MSc

The spider mite, Tetranychus truncatus Ehara and the black cowpea aphid, Aphis
craccivora Koch are two predominant sucking pests infesting several vegetable crops
in Kerala. Farmers depend heavily on chemical pesticides for the management of these
pests. However, due to the adverse effects of synthetic pesticides on environment and
non-target organisms, there is a growing need for environmentally benign alternatives.
This has generated interest in the development of biopesticides, based on botanicals.
Tagetes minuta L., commonly known as Mexican marigold or wild marigold or
southern cone marigold is a plant whose pesticidal properties have been well
documented.
The study was carried out in the Department of Agricultural Entomology, College
of Agriculture, KAU, Vellanikkara during 2020-2022. The objectives of the study were
to identify the bioactive fractions of T. minuta and to evaluate the bioefficacy of the
solvent fractions against T. truncatus and A. craccivora. Tagetes minuta plants
cultivated in the Department of Floriculture and Landscaping, were harvested at the
flowering stage followed by shade drying and grinding. The powdered botanical was
extracted sequentially using hexane (non-polar), chloroform (medium polar) and water
(highly polar) as solvents. The hexane, chloroform and aqueous fractions of T. minuta
were evaluated for their efficacy against T. truncatus and A. craccivora in the laboratory
at five different concentrations viz., 0.025, 0.05, 0.1, 0.15 and 0.2 %.
The ovicidal and adulticidal effects of the solvent fractions were evaluated
against T. truncatus by topical application method. Considerable ovicidal action was
exhibited only by hexane fraction at higher concentrations (0.1, 0.15 and 0.2 %), with
cent per cent egg mortality at 0.2 %, after 120 h of treatment. However, all the three
solvent fractions caused appreciable mortality of adult mites, in a concentration
dependent manner. The hexane fraction at 0.2 % and 0.15 % were significantly superior,
causing 91.67 and 90.00 per cent mortality, respectively, followed by 0.2 % chloroform
(66.67), 0.15 and 0.10% (56.67), and 0.2 % aqueous fraction (53.33), after 120 h of
treatment.
The efficacy of the solvent fractions (0.025, 0.05, 0.1, 0.15 and 0.2 %) against A.
craccivora was evaluated in the laboratory on cowpea seedlings grown in paper cups.
Though the highest reduction in aphid count after 120 h of treatment was recorded in
hexane fraction at 0.2 % (90.00 per cent), chloroform (86.67) and aqueous fraction
(85.00) also recorded reduction in aphid count on par with it, at the same concentration.
Qualitative phytochemical analysis of the hexane and chloroform fractions was
carried out by GC-MS/MS to identify the major bio- active constituents. The two
compounds, 4-o-methylphorbol 12, 13- didecanoate and milbemycin b were found in
both the fractions, while 4α-phorbol 12,13-didecanoate was found in hexane fraction
only.
The best concentrations of the three botanical fractions of T. minuta were tested
for efficacy against T. truncatus on potted plants of amaranthus along with neem oil
emulsion (2%) and horticultural mineral oil (2.5%). After 14 days of treatment, the
hexane fraction recorded the highest reduction in mite population (88.78 %) followed
by chloroform and aqueous fractions which were on par with each other (77.97 and
76.96 %, respectively). Hexane fraction was superior to neem oil emulsion, while
chloroform and aqueous fractions were found to be on par with neem oil emulsion
(76.44), which is a widely recommended botanical preparation for mite pest
management in different crops.
The best concentrations of the three solvent fractions of T. minuta identified
in the laboratory study were evaluated against A. craccivora on cowpea, in a pot culture
experiment. By 10th day of treatment, the field efficacy of hexane fraction (78.95 %
reduction) was comparable with that of neem oil emulsion (87.59 %), while chloroform
(72.43 %) and aqueous fractions (68.20 %) were comparable with azadirachtin 1 EC
(67.97 %).
The results of the study indicate that Mexican marigold possess excellent
acaricidal and insecticidal properties and that the plant can be explored further, for
utilization in the management of spider mites as well as aphids.