Insect Watch

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The Colorado potato beetle is a major pest all across Canada.

[deck]What are the leading pests potato growers need to watch out for this season — and what they can do to control them.[/deck]

Every year, potato producers across Canada are challenged by insect pests damaging their crop. The size of the pest population is influenced by factors such as overwintering survival, host plant availability and climatic conditions, and often varies widely between regions and years.

To mitigate damage to the crop, it is essential to monitor for insect pests, especially the most significant ones. Some important pests to watch for in 2015 are:

  • Colorado potato beetles
  • Potato flea beetles
  • Aphids
  • Wireworms
  • Potato leafhoppers

Colorado Potato Beetles

This beetle is a major potato pest all across Canada. The adults overwinter in the soil along hedgerows and emerge in early spring, when they lay clusters of yellow eggs on the underside of leaves of newly emerged potato plants. The eggs hatch within a week and the larvae begin feeding on the leaves, passing through four larval stages before pupating in the soil.

The beetles emerge from the pupa as new adults in late summer. At this stage they’re known summer adults and they feed on the leaves before moving to hibernating sites. Both larvae and adults feed extensively on potato leaves, which can result in complete defoliation of the plants if populations are left uncontrolled.

There are several insecticides registered for controlling the Colorado potato beetle in Canada. Insecticides applied in-furrow at planting are systemic and control both the larvae and adults over the summer.

For foliar insecticides, it is best to scout the field beforehand to determine if the economic threshold has been reached before applying pest control products. This threshold is typically defined by the Colorado Potato Beetle Equivilent (CPBE), which can be calculated using this formula:

CPBE = (no. of spring adults x 1) + (no. of small larvae x 0.125) + (no. of large larvae x 0.333) + (no. of summer adults x 0.625)

An economic threshold of 1.5-2 CPBE is recommended before application of a foliar insecticide.

Potato Flea Beetles

These are tiny black beetles that overwinter as adults along the field edges under leaf litter and debris. They emerge in early spring and begin feeding on the leaves of potato plants. The beetles are of greater economic importance in the Maritime Provinces compared to other potato growing regions in Canada, where populations may not reach economic injury level.

Flea beetles lay their eggs in the soil where the larvae hatch and feed on root hairs. Pupation occurs in the soil; new adults emerge in late summer and begin feeding on the leaves before moving to hibernating sites. Adults cause most of the damage by feeding on leaves, creating small holes that give the leaves a typical “shot hole” appearance.

Applications of in-furrow systemic insecticides at planting can effectively reduce adult flea beetles populations in the spring. The use of thresholds is recommended for foliar insecticide applications.

Scouting fields and counting the number of shot holes on the fourth terminal leaf (counting down from the apex of the plant) is used to establish damage levels in terms of percentage of leaf eaten. Low is 0-5 per cent, medium is 5-10 per cent and high is more than 10 per cent. This information is used to correctly time foliar insecticide applications.

Aphids

Aphids are a major concern to potato growers because they are capable of transmitting viral diseases such as potato virus Y (PVY) and potato leaf roll virus (PLRV). The most commonly found aphids associated with potatoes are:

  • Green peach aphid, considered the most important aphid pest because it is a very efficient vector of PLRV. This a late-infesting aphid arrives on winds blowing in from the south late in the season
  • Fox glove and Buckthorn aphids, which are good vectors of PVY
  • Potato aphid, which is usually the first colonizing species and the most abundant across Canada

Systemic insecticides applied in-furrow at planting effectively control aphids in spring and early summer; however these insecticides lose their efficacy later in the growing season. Aphids such as the green peach aphids that arrive late in the season need to be controlled by foliar sprays.

Scouting and the use of yellow pan traps are useful for determining the arrival and presence of aphids in a potato field and can help in correctly timing foliar insecticide applications. It’s important to look for the weed known as hairy nightshade, which is an exceptionally good host for aphids. Removing this weed will reduce the build-up of aphid populations in the field.

Wireworms

Tuber damage caused by wireworms is an increasing concern for potato producers across Canada. Wireworms are the larvae stage of click beetles. The adults emerge in the spring and lay eggs in the soil, but they do not cause any damage to potato plants.

The larvae, which can live for five years in the soil depending on the species, damage potato tubers by making holes and tunnels while feeding. This damage can result in harvested tubers being unmarketable for the table and processing markets.

Control of this pest is very difficult because of its long life cycle and large host range. For potatoes, there are three registered insecticides in Canada that can be applied in-furrow at planting:

  • Thimet (active ingredient: phorate) which is scheduled to be taken off the market by the end of 2015
  • Titan (active ingredient: clothianidin)
  • Capture (active ingredient: bifenthrin)

The efficacy of these insecticides varies across different regions in Canada, and some may only suppress wireworm feeding but not kill them.

Potato Leafhoppers

The potato leafhopper does not overwinter in Canada but migrates in on southerly winds from the United States. This pest is mainly an issue in Ontario and Quebec and but occasionally it can cause problems in Manitoba.

Both the nymph and adult stages of the leafhopper feed on the stems and leaves of potato plants. The damage is characterized by yellowing and the eventual death of affected foliage. The initial damage appears as yellowing at the tips and margins of leaflets, and these symptoms are typically called ‘hopper burn’. Dry conditions can exacerbate symptoms.

There are two generations (egg to adult) in one growing season in southern Ontario but only one generation in the rest of Canada. A threshold of 10 nymphs per 100 mid-plant leaves is recommended before control measures are considered. Several registered foliar insecticides are effective against potato leafhopper.

Regardless of which insect pest is present, the best strategy for mitigating economic losses is for potato producers to scout and monitor their fields for pest insects and apply the appropriate control measure at the correct time.

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