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Figure 1: Saw fly defoliation, early stage. Photo by Valerie West.
Bossier and Webster parishes were host to an unwelcome pest this spring. Pine sawflies (Neodipiron spp.) left their mark across hundreds of acres in these parishes, alarming many landowners.
Midrotation stands of loblolly pine timber in Bossier Parish, are under attack from pine sawflies. In Figure 1, the early stage of defoliation is beginning at the tops of the trees. Figure 2 shows the late-stage of defoliation where most of the needles have been consumed by sawfly larvae.
Sawflies are not flies, but rather a subgroup of nonstinging wasps. Their larvae are caterpillarlike and can be mistaken for the larvae of moths and butterflies (Figure 3). Female sawflies use their saw-shaped ovipositor to insert eggs into the host plant tissue. They can lay more than 100 eggs each. The larvae emerge from the eggs and begin to aggressively feed, often defoliating the host plant over the period of a month or more.
Once the larvae have reached full size, they drop from the host plant to the ground, where they create cocoons and metamorphose into sawflies. This pattern can occur up to five times per season here in Louisiana and other parts of the southeastern U.S. depending on the species of sawfly. The main host plants for the sawfly in Louisiana are the southern yellow pines. Pine plantations, in particular, are vulnerable to sawfly outbreaks due to the availability host plants. Fortunately, while ugly to look at, sawfly damage in mature stands will reduce tree growth in the short-term but does not have a long-lasting impact on the overall health and productivity of the stand unless another environmental factor (e.g., drought) compounds stress to the trees (Figure 4). In young stands, sawfly damage causes larger growth losses and can result in the death of seedlings if left untreated. Sawflies also feed on fully extended needles, so while they remove all the current needles from their hosts, any new growth flushes will not be impacted once the larvae are finished feeding.
Treatments can be spot applied to small patches of sawfly or broadcast applied to larger infestations when the larvae are first observed. Insecticide treatments that include spinosad, carbaryl, pyrethrins, neem oil and insecticidal soap are all effective for controlling sawfly larvae when used following the manufacturer’s instructions. Treatments may need to be applied more than once per season depending on the severity and reoccurrence of outbreak.
Valerie West is a forestry extension agent in the LSU AgCenter Northwest Region.
