May 2, 2020
“Don’t let the worms spoil your apple”, warns the Wormy Apples children’s board game. The game is won by pulling all of the pesky worm pieces out of the plastic apple. If “worms” have been spoiling your homegrown apples, there are measures you can take to effectively control these pests and enjoy undamaged fruit.
Most likely the “worms” in your apples are actually codling moth larvae. The codling moth is a common insect pest and apples are its preferred food, although pears and English walnuts can also be targets. Often the first indication of a codling moth infestation is the destruction you find inside a ripe apple. The larva may no longer be there but it has left an unappetizing brown mess behind. Preventing this invasion requires some knowledge of the life cycle and habits of the codling moth.
The adult codling moth emerges from its cocoon in mid-March to early April. It has spent the winter pupating in a protected site under tree bark or in debris around the tree base. The moth is small and easily camouflaged with mottled gray wings tipped by a coppery brown band. It rests during the day and becomes active for a few hours before and after sunset, mating when sunset temperatures are above 62 degrees. The female moth then lays around 70 tiny, disc-shaped eggs on leaves or developing fruit. Eggs hatch in a week or two, depending on the temperature, and the newly hatched larvae immediately begin to burrow into the fruit creating entry wounds known as “stings”. As a larva tunnels into an apple, it leaves behind the reddish brown excrement called “frass”. When the larva reaches the core, it feeds on the developing seeds and goes through its stages of maturation until it is fully-grown and ready to emerge and spin a cocoon. Larvae that surface in late spring and early summer can pupate within their cocoons during the same season and emerge as a second generation starting the sequence again.
Awareness of the codling moth life cycle therefore is critical in the success of early and consistent control methods. In many cases, nonchemical management approaches can successfully limit codling moth damage without the need for pesticides. Sanitation is the first step. This starts with prompt removal of debris and fallen fruit that can provide cover for codling moth pupa and harbor larvae. Beginning about six to eight weeks after bloom, developing fruit should be inspected for stings and infested fruit picked and destroyed, especially in May and June. Thinning the fruit also helps since the larvae tend to invade where fruit is touching. Along with sanitation, bagging the thinned young fruit on the tree can effectively protect it from the codling moth. Use small brown paper bags to cover the fruit when it is ½ to 1 inch in diameter. Slip the fruit through a slit cut in the bottom fold of the bag and staple shut the open end. Remove the bags and allow the fruit to ripen about a week or two before harvest.
When it is not possible to keep codling moth levels down through nonchemical methods, insecticide spray applications may be necessary. Cyd-X and Spinosad are both safe biological agents that are available to home gardeners although Spinosad is more toxic to beneficial insects than Cyd-X. Timing is essential as sprays should be applied before or just as eggs are hatching and spraying needs to be repeated over the egg hatching period for each generation. Once the larvae have entered the fruit they are protected from insecticides. Home gardeners can detect the beginning of egg hatch by looking for the first stings or use pheromone traps and degree-day calculations to time sprays. You can find more about this method and codling moth control on the UC Integrated Pest Management website, ipm.ucanr.edu and click on the link Insects, Spiders & Others.
Keeping those worms out of your apples is more challenging than a board game, but with careful timing and persistence you can savor those juicy ripe apples from your own tree.