Larvae of several moths (order Lepidoptera) occur incidentally in forest nurseries and, although often causing cutworm-like damage, they are not true cutworms. This designation applies only to larvae, of moths in the family Noctuidae, of which the variegated cutworm Peridroma saucia is probably the most common and damaging in local forest nurseries.
Some cutworm damage likely occurs in all nurseries every year. All coniferous species in both bareroot and container nurseries throughout the province may be attacked, but severity of infestation varies greatly. Damage is generally confined to young, succulent seedlings before their stems become woody. A single larva can destroy many seedlings each night, thus only a few cutworms per m2 can destroy thousands of seedlings in a few weeks. Cutworms feed on foliage (Figure 86) and often cut through the stems just above ground, leaving a short stump. The following are indicators of cutworms: (i) stems on which the needles have been consumed, (ii) sunken or depressed areas on the stems that look like fungus-caused lesions (these are old feeding sites), (iii) stems cut off below the soil so it appears that the seedling did not germinate, and (iv) stems cut off at ground level. The latter damage may be confused with damping-off.
Cutworms have recently become a problem on 2+0 container stock in compounds. Damage on such seedlings is minimal, as they can withstand some defoliation. On summer-lifted seedlings, however, cutworms could be carried to the outplanting site, damaging seedlings there.
Adults are dull-colored, heavy-bodied, night-flying moths about 18 mm long. Their wings are folded tent-like when resting. Larvae are large, soft, fat, worm-like, dull-colored caterpillars up to 4 cm long, with hairless bodies and shiny heads. They characteristically assume a curled position (Figure 86), especially if disturbed. Variegated cutworm larvae (Figure 88) have a single line of four or more yellowish orange dots on the upper body surface. Cutworms pass through egg, larva, pupa, and adult stages and, depending on the species, can have one to three or more generations per year. The number of variegated cutworm generations per year varies according to climate. Locally, one or two normally occur, but this may increase in certain localities in warmer years. The spring generation is the most damaging, because its occurrence coincides with seedling germination. The larvae, which feed at night and hide during the day, are often difficult to detect. They may be easier to locate in container stock because they hide in the container cavity, disturbing the covering surface, or in moist areas between styroblocks. Depending on the species, cutworms can overwinter in most life stages.
Fallow, bareroot fields should be kept weed-free to reduce egg-laying females, which are attracted to certain weeds. However, once cutworms are present, killing weeds may drive larvae from preferred weeds to seedlings. The use of light traps in greenhouses has effectively reduced populations of adult moths. Greenhouses can be insect-proofed to exclude moths. In container nurseries, small outbreaks can be controlled by removing and destroying the cutworms from the growing medium. In both bareroot and container nurseries, poison baits with natural attractants (such as apple pomace or bran) may be used to kill larvae. Where outbreaks are large, insecticides are often used; but where damage is confined, only those areas where seedlings have been damaged should be sprayed. Sprays are most effective against young larvae. They should be applied under warm moist conditions late in the day or in the evening when cutworms are most active.
Banham, F.L. and J.C. Arrand. 1970. Recognition and life history of the major insect and allied pests of vegetables in British Columbia. B.C. Dep. Agric., Bull. 70-9, Victoria, B.C.
Furniss, R.L. and V.M. Carolin. 1980. Western forest insects. U.S. Dept. Agric., For. Serv., Misc. Publ. No. 1339, Washington, D.C.
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Principal, locally grown hosts |
Host age and season when damage appears |
|
Nursery type and location |
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|
|
|---|---|---|---|---|---|---|
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|
|
|
Bareroot |
|
Container |
|
|
|
Age |
Season |
Coastal |
Interior |
Coastal |
Interior |
|
All species |
1+0 |
Through out growing season |
Yes |
Yes |
Yes |
Yes |
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Figure 86. Typical cutwormn and cutworm-damaged seedling (cut off at groundline).
Figure 87. Life history of cutworms (up to three or more generation per year).
Figure 88. Varigated cutworm Peridroma saucia.