Black Beetle

What are Black Beetles?

The Black Beetle is an introduced pest species. In NZ it’s mostly found in the upper North Island. Larvae are easily mistaken for native NZ grass grubs (Costelytra giveni) or bronze beetles (Eucolaspis brunnea).

Damage to garden plants is caused by both the adult beetles and their larvae. Adults mostly feed on plant shoots and stems, just above or below the soil surface (they can burrow as deep as 15cm). Adult beetles fly in swarms, most often on warm nights – especially when they emerge from their pupae in autumn, with another peak while they lay eggs in spring. Flying beetles are attracted towards outdoor lights.

Black beetle larvae feed on grass and plant shoots below soil level, causing dead patches, yellowing and severed stems. The worst damage is concentrated from summer to early autumn (December - March).

The time of year is the easiest way to diagnose black beetle larvae in a lawn, because other grub species cause grass damage from autumn, stretching into winter. This summer activity means black beetle is a particular pest of warm season grasses, like kikuyu and couch.

Symptoms

Lawn Symptoms:

• Wilted grass blades - despite regular, deep watering.
• Yellowing leaves.
• Dry, straw-like stems and leaves.
• Severed shoots just above or below the surface of the soil, with a frayed or 'shredded' appearance.
• Dead or dying patches of grass.
• In patches of turf where roots have been chewed, turf can be easily lifted, similar to rolling back a corner of carpet.

Plant Symptoms:

• Wilted foliage, including new growth - even after regular, deep watering.
• Stunted growth.
• Reduced plant health.
• Severed shoots just above or below the surface of the soil, often with a frayed or 'shredded' appearance.
• Flattened, toppled or partially uprooted plants.
• Ringbarking of stems at the base of plants.
• Affected seedlings and young plants can be easily lifted from the ground, due to a chewed or poorly developed root system.
• Holes in tubers, e.g. potatoes and kūmara.
• Unexplained dead plants.

Plants Impacted

Lawns

Most grass varieties are susceptible to damage, severely impacting lawns and pasture. Warm-season lawn types are at higher risk, due to the grub season. 

• Kikuyu
• Couch
• Fescues (tall fescue and fine fescue)
• Ryegrass

Adult beetles are known to feed on fresh-sown lawn seed during autumn, so a preventative application of Yates Lawn Grub & Porina Kill + Protect is recommended alongside sowing.

Vegetables

Young plants and seedlings are vulnerable to attack by beetles.

Ornamentals

Ornamental garden plants and potted plants may also be impacted.

The Black Beetle Lifecycle

Usually, there is just one generation of Black Beetle per year. However, depending on environmental conditions, it can take up to 2 years to complete their lifecycle.

• In spring, the female beetle lays small (1.8mm) round eggs just underneath the surface of vegetation. She will lay 6-12 eggs at a time, laying about 30 eggs in her lifetime.

• After 2–5 weeks, eggs hatch and grub-like larvae emerge. Larvae develop through 3 larval stages (instars). The first instar feeds on decaying organic matter just above the soil surface. At the second instar larvae burrow into the soil to feed below ground on plant and grass roots, which is when they do the most damage. Larvae are generally active from summer to early autumn (December - March).

• From late summer to early autumn, mature larvae dig a chamber in the soil to pupate, staying inactive while they gradually transform into adults.

• After pupating for about 2 weeks, adult beetles emerge in late summer and autumn. Over winter, adult beetles become less active. In cold conditions, they hibernate in the soil, then emerge in spring fully mature and ready for mating. Adults generally survive for about 10 months, dying soon after reproducing.

How to Identify Adult Black Beetles

Beetles are glossy black, oval-shaped and 12-15mm long. On the underside of the body are light-brown bristle-like structures (setae). The short, broad head is angled downwards, with short, golf-club-like antennae on each side.

Legs are lightly covered in spines. Beetles have 2 pairs of wings, the outermost are hardened forewings (elytra) that protect the delicate membranous flight-wings while they're on the ground. Adult beetles are slow crawlers. 

How to Identify Black Beetle Larvae

The grub-like larvae have a creamy-white body, with an orangey-brown head. The hind section of the body is generally swollen and grey-blue to green in colour, due to the visible soil in their gut. The segmented body grows up to 25mm long. Larvae have a distinctive 'curling into a C-shape' behaviour when disturbed. The larvae are easily mistaken for grass grubs, so the time of year is the best hint for telling them apart: black beetle larvae are expected between summer and early autumn (December - March)