Grass blue butterfly outbreak

Posted in PulseCheck–Coastal on Jan 23, 2019

Major grass blue butterfly and soybean aphid outbreaks threaten coastal soybeans in SE Qld

Hugh Brier, Queensland Department of Agriculture and Fisheries, Kingaroy, and others

Take home message – check your crops now.

A major outbreak of grass blue butterfly (GBB) (Zizina labradus) in late vegetative and flowering soybeans has been reported in the Maryborough/Hervey Bay region.

Some crops inspected by the authors had 10-12 GBB larvae per square metre with many plants denuded of buds and flowers. Other crops had less damage and fewer larvae (4-5 per square metre), but had 5 or more adult butterflies per square metre. In these crops, the adults are a major risk as their egg laying will potentially result in very high subsequent GBB larval populations. Entomologist Hugh Brier said this is by far the worst GBB outbreak he had ever seen in his 40+ years career.

The green larvae are slug-like but have legs (visible when they are turned over) and are broader at the head end.

Some crops inspected also had worrying numbers of soybean aphids, with above-threshold populations (250 per plant) seen on many plants. Consequently, growers/consultants are urged to inspect their crops for both pests.

While GBB are not uncommon in coastal soybeans, most outbreaks normally start well before flowering and usually cause easily-visible leaf and terminal damage. However in the infested flowering crops, damage was not immediately obvious when looking at the top of the canopy, but was detected when sampling. This was because beat sheet sampling dislodges not only the larvae, but also the damaged and dead buds and flowers.

While GBB larvae are quite small (reaching only 10 mm in length), it is their feeding behaviour that makes them so damaging, the larvae attacking the base of bud and flower clusters. Consequently one ‘chew’ by a caterpillar can kill a cluster of buds and flowers, which subsequently dry up and are easily dislodged. In some crops, sizable helicoverpa populations were also present, inflicting similar bud/flower damage, but also inflicting a lot more foliar damage.

Note that as soybean flowers progress to pod the pod stage, the petals die and turn brown, but that the surrounding bracts and buds stay green. In contrast, where there is GBB damage, feeding damage at the base of the bud/flower cluster kills all structures, turning them brown.

Damaged buds in Hervey Bay soybean crop, January 2019. Photo: H Brier

For both caterpillar pests, the severe bud and flower damage may have been escalated by recent hot dry weather, which stresses the plant’s leaves. This makes leaves less attractive to larvae, thereby reduces larval leaf feeding but leads to increased feeding on terminals, buds and flowers.

At the time this article was being written, many of the latter crops were or had recently been sprayed with Altacor (chlorantraniliprole), which is registered in soybeans against helicoverpa. Collections of both helicoverpa and GBB larvae post spray showed this pesticide was effective against both species.

Accordingly an emergency use permit has been secured (PER 87644) for Altacor, or other registered products containing 350 g/kg chlorantraniliprole as their only active constituent, against grass blue butterfly (GBB) in soybeans at the helicoverpa-registered rate of 70 g/ha. This permit is valid from 23 January to 30 April 2019.

The reason Altacor is the permit product of choice is because it will give lengthy protection of crops against larvae hatching from eggs laid by GBB adults currently present in many crops at very high densities. Note that no other insecticides are currently registered in pulse crops against GBB, and that Altacor has minimal impact on beneficials preying on GBB and other soybean pests.

One key point to remember with Altacor is that larvae are not killed immediately. Rather, they cease feeding and become increasingly moribund (shrunken) before dying after 4-5 days. While the moribund state is relatively easy to detect in helicoverpa, in GBB larvae, ‘moribundity’ is less obvious. If in doubt, collect larva and place in ventilated but escape-proof containers with freshly picked leaves from the sprayed crop. If larvae don’t move or feed much, then they are moribund and will eventually die.

Note that Altacor is an ingestion product and requires a minimum spray volume of 100 L of water/ha, but preferably 200 L/ha in densely-canopied crops in narrow 50 cm rows, as are common in coastal crops. When using Altacor, remember to add a non-ionic surfactant at 125 g a.i./100 L of water and adhere to other label guidelines.

It is important to not confuse the beneficial hoverfly larvae with GBB larvae. The two are similar in outline (both are slug-like), but the hoverfly taper towards the head whereas the GBB larvae have a broad rounded head end. Hoverfly larvae also lack legs.

To confirm that you have pest (the grass blue larvae) and not the beneficial, flip them over to check for the (normally hidden) head capsule, 3 pairs of proper legs (at the head end) and 4 pairs of ventral prolegs. These legs are positioned towards the larva’s mid-line and are not visible from above. Another way to confirm identity is to place the larvae in a jar with some leaves – the hoverfly larvae will not eat the leaves but instead rear up to look for prey while the plant pest will eat the leaves. Photo: The Beatsheet Blog

Soybean aphid

The other pest of concern is the bright-green soybean aphid, which is at near or above-threshold levels in many crops. The US-based threshold is 250 aphids per plant. As a rule of thumb, if you can see aphids on the main stem you are well over threshold.

Many aphid-infested crops also have significant ladybird and hoverfly larvae populations, both of which are key aphid predators. The IPM aphid strategy is to watch crops closely during the vegetative and early flowering stages, to see if the predators are bringing the aphids under control. Critical to this strategy’s success is to only use soft insecticides during the vegetative stage to foster these and other beneficials.

However, if aphid populations are still increasing, action needs to be taken as above-threshold aphid populations lead to very uneven plant and pod maturity, making harvest extremely difficult. The softest and preferred aphid option is pirimicarb (PER 85152 – valid till 31 July 2019).

Some crops may require a dual Altacor/pirimicarb spray. These products are compatible but the preferred mixing protocol is to dissolve the pirimicarb granules first and pour into the spray tank, and then dissolve the Altacor granules and add them last to the tank. Note that both are rainfast after 2-3 hours.

More information

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