The five worst walnut tree pests are the walnut husk fly, codling moth, navel orangeworm, walnut twig beetle, and fall webworm. Each one attacks your tree at a different point in the growing season. Knowing which walnut tree pests show up and when gives you the best shot at saving your crop before the damage spreads.
I first spotted walnut husk fly damage during a late August harvest when I cracked open a nut and found the hull stained jet black inside. Small white maggots were tunneling through the soft husk tissue just beneath the outer skin. The smell was terrible, and every nut from that side of my tree had the same dark staining that year. Walnut husk fly control starts with catching the adult flies before they lay eggs on your developing nuts.
Good walnut pest identification means knowing what to look for on your tree at each point in the year. UC IPM tracks 19 key invertebrate pests that follow a seasonal cycle through your orchard. Aphids and mites attack during spring leaf growth. Codling moth larvae bore into your green husks through the summer months. Navel orangeworm moves in right at harvest time when husks start to split open. Each pest leaves distinct marks on the nuts, leaves, or bark that tell you what you are dealing with.
Walnut Husk Fly
- Season: Adults emerge in late June through August and lay eggs under the husk surface of your developing nuts.
- Damage signs: Black-stained hulls with soft, rotting tissue and small white maggots visible when you peel the hull back.
- Impact: USU Extension calls this the most common insect pest of walnuts, and it can ruin up to 80% of your crop if left unchecked.
Codling Moth
- Season: Two generations per year with larvae active from May through September in most growing regions.
- Damage signs: Small entry holes in the husk with frass (insect waste) packed around the tunnel opening near the stem end.
- Impact: Larvae feed on the kernel itself, making infected nuts useless for eating or selling at market.
Walnut Twig Beetle
- Season: Active from spring through fall, with beetles boring into branches and carrying a deadly fungus.
- Damage signs: Tiny entry holes in bark surrounded by dark staining, plus wilting branch tips that die back over weeks.
- Impact: Carries thousand cankers disease that is nearly 100% fatal in black walnut trees within 2 to 4 years of infection.
Navel Orangeworm
- Season: Larvae enter nuts through split husks at harvest time from August through October.
- Damage signs: Webbing and frass inside cracked shells with damaged kernels that show dark feeding spots.
- Impact: Causes major crop loss in late-harvested orchards where nuts sit on the ground too long after hull split.
Fall Webworm
- Season: Caterpillars build silk webs on branch tips from late summer into early fall across your canopy.
- Damage signs: White silk tents covering clusters of leaves, with caterpillars feeding inside the web structure.
- Impact: Strips leaves from branch tips, which reduces your tree's energy reserves headed into winter dormancy.
You should hang yellow sticky traps in your trees by early June to catch adult husk flies before they start laying eggs. Place your traps at head height on the south-facing side of the canopy where flies tend to gather. Check them each week and count the flies you catch. When you see 5 or more flies per trap per week, it is time to spray. I use this method every year and it gives me a clear window for when to act instead of guessing.
Practice good orchard sanitation after each harvest to break pest life cycles for the next year. Pick up every fallen nut and hull from the ground beneath your trees. Dispose of any damaged or infested nuts away from your orchard rather than composting them on site. Prune out any dead or dying branches that could harbor twig beetles or other boring insects. These cleanup habits cost you nothing but time, and they make a noticeable difference in your pest pressure the next season.
Read the full article: Growing Walnuts: 7 Key Steps
