Importance of pollinators
Production of cashew is hampered by several biotic as well as abiotic factors. Inadequate pollination is a primary factor for poor nut set and yield. As cashew is a cross-pollinated crop, it requires insects especially bees for pollination and fruit set. Knowledge of pollinators of cashew in a particular region is very much important to devise suitable conservation measures for them.
Cashew (Anacardium occidentale L.) is a cross-pollinated tree crop. Cashew bears both staminate (male) and hermaphrodite (bisexual/ perfect) flowers on the same inflorescence. In general, inflorescences last around 100 days in a flowering season that varies from 5-7 months, with each tree producing hundreds of inflorescences during its flowering period. The flowers are pentamerous, small, white or light green at the time of opening, and later turn to pink. The androecium consists of a fully developed long stamen and 7 to 9 staminodes with pink anthers. The staminodes possess short filaments and are hidden in the lower half of the flower. In hermaphrodite flower, the long stamen itself has a short filament and its anther is far below the level of stigma. Due to the coincident flowering of male and hermaphrodite flowers on the same inflorescence, self-pollination may also happen to a lesser extent.
Anthesis occurs between 9.00 and 14.00 hours depending on the sunshine and over 80 per cent of the perfect flowers remain open between 10.00 and 12.00 hours. The peak period of anther dehiscence occurs between 9.30 and 11.30 hours. As the pollen grains of cashew are sticky in nature, the possibility of wind pollination is difficult. Though cashew flowers profusely, only less than 10 % of the total flowers are hermaphrodite in nature in the majority of the genotypes. Several workers have reported that pollination in cashew is inadequate in nature and the potential yields are not realized. Thus pollinators are very essential in obtaining good yields.
Pollinators of cashew
Earlier studies on cashew pollinators reported many insect species like ants, thrips, butterflies, flies, wasps and bees visit cashew flowers and are pollinating agents of cashew. However, their importance has not been critically determined. Less quantity of nectar produced by the cashew flowers favours mainly small and metabolically undemanding pollinator species. Insect visitors documented on cashew flowers at Puttur include 40 species belonging to 13 families of three insect orders. Among them, eight species of bees belonging to Apidae and five species belonging to Halictidae are the important pollinators of cashew at Puttur, Karnataka.
Bees visit the flowers from morning 8.00 hours depending up on the sunshine, and up to 17.30 or 18.00 hours in the evening. Peak foraging activity is between 11.00 and 13.00 hours. It is important to note that the peak foraging period of most of the pollinator bee species coincides with peak anthesis of flowers and stigma receptivity period which is very much advantageous for effective pollination in cashew. In nature, multiple bees may visit the same flower many times ensuring successful pollen deposition on the stigma of hermaphrodite flowers. Bee exclusion studies by caging of inflorescences reported zero or very less nut set in cashew inflorescences, which shows the importance of pollinators in cashew.
As per the available reports, keeping colonies of Apis mellifera in cashew plantations of Brazil, Apis cerana indica in North Karnataka and Tamil Nadu has increased the cashew yield considerably (10-30%). Studies taken up at ICAR-DCR, Puttur showed that under confined studies, effective pollination in cashew flowers and good fruit set were recorded due to pollination by Apis cerana indica, Braunsapis mixta and Tetragonula iridipennis.