Plain pumpkin beetles (Aulacophora abdominalis) are an orange oval shaped beetle slightly larger than a ladybug (approx 6-8mm long). They leave a distinctive circle bite mark on the leaves of cucurbits (pumpkin, melons, zucchini etc). If they have been attacking a plant for a while without detection there may be skeletonised leaves where only the leaf veins are remaining. Their eggs are laid in clusters in the soil near host plants which then hatch into grubs that eat the plant roots. The adult beetles are regularly seen eating together on the same plant. Plain Pumpkin Beetles attack all parts of the plant; the grubs eat the roots and the beetle eat the leaves, flowers and fruit. While a plant can survive the attack it can also make them more susceptible to damage from other organisms such as fungi.
Females lay their yellow, oval eggs into the ground near the host plant. In 5-15 days the eggs hatch and the cream white larvae burrow into the soil to feel primarily on roots. They moult 4 times over the course of a month, increasing in size each time before entering a pupal stage in an underground chamber where they will stay for 7-20 days before emerging as an adult beetle. The females are capable of laying up to 500 eggs over their lifetime (which can be as much as 10 months!) meaning several generations can exist simultaneously.
Plain Pumpkin Beetles can be flighty making them at times difficult to catch however if caught they are fairly easily squished. Females can be deterred from laying their eggs by utilising mulch (making it difficult to lay eggs in a prime position). Fast growing varieties of cucurbits are less likely to be impacted than slower varieties as they can outgrow the damage.
Predators of Plain Pumpkin Beetles include tachinid flies, spiders and a few soldier beetles amongst others. Some predators, such as birds are warned off by the beetles defensive colouring.
As with most insects soapy water will damage their waxy coatings leading to dehydration. More chemical applications such as pyrethrum or neem oil can also be used, however as always remember such treatments despite being natural, can harm beneficial bugs as well, potentially damaging your eco-system and becoming more reliant on your intervention.
Following an attack
If you’ve noticed an attack on your plants your next steps will depend on the condition of your plant. Larger plants can sustain more damage before the harvest is affected however seedlings are much more susceptible to smaller attacks. Once the number of beetles are under control, applying a fertiliser, seaweed tonic or weed tea will help support the plant to recovery. Following the harvest, Some light soil tilling may uncover eggs and it is recommended to avoid planting the same family of plants in that area if you had issues with the beetles in the past or minimum of 2 months.