The Biology of Neodiprion rugifrons (Hymenoptera: Diprionidae), a Sawfly Infesting Jack Pine in Wisconsin1

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Abstract
Neodiprion rugifrons Middleton has completely defoliated and killed 3- to 20-ft jack pines planted in southern Wisconsin. Adults emerge annually during May and June, and oviposit on foliage 1 or more years old. The larvae that eclose from these eggs feed only on this older foliage. Host damage is slight, as spring shoot growth is virtually complete before feeding on older foliage is finished. In years when a second generation develops, its adults emerge during July and August and oviposit principally in current-year foliage, and larvae eclosing from their eggs feed on foliage of all ages. Trees die when they are completely defoliated by second-generation larvae during September and October. The tops of trees retaining 10% or less total crown foliage often die the following spring; terminal shoots completely defoliated are significantly reduced in spring growth and often die; and terminal shoots defoliated except for a few needles near the terminal bud show significantly reduced spring growth. Terminal growth in trees with less than 10% total crown defoliation and undamaged terminal shoots is not significantly reduced. The sex ratio of ♀ : ♂ is approximately 2:1. Newly emerged females contain 20–150 eggs, and consistently produce 90–130 eggs when larval rearing conditions are favorable. Males are attracted to virgin females, and mating occurs soon after adult emergence. Mated females lay 90%–95% of their eggs, typically placing them at random within a compact group of about 25 needle fascicles near the tips of 1 or more shoots. The eggs develop in 2–3 weeks, and larvae feed for 4–6 weeks. Females have 5 feeding larval instars, males only 4. Development of male larvae is similar whether they arise from mated or from nonmated females. Cocoons are spun in soil to a 3-in. depth, where the last-stage larvae may remain in diapause for 1 or more years. The major recognizable control factor was dipterous parasitism of feeding larvae.

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