Managing insect pests in sunflower crops
Monitor and manage insect pests, in particular Rutherglen bug and helicoverpa.
The main seedling pests are brown cutworms, wireworms, false wireworms and brown field crickets. Check for insect pests regularly and thoroughly before planting and during establishment. Use germination seed baits (gsb) prior to planting. If there is a history of soil pests, use insecticide seed dressings where possible and spray when insect populations exceed the economic threshold.
The major pests of sunflowers in the reproductive stage are Rutherglen bug (Nysius vinitor) and Helicoverpa spp. Green vegetable bug (Nezara viridula) is a minor pest.
Rutherglen bug (Nysius vinitor)
Rutherglen bugs (RGB) reduce sunflower yield and oil content by sucking the developing seed, which reduces seed weight and changes oil composition. The critical times to monitor Rutherglen bug populations are at budding and seed fill. Moisture stress will exacerbate the effect of Rutherglen bug damage.
When a Rutherglen bug pierces the seed it leaves brown marks on the seed that make confectionary seed visually unattractive. It can be difficult to maintain numbers below the threshold of five adult Rutherglen bugs per plant.
Rutherglen bugs breed in inland areas on a wide range of host plants, including winter weeds. They move into crops when these weeds dry off. Female Rutherglen bug lay up to 400 eggs, which hatch approximately one week later. The nymphs are wingless, with a pear shaped body and are reddish brown in colour. Nymphs develop over three weeks, before gaining wings, changing their shape and size and becoming adults.
Females use the developing sunflower seed as a protein source to initiate egg laying. Eggs are laid between the seeds and dead florets up to two weeks post-flowering. This means a second generation could be mature enough to lay a third generation by the time the crop reaches physiological maturity. Damage can continue until harvest, depending on seed hardness.
RGB management aims to prevent population explosions, so an understanding the lifecycle of the Rutherglen bug helps when making spray decisions. Adults will not start breeding until a protein source is available in the form of developing sunflower seed and by late February breeding ceases, even if crops are still available.
Rutherglen bug infestations can be extremely patchy making monitoring difficult, however DAFQ recommends random sampling of 20 heads per field to estimate the number of bugs per head and calculate a field average.
The most effective pesticides are synthetic pyrethroids, which have 3–5 day residual effect and severely disrupt natural predator populations. As adults are winged, re-infestation can occur rapidly after treatment. If crops require spraying, best results are achieved with application prior to petal drop and before the heads turn toward the ground.
There are two species of helicoverpa that occur in sunflower—Helicoverpa armigera and H. punctigera. Heicoverpa cause damage from late budding until late seed-fill by:
leaf feeding and feeding on the stem during budding
eating florets and developing seeds
boring holes into the back of sunflower heads.
Sunflower can tolerate large numbers of helicoverpa caterpillars. There is no significant yield reduction from helicoverpa feeding on leaves, seeds or florets in the absence of secondary head rots.
At budding, more than four 7 mm long larvae per head is the threshold for spraying. Natural mortality rates of 30% for larvae less than 7 mm are common and should be taken into account. When expected mortality is included in the calculation, the threshold for <7 mm long larvae is 6 larvae per head.
Select control options that are compatible with the insecticide resistance management strategy for your region. Larvae are difficult to control when they are feeding on the sunflower face and under bracts, especially once the heads turn to face the ground. Crops should therefore be sprayed before the heads turn down.
Green vegetable bug (Nezara viridula)
Green vegetable bugs (GVB) tend to feed on the upper stems and heads and when present in large numbers cause wilting, shrivelling and deformed heads. Occasionally they feed on developing seed. The current threshold is one mature bug or 5th instar nymph per plant.
Females lay 30–130 eggs in a raft on the leaf surface. The eggs hatch in 5–21 days and it takes 30 days to progress through the five nymphal instars. Adult life spans vary from several weeks to four months, with summer reducing their longevity.
Green vegetable bugs are sap sucking insects that have a wide host range. If they gather around the peduncle, water and nutrient supply to the developing head will be reduced.
Integrated pest management
IPM is the term used for a range of tactics to prevent pests from reaching damaging levels in crops. Insect pest management in the grains industry is heavily reliant on insecticides to control insect pests. However, for some pests there are non-chemical options or monitoring strategies that can help avoid pest outbreaks and damage. Using as wide a range of tactics as possible to deal with pests reduces the reliance on a single method of control, such as insecticide sprays.
Timing is critical in sunflower pest management as different insect pests attack sunflowers during the seedling, budding and seed-fill stages.
Identifying when sunflowers crops are susceptible to pests is the first step in good pest management. Sunflower growers throughout Australia contend with a number of insect pests at various stages of crop development. Most of these pests feed and breed on other crops and weeds but also infest sunflowers, causing damage at susceptible crop growth stages.
The basic sampling recommended is the examination of at least five consecutive plants in a row at six widely spaced locations throughout the crop.
The six locations chosen for sampling should represent areas that are typical of the field and should not include areas such as atypical wet/low areas, hills or outside rows, which are not likely to be representative of the rest of the field.
Rutherglen bug (RGB) infestations can be extremely patchy making monitoring difficult, however DAFQ recommends random sampling of 20 heads per field to estimate the number of bugs per head and calculate a field average.
Insecticide use (best practice)
Insecticides are a key tool in an integrated pest management (IPM) program for sunflowers. However, unnecessary or poorly timed spray applications, or selection of the wrong pesticides can flare secondary pests, hasten the development of pesticide resistance, contaminate the harvested product, increase costs and reduce profitability.
Be aware of the potential of synthetic pyrethroids and carbamates to flare helicoverpa, whitefly, and loopers. Repeated synthetic pyrethroid use will select for higher levels of resistance in H. armigera. Both short and long term factors must be considered when using insecticides in sunflowers.
Protecting beneficial species
Beneficial insect species include natural enemies of insect pests (e.g. predators, parasites and parasitoids), pollinators and stubble decomposers.
Conserving or encouraging natural enemies helps to regulate pest densities, particularly of minor pests. To achieve a healthy population of natural enemies in a crop, keep in mind the following:
Where possible, use the least disruptive option/s first. This strategy allows natural enemy populations to build up in the crop and have maximum impact on the pests.
Non-chemical control options that disrupt pest build up (e.g. pupae busting, planting into stubble, strategic cultivation, crop rotation) are extremely useful in avoiding pest outbreaks, reducing the need for in-crop spraying.
Non-crop vegetation, particularly native vegetation, provides important habitat (e.g. shelter, pollen, nectar, prey) for natural enemies. There is evidence that weedy vegetation hosts more pests than natural enemies, and that there is movement between native vegetation and crops (Shellhorn et al. CSIRO).
Even selective insecticides will kill one or more groups of natural enemies (e.g. bugs, wasps). The more applications, the greater the cumulative impact on natural enemy populations. Eggs and/or larvae may escape the first application, but not subsequent ones.
Current hybrid sunflowers are largely self-pollinated however several types of native, wild and hive bees forage in sunflower crops. To prevent killing bees, avoid spraying products that are broad-spectrum, or have impact on bees, during flowering. Use chemicals with a short residual pre-flowering. If possible, spray while bees are not actively feeding and close up or move any nearby hives when spraying occurs.