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 NZ grass grubs, although they're a different species to native grass grubs (Costelytra giveni) and 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 and young shoots - 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 attack, 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

Vegetables

Young plants and seedlings are very 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, after mating, the adult female lays eggs just underneath the surface of vegetation. Eggs are small (1.8mm), round and cream-coloured. Females lay between 6 and 12 eggs at a time, laying approximately 30 eggs in her lifetime.

• After 2–5 weeks, eggs hatch and grub-like larvae emerge. Larvae develop through 3 larval stages (instars), shedding their exoskeleton at each stage. After hatching, the first instar feeds on decaying organic matter just above the soil surface. At the second instar larvae burrow into the soil. At the second and third instar stage, larvae 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 create an ‘earthen cell’. Larvae ‘clear their guts’ of food and become a pale-yellow colour. As pupae develop, they become slightly darker and redder in colour and shorter (15mm long) and more rounded in appearance. During the pupal stage, pupae don't feed, staying inactive while they gradually transform into adults.

• After pupating for about 2 weeks, adults 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 head is short, broad and angles downwards. At the front of the head are chewing mouthparts. On either side of the head are a pair of compound eyes and short, golf-club-like antennae. Behind the head, on the upper side of the body, is a large trapezoid-shaped plate (pronotum) which is the front segment of the middle section of the body (thorax). 

Attached to the underside of the thorax are 3 pairs of legs which are lightly covered in spines. The upper portion of the leg is slightly flattened while the lower leg is thinner and thread-like. However, in males, the lower portion of the front legs are also slightly flattened. At the tips of the legs are claws. The hindlegs are the largest pair which are used for digging. Adult beetles are slow crawlers, on the ground. 

Adults have 2 pairs of wings, the outermost wings are hardened forewings (elytra) and used to protect the delicate membranous hind-flight-wings. While at rest, forewings fold down over the flattened hindwings and meet in the centre of the body, forming a distinct straight line. At the top of this line is a small triangular-shaped plate known as the scutellum. Along the length of the forewings are straight, mostly parallel indented lines.

How to Identify Black Beetle Larvae

The grub-like larvae have a creamy-white body with an orange-to-brown rounded head capsule, and black jaws. The hind section of the body is generally swollen and grey-blue to green in colour, due to the visible contents of the gut. The body is long and heavily segmented, naturally curls into a C-shape, and grows up to 25mm long. At the front, along either side of the body, at each segment are a pair of orange-brown spots which are used for breathing (spiracles). On either side of the head are a pair of compound eyes and at the front of the head are chewing mouthparts. Attached to the underside and middle section of the body are 3 pairs of thin 5-segmented legs. Larvae are wingless.