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The symptoms of attack are similar for all Earias spp. Cotton infestation generally starts with shoot boring in the young crop. Earias insulana enters the terminal bud of the vegetative shoot and channels downwards from the growing point, or directly penetrates the internode. Only soft growing tissue is attacked. Extensive tunnelling results in wilting of the top leaves and the collapse of the apex of the main stem. The whole apex turns blackish-brown and dies. The result is bunched growth in young plants and death of the growing point in the mature plant. If only the apical bud is attacked, the damage may not be noticed until the main stem divides (twinning) when the axilliary buds take over growth (Kashyap and Verma, 1987; Reed, 1994).
As the buds and flowers appear they wither and are shed; they usually have a conspicuous hole where the larva has entered. The shedding of minute buds is often blamed on mirids but may be caused by very young Earias (Pearson, 1958). The bolls are also attacked, but only when they are unripe. The larvae usually bore deeply, filling the tunnel opening with excrement. The tunnel often enters the bolls from below, at a slight angle to the peduncle (Pearson 1958). Small bolls, up to 1 week old, turn brown, rot and drop. Larger bolls, 2-4 weeks old, may not drop, but open prematurely and may be so badly damaged they cannot be harvested. Bolls are vulnerable up to 6 weeks of age (Butani, 1976). The larvae tend to move from boll to boll and the damage they cause may be disproportionate to their numbers.
Secondary invasion by fungi and bacteria may conceal the E. insulana infestation. Earias spp. can transmit Xanthomonas malvacearum, causing bacterial blight of cotton (Borker et al., 1980). E. insulana was responsible for the establishment of 99% of the black fungus infection (caused by Rhizopus nigricans) present in a cotton crop (Nasr and Azab, 1969a). Young bolls or those without larval feeding holes rarely became infected.
Okra is attacked in a similar way to cotton. Initially, the terminal shoots are bored, with the attack moving to the flower buds and fruit as they appear. With severe tunnelling, the top leaves wilt and the whole apex of the plant droops, hampering further growth. Secondary branching may occasionally occur. When fruiting starts, larvae move to the flower buds, tiny fruits and eventually the mature pods. A severe attack causes the shedding of flower buds and reduced yield. When attacking the fruit, the larvae feed on the milky seeds and other contents of the pod leaving excrement-filled tunnels.
Much of the literature investigating cultural control of bollworms looks at mixed populations such as Pectinophora gossypiella. Earias spp. are often a secondary component and there is rarely any indication of the damage caused by each pest in isolation.
In India, long duration cultivars of cotton supply a host for Earias from June or July until February, with okra providing an important carryover crop from one cotton season to the next. Cotton plants not removed after the harvest also assist carryover because they sprout from the stump and continue to provide food for Earias. Irrigated cotton in summer also provides extra food for the Earias population. Legislation in some countries requires farmers to uproot and destroy harvested plants to ensure an adequate close season, but this is seldom enforced. However, in Cyprus, legislation requiring growers to destroy all okra plants before a fixed date was apparently ineffective in reducing damage, mainly because of the presence of wild malvaceous plants in the vicinity of the crop fields (Melifronides et al., 1978). Eradication of alternative host plants has also been attempted, but is of doubtful benefit because many are valuable sources of food, feed or fibre and their removal may reduce the pool of natural enemies.
Kashyap and Verma (1987) suggested that cotton should be inspected regularly and all wilted shoots removed, thus removing the larvae of Earias. Some farmers allow livestock to graze cotton during the vegetative stage with the same effect. Nasr and Azab (1969b) also emphasize the importance of removing infested shoots and suggest that it be done when the egg-masses of Spodoptera littoralis are being collected and destroyed. The benefits of topping are controversial, although Nasr and Azab (1969b) claimed that the removal of the topmost few centimeters of the cotton plant at the beginning of the season reduced infestation and encouraged lateral branches, increasing the yield, without affecting the quality of the fibre.
Other suggested cultural practices include deep ploughing (Faseli, 1977) and close spacing of plants (Abdel Fatah et al., 1980). High doses of nitrogen fertilizers have been found to increase infestation (Reed, 1994). Singh et al. (1997) suggest that missing out rows when sowing cotton, to provide a path for spray operations, lowers the incidence of E. insulana and other bollworms, as well as improving yields. Other studies have found that earlier sowings help reduce bollworm infestation (Bishara, 1969; Ilango and Uthamasamy, 1989; Abdalla, 1991).
Considerable resistance to Earias has been recorded in several wild species of Gossypium (Anson et al., 1948). G. hirsutum has been reported to be more susceptible than either G. barbadense (Badway, 1974) or G. aboreum (Butani, 1974).
Numerous trials have tested the resistance of various cultivars and reduced susceptibility has been found in many of them. Those with high levels of tannin and gossypol (Sharma and Agarwal, 1984; Mohan et al., 1994) frego-bract and okra-leaf characters (Thombre, 1980) and red pigmentation (Duhoon and Singh, 1980) have been found to be less susceptible than many commercial cultivars. Hirsute varieties (Agarwal and Katiyar, 1974) and glandless varieties (Brader, 1969) have been found to be more susceptible. Singh et al. (1974) reported that tall plants with larger top leaves and bolls in clusters carried more bollworm attack. Conversely, dwarf varieties with early flowering habits have been found to escape the damage of spotted bollworm (Wankhede and Sadaphal, 1977).
Khambete and Desai (1996) found some okra varieties had a certain amount of resistance. Nerkar (1991) discusses the possibility that resistance might be found in wild relatives of okra such as Abelmoschus spp.
A number of studies have investigated the potential of various parasitoids against Earias spp.
The failure of the cotton crop in the Punjab in 1905, owing to infestation with Earias, was attributed to the absence of Bracon greeni, which at the time was considered to be so efficient as a biological control that it was specially transported from Delhi to Lyallpur (now Pakistan) (Lefroy, 1906). Ahmad and Ullah (1939) suggested that the rains, by lowering the temperature and enhancing relative humidity, benefited B. greeni at the expense of Earias and enabled the parasitoid to keep the host under check. Khan and Verma (1946) reported a very high population of B. greeni from July to September in India (in Sekhon and Varma, 1983).
There have been several field trials involving parasitoid release. Trichogramma australicum and B. greeni were mass bred in Pakistan and released against Earias spp. with limited success (Habib and Mohyuddin, 1981). Releases of T. brasiliensis also increased the cotton yield in India, but further releases were necessary (Sangwan et al., 1972; Simmonds, 1974; Tuhan et al., 1987). In another study, T. brasiliense, T. pretiosum, T. achaeae, and the braconids Chelonus blackburni and B. kirkpatricki were released leading to a significant reduction in the damage to cotton. It was concluded that the use of natural enemies could be incorporated into an integrated control programme for cotton pests (Pawar and Prasad, 1985; Prasad et al., 1986).
Although parasitoids can control Earias, it appears that large numbers are necessary and the evidence suggests that these high levels have to be artificially maintained. Stam and Elmosa (1990) found that parasitoids were relatively unimportant in controlling lepidopterous pests in cotton in Syria, but that the use of pesticides reduced the numbers of predators, resulting in a reduction in seed cotton yields.
There are fewer studies on other types of biological control. Bacillus thuringiensis has been used with some success. Glazer et al. (1992a,b, 1993) have looked at control using entomopathenogenic nematodes under laboratory conditions. Li et al. (1984) found that placing nests of the wasp Polistes antennalis [P. chinensis antennalis] in cotton fields in China gave effective control. Croizier et al. (1983) report a lethal cytoplasmic polyhedrosis virus was observed in the larvae of E. insulana in Cameroon. Atger (1970) also mentions undefined viruses attacking E. insulana.
Cork et al. (1988) identified six pheromone components detected by the male moth. A series of trials in Pakistan, using a slow release 'twist-tie' formulation, containing the major components of E. insulana pheromone, has successfully disrupted mating and controlled the pest at least as well as conventional pesticides (Critchley et al., 1987; Chamberlain et al., 1992, 1993; Hall et al., 1994). Nakache et al. (1992) had similar results in a trial in Israel and concluded that the use of pheromones was a viable control method. Kehat and Dunkelblum (1993) tested various traps and dispensers for pheromone control in Israel.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
The importance of Earias species varies considerably and, in some countries, has changed over the years. There appears to be some confusion concerning the impact of the pest. Often the literature does not distinguish between Earias spp and other bollworms when estimates of damage are given. In addition, the results of studies carried out in research farms are apparently not comparable to the losses in farmers' fields (Reed, 1974).
Sufficiently long periods in which conditions are naturally unfavourable appear to be adequate for reasonable control of the pest. Earias insulana used to be a major pest of cotton in Egypt, but changing farming practices and the invasion of the pink bollworm, Pectinophora gossypiella, upset the food supply sufficiently to reduce E. insulana to a minor pest (Pearson, 1958). The cold winter in northern Iran also effectively keeps populations of E. insulana in check (Heidari et al., 1981). Pearson (1958) suggests that even in areas with ideal conditions, cotton may not be the first choice of host for E. insulana.
E. insulana is regarded as an important pest in most of the cotton-growing areas of India. It is most common in the Punjab, becoming less common further south. However, field tests carried out in the Indian Punjab in 1985-1986 to investigate the damage to different cotton varieties caused by several bollworms, including E. insulana, concluded that the insects may not contribute significantly to loss of yield in cotton (Dhawan et al., 1990).
Damage by E. insulana is of great economic importance to growers of okra on the south-eastern coast of Cyprus (Melifronides et al., 1978). Faseli (1977) reported that in the south Khorrasan region of Iran, E. insulana caused about 80% of the damage to cotton. Stam and Elmosa (1990) report that E. insulana was the most damaging pest in the Syrian cotton agro-ecosystem from 1980 to 1983.