Effects of season, sowing date, nitrogenous fertilizer and insecticide spraying on the incidence of insect pests on cotton in the Sudan Gezira

SUMMARYThe effects of the sowing date of cotton and application of nitrogenous fertilizer to it on the incidence of insect pests were investigated over a 5-year period. Response to frequent insecticide spraying was also determined and the results were related to previously reported interactions between nitrogen, sowing date and insecticide application in their effects on yield and its year-to-year variation. Numbers of jassids (Empoasca lybica de Berg) and whiteflies (Bemisia tabaci Genn) were considerably greater on the fertilized plots. The pattern of pest population change over the season varied with sowing date and also from year to year as did the relative numbers of jassids and whiteflies. Effects of spraying on the yield obtained under different agronomic treatments and in different years were clearly related to effects on numbers of insect pests.

The distribution of Empoasca lybica (de Berg.) (Hemiptera, Cicadellidae) on cotton in the Sudan

The distribution on cotton plants of the eggs, nymphs and to a lesser extent the adults of Empoasca lybica (de Berg.) was studied on the varieties Bar XL1 and BAJ 7/57 in the Sudan during 1961–64. Counts of jassid nymphs in the process of hatching from eggs buried in the leaf veins showed that hatchlings were most numerous on the third to fifth expanded leaves from the main-stem apex and that most eggs had been laid either close to the point of insertion of the petiole into the leaf lamina or about half-way along the length of the veins on the underside of the leaves. The greatest number of hatchlings was found in the central leaf vein. It was concluded that eggs are most frequently laid in this position on newly expanded leaves at the apices of the main stems or side branches.The distribution of the early instars on the main-stem leaves was peaked, with maximum numbers occuring on or about the fifth expanded leaf from the apex; a similar distribution with a less prominent peak nearer to the apex leaf was found on side branches. The later instars, being more mobile, had a more even distribution. In the over-all distribution of nymphs on a typical cotton plant, a combination of the patterns found on main-stem and side-branch leaves, the greatest numbers of jassids occurred in the upper half or upper third of the plant. Co-variance analyses showed that the relation between numbers of jassids per zone and number of leaves per zone was not close and that the manner in which the plants were divided into zones for analysis of the relation greatly affected the correlation coefficients obtained.Regular counts of nymphs and adults on the upper surface of leaves throughout a period of two complete days showed that both nymphs and adults moved to the upper surface of leaves each day between 1400 and 1800 hr. local time and returned to the under surface at about 0300–0400 hr. Neither the environmental factor that stimulated this movement nor the way in which it benefited the insect was clear.

Some factors affecting numbers of Empoasca lybica (De Berg.) (Homoptera: Cicadellidae) infesting cotton in the Sudan Gezira

The Cicadellid, Empoasca lybica de Berg, is an important pest of cotton in the Sudan Gezira, where over 300,000 acres of cotton are grown annually under irrigation. Cotton is sown in mid-August, and the plants are uprooted and burnt the following May. The life-cycle of E. lybica from egg to gravid adult takes 16–24 days, and the adults live for up to 40 days. There is no diapause. During the 100 days from late August to early December when breeding on cotton is of economic importance, a single male and female could give rise to some 50,000 progeny.During May to July, when crops are confined to irrigated gardens and river banks, E. lybica is widely distributed in such places and can be found also on tree hosts, which are numerous especially in the southern Gezira and along river banks. There is circumstantial evidence of displacement over long distances, and the great majority of catches of E. lybica in sticky traps were made before the increase in population on cotton that occurs from September onwards.Of the 53 species of host-plants that have been recorded, only Solanum dubium, Rhynchosia memnonia, Hibiscus spp. and Abutilon spp. are of importance in the ecology of E. lybica. The first two especially are common weeds in fallows, which comprise more than half the land under rotation. Populations of E. lybica in Gezira fallows at the time of cotton germination tended to be greatest where pre-sowing rains (i.e., those falling from 1st July to 15th August) were highest. Correspondingly, initial infestation of cotton was highest in seasons and places receiving the most pre-sowing rains, although density of infestation in any place was affected by sowing date and proximity to irrigated fields and gardens which supported weed host-plants.In order to develop a system of sampling for infestations of E. lybica in the cotton crop, the distribution of nymphs on cotton plants was examined. It was found that nymphs were most numerous in the leafiest zones of the plant and a random choice of leaves seemed an appropriate means of sampling for infestation. The distribution of nymphs within and between cotton fields was also investigated and a standard sampling procedure adopted.Peak infestations on cotton could not be predicted from the level of initial colonisation, or from surveys a month later. Peak infestations were usually inversely related to the level of initial colonisation, especially when comparisons were made between seasons, as at the Gezira Research Farm. That is to say, high levels of initial infestation, which occurred in seasons of good pre-sowing rains, tended to be followed by low rates of increase, and in years of poor pre-sowing rains, initial infestations tended to be low and rates of increase high.The relationship of these findings to those of Cowland & Hanna (1950) and Hanna (1950) are discussed; the hypothesis that pre-sowing mud-splash is a major factor controlling numbers of E. lybica in the Sudan Gezira is discounted, although it is accepted that this factor temporarily reduces populations.The rate of increase of infestations of E. lybica was found to be positively correlated with the concentration of nitrogen recorded 2–4 weeks previously in the cotton leaf. This concentration affected not only the rate of increase of the initial colonisers, but also the rate of recovery of populations during November and December after spray-applications of DDT. The nitrogen concentration in the leaf was increased by nitrogenous fertiliser, with a corresponding increase in infestations of E. lybica. It was also found to be negatively correlated with pre-sowing rains, which, if low, prevent the nitrate in the top 12 in. of Gezira soil being washed to lower levels, but the data presented provide no evidence that the relationship is causal.It is concluded that localities and seasons of poor pre-sowing rains favour a high rate of increase of small populations of E. lybica because of high nitrogen concentration in cotton leaves during September and October. This tendency is augmented by application of nitrogenous fertiliser. A regression equation relating the peak infestations of E. lybica with pre-sowing rainfall and with nitrogenous fertiliser is given and the infestations computed from this are shown not to differ significantly from those recorded in the Gezira as a whole, and in the four main divisions of it separately, during the eight years 1949–1956.

The Yield Response of Cotton in the Sudan Gezira to DDT Spray

In the Sudan Gezira, some 240,000 feddans of Egyptian-type cotton (Gossypium barbadense) are grown annually by gravity irrigation from the Sennar Dam, and this is sprayed with DDT, 6–10 weeks after sowing, in order to control the cotton jassid, Empoasca lybica (de Berg.), which is considered to reduce the yield. The seasonal yield response to DDT was estimated by comparing the yields of single sprayed and unsprayed 90-feddan fields, pairs of which were selected in each of the 40 (now 44) Blocks into which the cotton estate cropped by the Sudan Gezira Board is divided for administrative purposes. In order to establish that this estimate adequately represented that of the estate as a whole, it was necessary to show the validity of three assumptions: that the sample was representative; that the unsprayed plots, though surrounded by sprayed cotton, were sufficiently large to behave in respect of yield as if their environment had not been sprayed; and that the selection of the fields was effectively at random.Examination of the yield history of the fields that were sprayed showed that there had been a slight bias towards the selection of fields that yielded better than the mean of the locality, and although this bias did not change from place to place or year to year, it did increase with level of yield. It is not, however, considered big enough to invalidate the first assumption.The second assumption was tested by a special experiment reported elsewhere, which produced no evidence to suggest that a 90-feddan field would be affected in respect of yield and insect pests by proximity to a differently treated neighbouring field.The third assumption cannot be proved, but during two seasons when fields were selected by a random procedure, the results did not differ seriously from those in years when the choice was not completely random. Moreover, when the yields of fields selected for treatment in one year were examined for the season when the fields were last cropped with cotton, and spray treatments were identical, they were found not to differ significantly, although the differences were increased and became significant in the year when sprayed and unsprayed treatments were applied.Accordingly, the yield of the unsprayed sample is taken to indicate what the yield of the whole estate would have been had all the cotton been unsprayed.The yield response to DDT spray of Gezira cotton, as thus estimated over the period 1949–50 to 1956–57, was shown to vary from season to season. A highly significant part of this variance was correlated with the amount of ram falling in July, some six weeks before the crop was sown, but the response was also significantly affected by both seasonal and site effects that were also correlated with this early rain. A series of experiments in 1956–57 showed that Domains Sakel cotton, which is grown in the drier, northern part of the Gezira, tended to give an increased response to DDT spray at higher levels of nitrogenous fertiliser, and that a very highly significant part of this response was due to the control of jassid, E. lybica, and thrips, (Caliothrips spp.). X1730A, grown in the wetter, southern part of the Gezira, gave no over-all response to spraying, nor was the response to nitrogenous fertiliser affected by spraying. When the yields of these varieties were examined separately over eight seasons, it was found that Domains Sakel gave a mean yield response to spraying of 1·11 kantars of seed cotton per feddan, which did not vary significantly from season to season, but that X1730A gave a yield response which was less when pre-sowing rains were good and nitrogenous fertiliser was applied. Although the regression on July rains and on fertiliser of the response of X1730A to spray failed to reach significance at P = 0·05, it is considered that further data should enable a prediction of yield response of X1730A to DDT spray to be made by examination of these two factors.Since 1935–36, yields of cotton in the Sudan Gezira have been positively correlated with the amount of rain falling between 1st July and 15th August of the year of sowing. Comparison of the coefficient of regression of yield on this rainfall in the period (1935–36 to 1947–48) before sprays or fertiliser were applied, with that of a more extended period (1935–36 to 1954–55) including six seasons in which sprays and fertiliser were applied, shows that the latter is only about one half the former. After allowing for differences in varietal response to spraying and fertiliser, it can be shown that this decrease in the deleterious effect of a deficiency in pre-sowing rains, is almost exactly accounted for by the beneficial effect of DDT spray.It is concluded that at least half of the deleterious effect of poor pre-sowing rains on Gezira yields is a pest effect which can be eliminated by DDT spray.The study thus provides a means whereby DDT spray may be applied selectively to those areas of the Sudan Gezíra where its effect is likely to be most profitable and suggests, moreover, that such a policy would reduce the considerable seasonal fluctuations in cotton yields which in the past have characterised cropping in this area.

Large-scale Spraying of Cotton in the Gash Delta in eastern Sudan

Following increased yields of cotton in the Sudan Gezira by DDT spraying, during the 1952/53 season 22,300 and 6,700 feddans of X1730A cotton were sprayed once and twice, respectively, in the Gash Delta of eastern Sudan, where nearly 60,000 feddans were grown under controlled flush irrigation. Each spray consisted of 1 lb. technical DDT per feddan and was applied by aircraft in 2 gals, of spray per feddan.Systematic observations were made on the incidence of cotton pests in 30 observation stations scattered throughout the Gash Delta, such stations being selected more or less at random from cotton sprayed once and twice and unsprayed, ten amongst each of these three treatments.A single DDT spray applied 50–70 days after sowing gave entirely satisfactory control of the Jassid, Empoasca lybica (de Berg.), and the thrips, Hercothrips fumipennis Bagn. & Cam. and H. sudanensis Bagn. & Cam., throughout the growth of the crop. There was little lasting control of the flea-beetle, Podagrica puncticollis Weise. The second spray, applied 70–90 days later, had little effect on any of these pests which were then present in low numbers.The incidence of bollworms was observed between December and March, covering the important fruiting period. During these months, over 70 per cent, more larvae of Diparopsis watersi (Roths.) and 40 per cent, more larvae of Earias insulana (Boisd.) were observed on sprayed than on unsprayed cotton. Significantly more larvae of D. watersi were recorded on cotton sprayed twice than sprayed once; conversely, significantly fewer larvae of E. insulana were recorded on twice- than on once-sprayed cotton. Moreover the estimated total number of fruits damaged by or shed in association with bollworm attack between January and March was over 30 per cent, greater, and nearly 30 per cent, more bollworm damage to nearly mature bolls was recorded between December and March, on sprayed than on unsprayed cotton.Yields of seed cotton were significantly less from twice-sprayed than from the other treatments. Once-sprayed cotton yielded less but not significantly so, than unsprayed cotton. Yield was negatively correlated with the number of sprays but the correlation coefficient just failed to reach significance.Further analysis of the data from stations where yield differences were most marked indicated that D. watersi lowered the yield potential of the crop as expressed by fruit production and retention, as a result of continuous shedding of the damaged fruit. Attack, however, was concentrated on cotton with the best yield potential. The effect of spraying was to increase the numbers of D. watersi and thus to give rise to excessive shedding, increased production of fruit primordia, an increased number of damaged bolls, and finally reduced yield.It is concluded that any benefits which the crop enjoyed, as a result of elimination of leaf-feeding insects by DDT spray, were completely lost through increased bollworm attack, which moreover reduced the yield below that of unsprayed cotton.

The Cotton Jassid, Empoasca lybica (De Berg.) during the dead Seasons in the Gezira, Anglo-Egyptian Sudan

Regular surveys were carried out from May to July to find out where Jassids live during the cotton close season.A list of host plants on which Jassids can breed, or feed only as adults, is given; they exhibit preference for certain plants.The movement of Jassids to cotton was recorded. The initial infestation of cotton is very small but is rapidly built up within the cotton crop. Gardens are shown to be the chief sources of infestation and cotton fields nearest them show larger initial numbers.Attempts to exterminate Jassids in these gardens by spraying all plants with 0.1 per cent, or 0.5 per cent. DDT emulsion during the summer dead season were not successful. It is thought that reinfestation came from adults which had been present on the tops of Acacia arabica trees and had not been reached by the spray.

The Effect of Rainfall on the Cotton Jassid, Empoasca lybica (De Berg.) in the Gezira, Anglo-Egyptian Sudan

The Gezira Scheme is divided into three areas according to the incidence of the Cotton Jassid, Empoasca lybica.1. The Northern area of normal abundance.2. The Central area of occasional abundance.3. The Southern area of possible abundance.The causes for this difference in the number of Jassids in the three areas were studied and rainfall in showers over 10 mm. in July and August was found to be the important factor. A definite correlation was found between the number of Jassids on cotton and the amount of this rainfall.Some experiments were carried out which proved that the effect of rain was due to the splashing of mud from the soil on to the lower side of the leaves. The rain could therefore be effective if it is heavy enough to produce mud splashing.It was shown by a simple apparatus that mud splashing hardly rises more than 30–40 mm.The correlation between the number of Jassids and the amount of rainfall in July and August might be used as a basis for forecasting the extent of Jassid attack on cotton every year.