Spider Mites Put Up A Tough Fight – Here’s Why

December 4, 2011

The tiny two-spotted spider mite (Tetranychus urticae) causes much anxiety for farmers and has been, to date, a scientific mystery. It feeds on over 1,100 species of plants, including 150 greenhouse plants and crops, such cotton, corn, soybean and a host of vegetables and permanent crops.

Spider mite damage on cotton. (Photo: Jack Bacheler, North Carolina State University)

By one estimate, farmers spend $1 billion a year worldwide on crop protection materials to hold the prolific mite in check. It breeds rapidly, especially in hot weather, and multiple treatments often are needed to reduce crop yield loss.

What researchers are finding now is that this near-microscopic pest is quite adaptable, which explains its ability to quickly dodge or build resistance to certain chemical treatments.

How adaptable? Based on a multi-country study, it appears to have adapted genetic material from bacteria and fungi that increase it’s ability to both digest food and detoxify itself when exposed to miticides and insecticides.

In the latest issue of the journal Nature, an international consortium of scientists published information on the sequencing of the spider mite genome, in turn shedding light on how it is capable of such feeding frenzy, as well as other secrets of this tiny pest.

Here’s more info from an announcement:

Their findings offer hints about new, potential control approaches, thanks to greater insight about the biological interactions between plants and herbivores that feed on them.

The group of researchers included teams from the U.S., Canada and Europe.

A spider mite can feed on an astonishingly large number of plants because it withstands the toxins that plants produce, a point stressed in the group’s findings.

“This, in itself, is an amazing feat,” according to an announcement of the findings. “However, among arthropods (animals with exoskeletons, such as spiders, ticks, crustaceans and insects), the spider mite holds first place in the number of pesticides it is resistant to. Mites become resistant to new pesticides within two to four years, meaning that control of multi-resistant spider mites has become increasingly difficult.”

Having the sequence of the spider mite genome has shown light on the genetic basis for its feeding flexibility and pesticide resistance.

Its secret? On one hand, it carries more copies of the genes involved in digesting and degrading plant toxins when compared to insects. On the other, the tiny pest seems to have incorporated genes from bacteria and fungi that are involved in digestion and detoxification.

In fact, researchers identified two groups of bacterial and fungi genes that are unique to the spider mite, suggesting that the tiny arthropod is adept at making the most of the innovation of transfer of genes between distant species (called lateral gene transfer – a rare occurrence in nature).

“Other groups of genes are shared, with aphids, for example (aphids are insects that also feed on crops),” according to the announcement. “By comparing the aphid genome with that of the spider mite, it seems that the bacterial genes moved first into the insects and from these were taken up by the spider mite.”

The name gives it away: spider mites make webs, for protection against predators and as a barrier against bad weather. However, their webs are different to those made by spiders: the genome sequence has revealed 17 genes involved in making web proteins.

These proteins make thinner fibers, but seem to be slightly more resistant to mechanical forces than other natural materials.

All these secrets came out of a very small genome – only 90 megabases.

By comparison:

  • The relatively simply fruit fly genome has 180 megabases.
  • The human genome has 3,000 megabases.

It is, indeed, the smallest arthropod genome sequenced so far, and reveals a remarkable evolutionary history: the spider mite has lost many genes that are shared amongst arthropods, but has accumulated species-specific genes, such as those that give it the ability to withstand toxins and pesticides.

Portuguese scientists were involved in analyzing immunity-related genes found in the spider mite genome. The spider mite belongs to the Chelicerata family, the second largest group of terrestrial animals. Chelicerates include spiders, scorpions and ticks. Chelicerates and insects make up the Arthropods. The spider mite is the first chelicerate to have its entire genome sequenced and analysed.


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