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Pests


Wire Worms

Wireworms are the larvae of the click beetle, so-named because of the audible click sounded when they flick themselves upright after falling on their backs. Wireworms are grassland insects, but when infected grassland is ploughed, subsequent arable crops are often attacked. Root crops such as potatoes are particularly susceptible as wireworms reduce the quality by burrowing into tubers, rather than affecting yield. Wireworms bore holes in tubers. The holes, which look as if they were made by stabbing the tuber with a nail, usually are lined with potato skin. Economic damage can occur at low population densities.

There are approximately 60 species of click beetle or "skipjacks recorded, but only a few of these can be classed as pests. By far the most important pest species are those belonging to the genus Agriotes. Three species, A. lineatus, A. obsurus and A. sputator are responsible for the vast majority of attacks.

Permanent grass is the most favorable for wireworms as it supports the largest populations. Wireworm populations in grassland of fifteen years duration have probably reached a stable level and are unlikely to increase further.

Young wireworms are initially white and about 1.5mm long. Young larvae need live vegetable material to survive and grow, and are likely to die of starvation within 30 days in the absence of food as they are unable to survive on soil organic matter. As they grow, their colour darkens to a shiny golden brown typical of all wireworms. Wireworms mature very slowly, passing through one to three instars each year.

If potatoes are to be sown in an infected field, growers should choose a variety that can be harvested in mid August or earlier, use a recommended insecticide, at planting and check the progress of damage from mid August onwards and lift at the earliest opportunity if damage is staring to occur.

The life cycle of most common wireworms takes 3 to 4 years under favorable conditions. Wireworms spend the winter in the soil either as partially grown larvae or as new adults in overwintering cells. Adults work their way to the soil surface in the spring when soil temperatures rise. These adults eat little if anything, and cause no economic damage.

As soon as the female emerges from the soil she mates and then returns into the soil to lay eggs at depths of 1 to several inches. Some females may not lay all of their eggs in one location but will return to the soil surface, fly to a nearby area, then re enter the soil and continue egg laying until they have deposited approximately 300 eggs. Because of these flights and the specific food and moisture requirements of larval development, spotty patterns of wireworms infestations are often found.


Digging Sods:

October/November and March/April are the best months for finding the worms. They are temperature sensitive and move downwards in the soil in mid -Winter. In frosty weather they have been found 1m down in the soil.

Sampling based on 20 x 10 cm diameter cores for a field between 4 and 10 ha is the most efficient method for estimating wiremworm populations. A single wireworm found in a 20 core or sod sample, represents a field population of 62,500 /ha which is still high enough to cause economic damage. Another significant drawback of soil sampling is that it is extremely labour intensive.


Holes in Tubers:

Wireworms tunnel into potato tubers leaving small, round holes on the surface and narrow tunnels into the tuber flesh. Superficially, wireworm damage may be confused with slug damage as slug entrance holes are similar to those made by wireworms. However, slugs often hollow out large cavities within the tuber flesh, whereas wireworms do not. Although damage does not affect yield, it causes a serious loss in quality tuber, which can render tubers unmarketable even when damage levels are relatively low. Wireworm holes may also provide access for slugs, or other secondary soil organisms such as millipedes or bacterial rots. Wireworms also attack seed potatoes, but this seldom affects the growth of the plant. Finding wireworms feeding on seed tubers shortly after planting is not a reliable indicator of likely damage to daughter tubers.


Control:

The most effective method of preventing wireworm damage is to avoid growing potatoes in infested fields. This requires a degree of forward planning to allow the implementation of risk assessment methods well in advance of final decisions being made on field choice.

Populations of wireworm decrease quickly under arable cultivation and it is unusual for large numbers of wireworms to persist for more than three to four years after grassland is ploughed.


Insecticides:

Firstly, wireworms are not generally found near the soil surface, so insecticides usually have to be applied pre-planting to ensure adequate incorporation of the active ingredient into the soil. Secondly, as Agriotes wireworm damage to potato tubers appears to occur late in the life of the crop, insecticides ideally have to be very persistent to ensure adequate control. Some control can be achieved using Nemathorin.


Slugs

Slug control is one of the most difficult problems in potato growing. Slugs not coming to the soil surface to take pellets is the kernel of the problem and also the fact that they prefer a tuber to a pellet adds to our troubles. The keeled slug and the garden slug are the main culprits. The grey field slug is of little importance to us as it stays mainly on top of the soil and only enters the tuber through a hole already made by another slug.

The keeled slug only comes to the surface to mate. This happens in late July or early August. Hence these are two periods when we apply pellets.

The garden slug is more mobile, and moves about on the soil surface and down through the soil.

Fields can vary very much as to the type and number of slugs present, so controlling them is very imperfect science.

New or different pellets are not likely to come on the market in the future. The best we can do is improve our knowledge of the field, slug species and varietal resistance.


Key Points:

  1. In some soils it is almost impossible to control slugs in a wet year.
  2. The ‘Keeled Slug’, the real potato slug, only surfaces in order to mate.
  3. Slugs prefer tubers to pellets, so they have little need to come to the surface.
  4. The field slug eats the potato leaves mostly and is no indication of the population of Keeled slugs in the soil.

No Tolerance:

Slugs often spoil maincrop potatoes by making holes in the tubers. Damage can be serious in autumns following mild, wet summers – especially in heavy soils. As no more than 5% of damaged tubers are allowed by the Gradings Standards only a very low level of slug damage can be tolerated, and though the actual loss in weight of individual tubers damaged by slugs is small, the economic consequences can be severe.

Roundback and keeled slugs are the main culprits; the field slug also contributes to the damage, although it is believed mainly to attack tubers, which have already been damaged.

Slug damage in potatoes is often confused with damage by cutworms. However, cutworms’ damage is most prevalent on light and medium soils after hot, dry summers. Slugs are more likely to be the problem in potatoes grown on heavier soils. Slugs typically make a small entry hole and eat right into the center of tubers, subsequently hollowing out a large cavity in which they can often be found resting. In contrast, cutworms make large entry holes and tend not to penetrate as far.

Slugs feed on tubers in the later part of the season, August to October.

Damage can therefore be partially avoided by lifting the crop as soon as possible after maturity.

Research has showed that delaying lifting from early October until early November resulted in slug damage increasing from 30% to 45%.


Control Measures:

Two main control measures are available:

  1. growing a resistant variety,
  2. using pellets.

A further culture method of limiting damage to potatoes is to lift the crop early – this entails yield penalties. It is not possible to eliminate damage entirely by choice of one of these methods alone, but where damage is anticipated, the use of more than one method is to be preferred.


Products for Slug Control

Product Active Ingredient Rate of Use (kg/ha)
Mini Slug Pellets Metaldehyde 15
Metarex Metaldehyde 5

Nematodes BrianRigney_PCNSurvey

Aphids 2015-Seed-Workshop-Poster