The 'might' of mites and the way out

Mites and ticks are small arthropods belonging to the class Arachnida.  Mites, along with crabs, scorpions, spiders and ticks belong to the arthropodan group Chelicerata, the second largest group of predominantly terrestrial animals.

Mites comprise spider mites, dust mites, gall mites etc. Among spider mites, the most important is the two spotted spider mite (TSSM), also known as red spider mite, a serious pest on more than 1,200 plant species including fruits, vegetables, trees, shrubs, herbs, herbaceous perennials and ornamentals.

The long running list of crops affected by the mite includes apple, cotton, soybean, okra, ashwagandha, cucumber, rose, strawberry and brinjal.

They are polyphagous or generalist herbivores, feeding on numerous species of plants, and possess the ability to detoxify or tolerate the diverse toxic compounds produced by plants to deter feeding. The TSSM has been recorded from several countries in Europe, Asia, Africa, Australasia, the Pacific and Caribbean islands, North, Central and South America.

Light infestations by this mite usually go unnoticed and cause stippling of mature leaves. But, heavy infestations cause discolouration and yellowing of infested leaves and damage buds and flowers, impairing plant growth.

The webbing of infested plant parts is one of the most characteristic signs of heavy mite infestation, facilitating their movement across the leaf surface, to protect against climatic factors such as wind and rain, and also from natural enemies and exposure to chemicals.

r-type and k-type
The adult female is eight-legged, 0.4-0.6 mm long, and pale yellowish or greenish with two dark spots, composed of food content, showing through the transparent body wall.

Males are smaller with more pointed abdomens. Females produce haploid males parthenogenetically (asexual reproduction) and diploid females when fertilised by the opposite sex. Life cycles span from nine to 20 days, comprising the egg and four mobile stages — six-legged larva, protonymph, deutonymph and the adult (latter three are eight-legged). Females can also lay eggs for a period of 20-30 days, up to 10 eggs per day.

Individuals remain in the same area where they are born, marked by secreting silk. Depleting resources force young mated females to disperse using “silk balloons”.

Species make up either of the two generalised life-history strategies (r-type and k-type) as given by American ecologists Robert MacArthur and Edward O Wilson. Spider mites are r-selected (more than 20 generations per year), focusing on more offspring at the expense of individual parental investment. Rodents, several insects and weeds are other classic examples of r-type.

Birds, large mammals and large plants are k-type species characterised by reduced quantity of offspring with an increased parental investment seemingly to promote success in particular environments. These life history strategies answer, why cows and elephants give birth to one calf at a time but mice can have about 10 litters per year.

TSSM feed on underside of leaves by sucking plant cell sap and obtain food by removing chlorophyll with their stylet-like mouthparts destroying 18 to 22 cells per minute. Like many of the serious arthropod pests of agriculture, TSSMs are polyphagous and can rapidly develop pesticide resistance. In nature, effective natural enemies on these mites include certain beetles and bugs.

TSSM infestation can be managed by cutting and burning severely damaged plant parts or spraying botanicals or synthetic acaricides at the early infestation levels.

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