Spongy moth Adults, pupa, and egg masses – Photo Credit Jason Brooks
By: David Dutkiewicz, Entomology Technician, Invasive Species Centre | Originally published by the Ontario Maple Syrup Producers’ Association
Spongy moth (Lymantria dispar dispar, formerly known as European gypsy moth) is an invasive forest pest that has been infesting North American forests since its introduction in Massachusetts by a French scientist in 1868. At that time, the consequences of this introduction and the impacts it would create over the next 150 years were unknown. In the late 1890s, the first major forest outbreak of this invasive pest was recorded, and spongy moth has been cycling through outbreaks periodically ever since. Spongy moth now spans across most of the Northeastern United States and into Ontario and Quebec. The range of spongy moth has now expanded to the natural distribution of oak throughout Ontario and Quebec. While oak is the preferred host of the spongy moth, it can survive and thrive off more than 300 different species of trees and shrubs, including maple, poplar, birch, white pine, and spruce.
Spongy moth has a giant appetite for such a small caterpillar and can consume over 1 square metre of leaf material over its life span. While that might not seem like a lot, multiply that 1 square metre by millions of caterpillars in the forest canopy and the trees become stripped of foliage very rapidly. The eggs over winter on the trees in cracks in the tree bark, branch splits, under eavestrough, sides of sheds, and any place that is somewhat sheltered from the elements. Egg masses are tan coloured and fuzzy, measuring approximately 1-2 cm in width and 2-4 cm in length with approximately 100-1000 eggs in each mass.
From these eggs, caterpillars emerge in late April to early May and start feeding on the closest available foliage. The caterpillars start out as small and black with lots of hair, which helps them to be carried by the wind, traveling from treetop to treetop within a forest. As the caterpillars feed, they grow and begin to develop into a speckled grey and white colour with five pairs of blue spots and six pair of red spots on their backs. Once they are at this stage in their development (usually in mid June), the caterpillars will wander in search of a place to pupate before developing into adults.
The pupae are brown in colour, usually with several white/yellow hairs along the abdomen. The pupation process takes several weeks to change, eventually emerging as an adult spongy moth in July and August. Male and female spongy moth do not look the same. Male moths can fly around in search of a mate and are brown in colour with black wave like arches along the wings. The female spongy moths are flightless, white in colour with fair black or tan wave like patterns similar to the males. Adult spongy moth do not eat and are only alive long enough to mate and lay eggs for the new generation to overwinter.
Life Cycle of spongy moth and different management strategies to use against the different life stages.
Spongy moth caterpillars have been known to heavily defoliate forests, causing stress to trees across the landscape. Deciduous trees can withstand 2-4 years of defoliation without death to the tree. However, repeated abuse on these trees can leave them vulnerable to other tree diseases or pest infestation. Deciduous trees do not store sugars in their leaves, allowing them to reflush leaves after spongy moth caterpillars have turned into pupae or adults. Unfortunately, coniferous trees do store their sugar reserves in the needles which are susceptible to mortality if spongy moth was to defoliate pine or spruce trees.
Defoliation of maple trees with a 75% or greater leaf loss will usually cause the tree to abort the defoliated leaf stems and re-grow with the buds intended for the next seasons growth. However, when the leaves reflush they are usually half the size and in fewer numbers of a normal year’s growth. This new growth adds stress to the leaves since they are growing during a hotter and drier period in the summer. Since these new leaves have a different growing season than non-defoliated trees, they will attempt to remain on the trees longer making them more susceptible to autumn frost. This does not allow for normal sugar and starch storage in the trees, usually causing a decrease in the amount available in the spring maple syrup season. This decrease in sap-sugar content means a greater amount of sap is needed to produce syrup as compared to a non-defoliated tree. Furthermore, defoliated trees require a greater recovery period from bark wounds such as taps that are used for drawing sap from maple trees. Tap holes drilled into trees that were defoliated for 3 consecutive years tended to be larger and showed possible signs of decay when compared to non-defoliated trees. This type of decay and internal wood bleaching could lead to a possible vector for other tree diseases.
The effects of spongy moth on trees in non-outbreak years is minimal and normally goes unnoticed by most of the population. Spongy moth has been in North America for over 150 years and during that time there have been a number of natural and introduced enemies that can combat spikes in population. Fungus (Entomophaga maimaiga), virus (LdmNPV nucleopolyhedrosis virus), and parasites (Ooencyrtus kuvanae) are all part of the biocontrol agents that control the spread of spongy moth. However, sometimes these biocontrol agents do not have time to react fast enough to curve a population outbreak, such as we saw in 2020. Spongy moth has only had three outbreaks throughout Ontario since the 1980s, with defoliation levels over 100,000 Ha, according the to the Ministry of Natural Resources and Forestry (MNRF) forest health conditions report. This historical data shows that with sharp increases in populations there is always an equally sharp drop in the population, with most of the outbreaks over 100,000 Ha of defoliation only lasting 2-3 years. In 2020, 500,000 Ha of defoliation occurred in Ontario, making it the largest outbreak of spongy moth in the province’s history. Fortunately, all types of biocontrol agents are currently present in the population and were collected by the MNRF forest health staff during the 2020 field season. The MNRF forest health staff collect insect and disease samples across Ontario to help identify forest pests and invasive species. These collections are used to help with the monitoring and management strategies of native and invasive pests to promote ecologically and economically sustainable forests.
Landowners have an active role to play in monitoring and managing spongy moth on their properties. Proper forest management and promotion of tree health is important for maintaining healthy sugar maple stands during years with environmental stressors like spongy moth. Aerial spray application of bioinsecticides such as, BTK (Bacillus thuringiensis kurstaki [Bacteria]) can help prevent defoliation of trees in forest settings. For ornamental and smaller trees, egg mass removal can help in controlling population levels in the following year. Routine management and maintenance of a forested areas is the best way to monitor health and vigour of trees. With invasive species like spongy moth in Ontario, early detection and management is extremely important to keep the sap flowing for years to come.
Spongy Moth Larvae – Photo David Dutkiewicz
References
Ministry of Natural Resources and Forestry (2019) Forest health conditions annual report summaries. Ontario, Canada
Wargo, P. M. (1977). Wound closure in sugar maple: adverse effects of defoliation. Canadian Journal of Forest Research, 7(2), 410-414.
Wargo, P. M. (1991) Remarks on the physiological effects of defoliation on sugar maple and some impacts on syrup production. Pp. 241– 251 in B. L. Parker, M. Skinner, T. Lewis, eds. Towards understanding Thysanoptera. Proceedings of the International Conference on Thrips. U.S. Department of Agriculture, Forest Service. General Technical Report NE‐147, Radnor, PA.
