Know your pest : Mealybugs

Arati N / S Gunamani Singh / Romila Ak / Aruna Beemrote / Ajitkumar N / Sushmita Th / A Gangarani Devi *

Mealybugs :: Pix - TSE

Insects are all around us. Whether we like it or not, we are compelled to share space and food resources with them in our houses (e.g., cockroaches) and crop fields (e.g., caterpillars).

We tolerate them as best as possible, but problems start when their numbers increase or they spread disease. Insects which cause economic losses to cultivated crops, including flowers and spread diseases are considered insect pests. Insects like honeybees, ladybug beetles and dragonflies are insects but not insect pests. They are beneficial insects.

One such insect that is causing economic losses and becoming a menace in Manipur is mealybugs. Farmers and kitchen gardeners are sure to be familiar with this insect pest. Due to climate change, increased trade and transport of planting materials in Manipur, the incidence and spread of mealybugs on all crops are fast rising.

Mealybugs are small, oval, soft-bodied sucking insects with a cottony appearance. They are not a single species but belong to different genera and species while falling into the common category of "mealybugs". In India, they were considered a minor pest but have become a major pest over the past decade.

In Punjab, they devastated the cotton crop in 2007-2008 and emerged as a major pest in cotton. They feed on almost all cultivated crops- fibre crops like cotton, vegetables, fruits, ornamental plants, and agricultural plants such as potato, tomato, and brinjal.

The use of synthetic chemical pesticides targeting major pests in these crops reduces their population and the natural enemies found in these agroecosystems, leading to the rise in the incidence and population of minor pests, such as mealybugs. This phenomenon is a fallout of depending and relying too much on chemical pesticides.

Mealybugs have a worldwide distribution of about 2000 species, with over 160 species labelled crop pests. Their host range is wide and varied. Some species, such as Papaya mealybug, are invasive alien insects in India. They are phloem feeders, sucking the sap from all plant portions, and they also transmit certain plant diseases, resulting in significant economic losses to economically important crop plants.

NATURE OF DAMAGE: Adult mealybugs are found on leaves, stems, and roots. They are covered in white wax, making them difficult to eradicate. On stems and leaves, they form colonies that develop into dense, white, waxy aggregates. The surplus sap is excreted as honeydew, which attracts ants and develops sooty mould, preventing the plant from producing food and depriving plants of essential nutrients.

Both nymphs and adults suck plant sap which causes the yellowing of leaves. They prefer young, new growth; leaves drop to occur in severe infestations. Insects suck sap from plants hampering photosynthesis and causing wrinkling and yellowing of leaves.

The insect secretes honeydew. Fungal growth, i.e., sooty mould, occurs on honeydew secreted by insects. Blackening of leaves is observed when sooty mould growth occurs. In severe infestation, the mealybug colony is cocooned in a mealy mass with a white, cottony appearance hanging from branches and twigs.

BIOLOGY OF MEALYBUGS: Different types of reproduction are found in different species of mealybugs, such as parthenogenesis (asexual single parent), biparental reproduction and ovoviviparity (i.e., the eggs hatch internally within the female and result in the delivery of living larvae).

The mature female lays eggs within a white waxy structure, ovisac typically in groups on the host plant's twigs, branches, or bark and occasionally on the plant leaves and terminal tips. Each egg sac can hold up to 600 eggs, with the majority being female, leading to a rapid increase in population. Eggs are minute.

Eggs hatch and transform into highly mobile nymphs known as crawlers. Nymphs of both sexes resemble adult females. Females have three nymphal instars, whereas males have 4, lasting 22-25 days. Under normal conditions, individual mealybugs may take up to 30 days to complete their growth through all the nymphal stages. There may be as many as 15 generations per year.

ANTS ASSOCIATION WITH MEALYBUGS:

Ants care for mealybugs for their honeydew secretion, like humans rear cows for milk. The female mealy-bug is not active and unable to fly. Ants help spread mealybugs and protect from predatory insects and other natural enemies. They also clean the colony from detritus on secreted honeydew. Ants spread crawlers.

MEALYBUGS MANAGEMENT:

1) Mealybug control frequently requires controlling caretaking ants. Mealybug populations remain small and slow to spread without caretaking ants. Ant nests can be killed using Chlorpyriphos 20 EC @ 2.5 ml/l or 5% Malathion dust @ 25 kg/ha.

2) Remove and burn all crop debris from contaminated farms. Field bounda- ries and farms should be clear of mealybug-attracting weeds.

3) Protection and conservation of beneficial insects and natural enemies by planting attractant plants such as marigolds, coriander and Thai basil.

4) Foliar spray of Verticillium lecanii or Beauveria bassiana (2 × 108 cfu/ml) at 5 g/ml per litre of water effectively reduces mealybug populations.

5) Hortimin and Lastraw are mineral oils and can be used @ 5ml/litre water to control whiteflies and other soft-bodied sap suckers. They should not be used under elevated temperatures beyond 25oC. They are non-insecticidal formulations.

6) They dissolve the waxy protective flocculent material surrounding the pest and render the insect vulnerable to abiotic stress while clogging the insects' breathing pores (spiracles).

7) Some ready-to-use salt-based organic mixtures for soft-bodied sap-sucking pests with a mealy/waxy coating are available for kit-chen, garden or home use.

8) As a persistent pest with fast reproductive ability, repeated sprays are necessary, which are much better than chemical pesticides to combat this insect.