Four insect pests in particular plaque apple trees and their fruits in Minnesota orchards. Following good cultural practices diminishes their numbers, as well as the damage they can do; trees can remain healthy if they are well-pruned and monitored and damaged fruits are promptly removed from the orchard. By keeping apple trees healthy and the orchard floor clean, you diminish reliance on chemicals.
Widespread across the state, the apple maggot (Rhagoletis pomonella) is also called the railroad worm. Infested fruits become inedible, having use perhaps only as cider or animal feed. Fruits reveal tiny holes where an adult fly punctures the apple's skin to lay eggs. The hatched larvae eat and burrow tunnels for three to four weeks in summer, causing misshapen fruits, eventually leading to premature drop. Once on the ground, the larvae burrow into the soil to pupate, becoming adults to fly away and impregnate other fruits.
Codling moth (Cydia pomonella) occurs most frequently in southeastern and central Minnesota, especially near large orchard farms. The adult moth lays eggs on small apples in early summer. When the eggs hatch the pinkish beige larvae or "worm" burrows into the fruit, eating it and leaving excrement in the hollowed out areas. When ready, the larvae burrows out of the apple and drops to the ground, leaving an inedible, destroyed fruit that cannot be consumed. The larvae then spins a cocoon and later becomes an adult.
Inflicting damage also on cherries, plums, and apricots, the plum curculio (Conotrachelus nenuphar) is a weevil also known by the name "snout beetle". Luckily, the firm skin and flesh of apples causes only superficial scarring, but any small holes or unhealed scars allow other insect pests to enter and infest the apple. Besides creating unattractively skinned fruit, the plum cucurlio causes immature apples to prematurely drop from the branches.
Asian Lady Beetle
Looking like our native ladybugs but with a voracious appetite for rotting fruits, the Asian lady beetle (Harmonia axyridis) recently gained infamy in Minnesota as annoyingly covering building facades in rural areas. These bugs do not cause harm to tree-grown apples, but any fruits fallen to the ground or already damaged by other insects or birds become ravaged by hungry mouths. Asian lady beetles cover rotting fruit on the ground under trees when their usual foodstuffs are scarce.