the tested action threshold levels in the pre-flowering and the flowering stages of cotton growth as compared to the control treatments. From the yields and number of sprays in the treatments it can be concluded that the already recommended action threshold 30 eggs or 10 larvae/100 plants is still be applied when flowering advances and bolls formed. Moreover, the standing action threshold seems lower than the actual level. Therefore, it is suggested to investigate higher action threshold than that in current use.


Introduction
The African bollworm (ABW), Helicoverpa armigera (Hbn.) (Lepidoptera: Noctuidae), is a polyphagus insect pest of a wide range of distribution. It is a pest in almost all cotton regions of the Sudan. Cotton is attacked by the ABW late September/early October and infestation continues to about February and three of four generations complete their development on cotton The effects on yield were found to depend not only on the number of larvae present, but also on their size, the timing of the infestation and the amount of damage done previously (Wallach, 1980). Abdelrahman (1995) showed that all the commercial cotton varieties can compensate the damage caused by H. armigera up to four weeks after the initiation of flowering. Eveleens and Abdelrahman (1980) mentioned that the first insecticide application can be delayed. As a result, during 1985-1992 a series of trails to raise the ETLs were conducted in different localities (Stam et al., 1994). Beside the ETL; 30 eggs or 10 larvae/ 100 plants which raised in 1993, also the IPM Project experimented the threshold 20 eggs and/or larvae per 100 plants in Gezira Scheme, Amara Kassir Block in an area of 5000 fed and gave good results. Thus, the impact of insect attack on the performance of the crop depends on the stage in which the host plant is attacked (Barendregt and van Lenteren, 1992). However, the effects on yield, depending on an effective action threshold level at different stages of cotton growth, have not been sufficiently evaluated in the Sudan. Therefore, this experiment was designed to study the effects of H. armigera on yield of cotton with reference to action threshold levels at different stages of cotton growth.

Materials and Methods
This experiment was conducted in Gezira Research Farm in seasons; 2001/2002, 2002/2003, 2003/2004, 2004/2005 and 2012/2013. The different stages of cotton growth were namely; the pre-flowering stage (X) {45-75 days after emergence}, the flowering stage (Y) {75-105 days after emergence} and the boll formation stage (Z) {105 days to early opening of bolls}. Four action threshold levels were proposed at the three stages of cotton growth in addition to infestation throughout (U) and Helicoverpa infestation free (T) as control treatments. These  All the treatments in this experiment except "U" and "T" were divided according to the phase of cotton growth into three stages: 1) Pre-flowering stage, 45-75 days after emergence.
3) Boll formation stage, 105 days after emergence to early opening of bolls.
These stages were equivalent to 3, 4 and 5 of the segmented growth stages of cotton set by Walker (1971).
For treatment "T" and the stage(s) of cotton growth decided to be free of H. armigera infestation, the deposited eggs were removed manually every 48 hours.
Different sampling procedures were carried out for H. armigera beside the natural enemies. Concerning H. armigera, the number of damaged and undamaged squares, flowers and bolls per 20 plants per sub-plot were weekly recorded. The percentage damage was calculated from the total number of fruiting bodies recorded. Eggs and larvae in randomly sampled 20 plants/subplot were weekly counted. The mean numbers of the important natural enemies (adult predators) at different sampling dates were recorded (Appendix 1).
The insecticides; Avaunt 150 SC (a selective insecticide recommended for the control of the ABW) at 0.105 litre/fed (15.75 g a.i.) and Tracer 240 SC (spinosad) at 0.104 litre/fed (25 g a.i.) were used. The insecticides were applied by a knapsack sprayer for spraying the treatments that reached the suggested action threshold levels.
The yield data and the damaged bolls were subjected to analysis of variance (ANOVA) and Duncan's Multiple Range Test whenever applicable. Meteorological data on temperature, rainfall and relative Humidity (R.H. %) during the five seasons were obtained from the Agrometeorological Services Department at Wad Medani (Table 1).