Volume 93, Issue 4
June 1984, pages 265-406
pp 265-273 June 1984
In a number of plant galls caused by members of Diptera, the larval cavity and/or the nutritive zone around it are inhabited by specific fungi. Careful investigations reveal that these fungi are not contaminants, but are brought by the gall insect itself. The fungi appear to lead a mutualistic life not only with the insect larva but also with the plant. Details regarding the fungi, and the nature of this three-way relationship, although fragmentary, are summarised, along with the author’s unpublished observations.
pp 275-282 June 1984
The interactions and inter-relationships between plant galls and the chalcids with them are often extremely intricate and it is not easy always to assess correctly a given species as a gall former or an inquiline or a parasite of another inhabitant of a gall. The chalcids show complex inter-relationships with other inhabitants of a gall in many instances. The existence of both phytophagy and entomophagy in the same species or in the same individual is an interesting phenomenon met with in some species of chalcids associated with plant galls. The host-finding and host-acceptance behaviour of some of the gall-chalcids are other interesting aspects discussed.
pp 283-292 June 1984
The active substance, cecidogen secreted from the saliva of gall midge larva and the larval feeding on the meristematic tissue of terminal and auxiliary shoot apices in rice plant are responsible for gall formation (silver-shoot) in rice. Gall midge occurs in many of the rice growing areas and causes considerable yield loss even up to 70% in kharif crop. Many resistant varieties are being cultivated but the occurrence of biotypic variation of the insect limits their extensive use. Integrating chemical and genetic resistance appeared feasible to check the pest. Seasonal occurrence, host variation, resistance mechanism, biological, cultural and chemical control methods are reviewed and future lines of research on this important pest have also been discussed.
pp 293-300 June 1984
Compared to other phytophagous insects, the gall insects are specialised in view of their imperative demand for a particular type of food in terms of a specific host; this is further supported by their specialised trends in establishing a coordinated, functionally efficient system involving (i) the biogenesis of the host plant organ, and (ii) the life-cycle of the gall-maker. The ’inherent potential’ (genetic-?) of the gall-insect to establish an independent, yet a discrete habitat, modifying the host plant tissue to enjoy a self-contained nutritional guild, facilitated by the host selection patterns, appears especially different from the other non-cecidogenous phytophages.
Besides the autecological factors such as the changing climate, chemical and physical changes in the host tissue, and patchyness of food plant resources, which are of great importance to the cecidogens, of particular significance are the community interactions within gall systems involving many arthropod participants like inquilines, parasites, predators, and other casual visitors. As specialised organisms and with limited population size per unit area, the gall insects seem to run the risk of random extinction, although their efficient development of strategies to survive appears significant.
pp 301-314 June 1984
The dynamic nature of the galls evoked by species ofMeloidogyne (root-knot nematode) involves many hypersensitive reactions. Energy relations in the galls of the suscep-tible hosts reflect the combat capabilities centering around functional resistance. Factors associated with resistance are indicated. The futurology of the problem of containing the infection is discussed. Chemotherapy consisting of imparting resistance in susceptible hosts is suggested, to achieve success in agricultural economy. Electrophysiological relations of pathogenesis and the future implications of these studies in the understanding of th6 disease are indicated.
pp 315-322 June 1984
Vector potential of mycophagous thrips as well as the nature of thrips-fungus interactions, in relation to the carrying of the fungal pathogens such asLasiodiplodia theobromae, Pestalotia sp.,Aspergillus sp., andPenicillium sp., are discussed.
pp 323-333 June 1984
Mycoplasma and allied diseases of forest trees in India have been described,mlo disease has been intensively studied in ‘sandal’ (Santalum album L.). Other forest trees showing typicalmlo etiology are ‘toon’(Toona ciliata), Acacia catechu, A. mearnsii, Eucalyptus grandis andE. tereticornis. Disease symptoms, collateral hosts, transmission tests with possible insect vectors and vector biology have been described for sandal spike disease. Reasons for contradictory results for sandal spike vectors have been discussed and possible lines of vector search for woody plants are described. Vector-host-pathogen interactions have been reviewed. During the feeding process, phloem cells are punctured, torn and disturbed by vectors. The hypertrophy of the plant cells around the site of puncture is caused by the action of injected salivary secretion. Vector borne procaryotic disease pathogens multiply within the vectors and circulate through a sequence of tissues and organs of vectors when the latent period of pathogen is long. These initiate both harmful and beneficial interactions within the vector. Possible lines of research to fill up the existing lucunae for insect vectors of mycoplasma and allied diseases of forest trees and vector-host-pathogen interactions have been discussed.
pp 335-338 June 1984
Leafhoppers in the familyCicadellidae and planthoppers in the familyDelphacidae are known to be active vectors of about 25 viruses infecting cereal crops. These leafhopper and planthopper transmitted viruses are discussed in this paper.
pp 339-347 June 1984
Although 120 diseases of fungal, viral, mycoplasma and bacterial origin have been reported in sugarcane, insect vector associations have been established with virus/mycoplasma diseases only. Thirteen species of aphids have been identified to be vectors of mosaic and another bug,Assamia moesta Westw. is associated as a mechanical carrier. Three species ofPerkinsiella transmit Fiji disease. The leaf hopper,Cicadulina mbila (Naude) transmits streak. The mycoplasma diseases, white leaf and grassy shoot are carried over byMatsumaratettix sp. andA. moesta, respectively. A mealy bug has been reported to be a vector of spike disease. The future lines of approach for insect vector associations are also discussed.
pp 349-357 June 1984
Information on the mechanism of viral acquisition by insect vectors and the interactions therein appears to be inadequate, and the differential responses of the vectors to different plant species appear to influence the virus transmissibility. High transmissibility is found in plants which have high susceptibility to both viruses and vectors. The vectors having low preferences for particular plant species show their inefficiency in transmitting viruses in those plants. Differential acquisition of virus is found even in different varieties of the same species. Different vectors of a particular virus may show differential capability of transmission in different hosts. The host specificity of the vectors also partially explains the virus specificity in plants. But in-depth information on the differential transmissibility due to vector-host interactions is not yet available. Further the loss of transmissibility by vectors due to feeding on some unfavourable hosts and due to mixed feeding of vectors and non-vector species of insects on some hosts, etc., have also not yet been adequately studied. Detailed study on the effect of insect vector-host interactions on the plant vims transmission may unravel a new biological method for the control of the plant virus disease in future.
pp 359-363 June 1984
Pollination is one of the most fascinating aspects of interaction between plants and insects. The extent of interdependence is regulated by phenology, floral characters as well as by form, structure and behaviour of the pollinators. During evolution flowers have developed various strategies for attracting insects. p ]We have studied the biology of pollination inLantana camara, a pernicious weed. The colour variant used in this study bears yellow flowers at anthesis which subsequently change to orange, scarlet and magenta. Lantana is self-compatible but needs insects for pollination. Thrips have been found to be consistent and regular pollinators. They visit only yellow flowers and avoid flowers of other colours. Colour change is triggered by pollination and functions in the partitioning of the pollinator and consequently helps in conserving pollinator energy. Whereas butterflies visit lantana in two seasons, thrips are associated with it all through the year and play an important role in seed production. By adapting to thrips pollinationLantana has become highly widespread.
pp 365-372 June 1984
That certain species of wasps developing inside the wild variety of figs are responsible for the setting and ripening of the fruits (caprification) of the cultivated variety of figs of the same species (Ficus carica L) is well known. Besides each species ofFicus having its specific agaonid pollinator and a close mutualistic symbiosis, several adaptive modifications in the agaonids andFicus species appear significant in the course of evolution of this relationship. Recent studies on the pollination ethology of the agaonids have shown that besides the passive mode of pollen transfer (topocentric pollination) an active and deliberate type of pollen transfer (ethodynamic pollination) exists, which is associated with the development of specialised structures like the mesothoracic pollen pockets and the fore coxal corbiculae. The ecophysiological changes in the interior of the ripening fig syconia also play an important role in pollination during the post-emergence behaviour of the males and females in some agaonids. Further lines of work in the fascinating area of fig-wasp research are indicated.
pp 373-390 June 1984
This article presents the recent trends in describing the bee-blossom relationships, in interpreting the bees foraging behaviour on the basis of optimal foraging theory and the nature of floral resource, the role of pollinator behaviour such as trap-lining, opportunism, territorial aggression, group foraging in determining the genetic make-up of the bee pollinated plants, and considering the pollination from the ecosystem point of view. The idea that honey bees and other social bees are adequate enough to pollinate the crops is considered as premature, and detailed studies to explore the pollination potential of wild bees together with their conservation and management are suggested.
pp 391-396 June 1984
Most butterflies with the characteristic long proboscis feed on floral nectar, and theHeliconius butterfly feed on pollen as well. They rely mostly on flower colour for locating and identifying the flowers. While foraging on nectar, they carry pollen on their body parts. Evidence is available indicating that the flower-visiting butterflies need not function in every case as pollinators.Leptidea synapis forages at the flowers ofViola andLathyrus without performing the reciprocal pollination service. The butterflies as a group have the tendency to visit a few flowers on a plant and then fly to another plant with the result of maximising xenogamy. The need for detailed studies over different geographical regions is stressed to fully evaluate the role of butterflies as pollen vectors.
pp 397-406 June 1984
Whiteflies (Bemisia tabaci) transmitted yellow mosaic, yellow vein mosaic and leaf curl diseases of economically important crops. Horsegram yellow mosaic virus and cowpea mild mottle virus have been isolated and characterised in India. Except these two viruses the causal agents of other whitefly transmitted diseases are not known.