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Citrus

A backyard citrus tree looks worse than it is most of the time. UC IPM says trees more than four years old tolerate citrus leafminer damage on new leaves with a negligible effect on growth and fruit. It also says that citrus tolerates extensive leaf curling from aphids without any effect on yield, and that natural enemies usually bring aphids down within about six weeks on mature trees. Young trees, and varieties that flush all the time, are the exception (UC IPM).

Honeydew is the other thing to watch. Sticky leaves and black sooty mould usually mean scale or mealybugs, and ants tending them attack the parasites and predators that would otherwise keep them down (UC IPM). The pest to report is Asian citrus psyllid, which spreads huanglongbing, a disease that UC IPM says can kill a tree in as little as five years and has no cure. Citrus trees cannot be taken out of a quarantine area, UC IPM asks people not to move citrus plants or clippings out of their area at all, and suspected finds should be reported at once (in California, to the CDFA Exotic Pest Hotline, 1-800-491-1899).

crop context · backyard citrus, new flush

What feeds on a backyard citrus tree

SUMMER AND AUTUMN FLUSH

Citrus leafminer

Silvery winding mines and curled new leaves. It looks bad, but UC IPM says mature trees rarely if ever need a spray.

ReleaseNothing to buyTolerate it on trees over 4 years old (UC IPM)

SPRING FLUSH

Melon (cotton) aphid

Curled new leaves and honeydew. Lacewings, lady beetles and hoverfly larvae usually keep them below damaging levels (UC IPM).

ReleaseHoverfliesWild — attract adults with open flowers

FEB–APR

Cottony cushion scale

Orange-brown scales with fluted white egg sacs on twigs, branches and trunk. The vedalia beetle has kept it in check in California since it was brought in from Australia in 1888–89.

ReleaseVedalia beetleWild — relocate 20 or more from another infested tree

WHEN ANTS ARE TENDING

Brown soft scale

Flat, yellowish-brown scales along stems and midribs, with honeydew and sooty mould below. Control the ants first, then let the parasitoid wasps and lady beetles work.

ReleaseMetaphycus waspsWild — conserve; M. helvolus is rarely sold

ALL YEAR ON POTTED TREES

Citrus mealybug

White cottony clusters in leaf joints. Outdoors, UC says natural enemies usually keep mealybugs below damaging levels; on a tree kept indoors they are out of reach, and a colony keeps growing.

ReleaseMealybug destroyer5 beetles per infested plant, or 2–10 /m² per release

SPRING AND AUTUMN

Snails

They climb the trunk at night to eat young bark, leaves and fruit. A copper trunk band, with the skirt pruned clear of the soil, keeps them off.

ReleaseDecollate snail3–5 per shrub, or about 100 per 1,000 sq ft (supplier); legal in 12 California counties only

SPRING AND AUTUMN FLUSH

Asian citrus psyllid

Mottled-brown adults tilted head-down on new flush; nymphs with curly wax tubules. It spreads huanglongbing, which has no cure.

ReleaseTamarixia wasp (released by the state, not sold)Nothing to buy — released by the state; protect the ones already there

Season calendar

US · QUALITATIVE READING · MILD-WINTER CITRUS AREAS

	J	F	M	A	M	J	J	A	S	O	N	D
Citrus leafminer	Jan: Quiet	Feb: Quiet	Mar: Quiet	Apr: Quiet	May: Quiet	Jun: Building	Jul: Pest pressure high	Aug: Pest pressure high	Sep: Pest pressure high	Oct: Pest pressure high	Nov: Building	Dec: Building
Aphids	Jan: Quiet	Feb: Quiet	Mar: Building	Apr: Pest pressure high	May: Pest pressure high	Jun: Building	Jul: Quiet	Aug: Quiet	Sep: Building	Oct: Building	Nov: Quiet	Dec: Quiet
Cottony cushion scale	Jan: Quiet	Feb: Quiet	Mar: Building	Apr: Building	May: Pest pressure high	Jun: Pest pressure high	Jul: Pest pressure high	Aug: Building	Sep: Building	Oct: Building	Nov: Quiet	Dec: Quiet
Snails	Jan: Quiet	Feb: Quiet	Mar: Building	Apr: Pest pressure high	May: Pest pressure high	Jun: Building	Jul: Quiet	Aug: Quiet	Sep: Building	Oct: Building	Nov: Building	Dec: Quiet
Vedalia beetle move	Jan: Quiet	Feb: Release window	Mar: Release window	Apr: Release window	May: Quiet	Jun: Quiet	Jul: Quiet	Aug: Quiet	Sep: Quiet	Oct: Quiet	Nov: Quiet	Dec: Quiet

Pest pressure high
Building
Release window
Quiet

Citrus leafminer · high pressure Jul–Oct
Aphids · high pressure Apr–May
Cottony cushion scale · high pressure May–Jul
Snails · high pressure Apr–May
Vedalia beetle move · release Feb–Apr

Starter program

A backyard citrus tree rarely needs anything bought in. Most of this plan is about what to stop doing, with a release only where one fits.

ALL YEARSticky trunk bandWrap the trunk with fabric tree wrap or heavy paper and coat the wrap with sticky material, so ants cannot reach the scale and mealybugs they protect (UC IPM)
FEB–APRVedalia beetleOnly where cottony cushion scale is present: if no beetles have arrived by the end of March, move 20 or more in from another infested tree
SUMMER–AUTUMNLeave the leafminer aloneDo not prune live branches more than once a year, and do not feed nitrogen in summer or autumn, when leafminer damages the new flush most (UC IPM)
POTTED TREESMealybug destroyer5 beetles per infested plant, on a colony with plenty of egg sacs, above 61 °F (16 °C)
CALIFORNIA ONLYDecollate snail3–5 per shrub, or about 100 per 1,000 sq ft (supplier), at least 3–4 weeks after the last snail bait; legal only in 12 counties

Setting up the garden

Most of what keeps a citrus tree clean is done once and then left alone.

Keep the ants off

UC IPM notes that ants attack the parasites and predators of scale insects. A sticky band round the trunk cuts off their route up.

Prune the skirt clear of the soil

Snails shelter in branches that touch the ground. With the skirt pruned up, a copper band round the trunk becomes their only way in, and copper stops them.

Do not spray a mature tree for leafminer

UC IPM says infestations on mature trees rarely if ever need insecticide, and that the sprays available leave residues that kill natural enemies.

Do not move citrus out of a quarantine area

Asian citrus psyllid and huanglongbing travel on moved plants. UC IPM asks people not to move citrus plants or clippings out of their area, and to report trees whose leaves yellow in a blotchy, lopsided pattern, green on one side of the leaf and yellow on the other.