- The two-spotted spider mite (Tetranychus urticae) is a polyphagous insect that also likes to cultivate corn.
- Both adults and larvae are harmful.
- They suck the juices from the plant, weakening the attacked organs, which dry out prematurely.
- IOR-PIB observations indicate that its most numerous occurrences are in southern and central Poland, but the arachnid also damages crops in the north of the country.
The harmfulness of the two-spotted spider mite is that it has a piercing-sucking apparatus, with which it sucks the juices from the attacked plant. Another argument why the spider mite is dangerous to plants is that the pest produces a large number of generations during the season (most often 4-5, but there can be more 6-8). One female lays about 100 eggs.
As stated by IOR-PIB in its phytosanitary report: the development time of one generation depends mainly on the temperature and ranges from several to several dozen days.
Therefore, the warmer the season, the greater the number of generations usually and therefore the greater the damage to the crop.
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Spider mites – a magnifying glass will come in handy
Spider mites are small arachnids (about 0.5 mm) that are sometimes difficult to see with the naked eye. A magnifying glass is useful for observing them. At first, the pest is red, and over time, through orange to yellow, they change into light green individuals.
– On corn, the red spider mite can be found from June to the end of the plant vegetation period, with the highest numbers of the arachnid occurring from mid-July to the second half of August – we read in a press release from IOR-PIB in Poznań.
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The harmfulness of the pest is increasing
In the era of climate change and due to the high area of corn crops, the pest develops without any major obstacles.
– In corn crops, it is able to inhabit up to 100% of plants in a field, but its population size can be observed to vary on large-area plantations – higher in the marginal strips and lower in the middle of the field, which later translates into the degree of damage to the leaves that the arachnid most often inhabits. Its greatest harmfulness is observed in the marginal strips of crops at a depth of several or a dozen or so meters. In such strips, its feeding can lead to the complete drying of plants. A similar symptom of the arachnid's feeding is observed, for example, in sugar beet crops – we read further in the IOR-PIB press release in Poznań.
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Is it possible to control two-spotted spider mites in corn?
The initial symptoms of spider mite feeding are difficult to observe. Usually, the grower is concerned about advanced symptoms of pest feeding. At that time, spots appear on the blades visible from the top of the leaves. At first, they are light yellow, but over time they expand and turn brown. The leaf dries prematurely and then no longer serves its function.
– Excessive transpiration during periods of problems with access to water contributes to the drying out of plants – adds IOR-PIB.
Then the plants experience drought stress more strongly.
According to scientists from IOR-PIB, in terms of harmfulness, the two-spotted spider mite is more destructive to corn leaves than aphids. Its presence also promotes the appearance of diseases, especially small leaf spot, corn rust and yellow leaf spot. It can also contribute to the development of bacterial and viral diseases in corn crops.
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As IOR-PIB points out, at the moment only one plant protection product is registered to control the two-spotted spider mite, in the form of a biopreparation based on the Beauveria bassiana fungus (strain ATCC 74040), but it is only approved for sweet corn.
In this situation, broadly understood prevention is key, including:
- using crop rotation,
- spatial isolation from other host plants,
- balanced fertilization (especially nitrogen),
- weed control,
- precise crushing of crop residues.
