Biosecurity Queensland has confirmed an outbreak of an exotic species of mealybug currently affecting cotton properties in Emerald is also present in cotton crops in the Burdekin region.
The exotic species of mealy bug is commonly known as the Solenopsis mealybug (Phenacoccus solenopsis).
We have received numerous enquiries about whitefly in the last week especially from the MacIntrye and Lower Namoi areas. Shortages of chemicals to control whitefly is currently of major concern to many growers and consultants in this region. Many enquiries also relate to species identification.
Whitefly samples identified from the MacIntyre and Lower Namoi areas have been 100% B biotype (SLW). Parasitism levels recorded from this area are low at this stage (between 0-10% parasitism).
Figure 1. B biotype, Bemisia tabaci (Photo: R. Lloyd)
Adults brown stink bugs (Dictyotus caenosus) are shield shaped, matt brown, and smaller than green vegetable bug (GVB), about 7 – 8 mm long (Plate 1). They may be confused with glossy shield bug (Cermatulus nasalis) which is a slightly larger predatory bug, and with rice spotting bug (Eysarcoris distinctus) which is smaller (5 – 6 mm) and has two pale elongated marks on the top (Plate 1).
While soybeans are more likely to be attacked by helicoverpa from flowering onwards, severe infestations can occur as early as the seedling stage. In recent years, severe early infestations have been reported on the Downs and in the coastal Burnett with the affected crops experiencing significant yield losses.
The latest published thresholds for helicoverpa in vegetative soybeans (Rogers and Brier, 2010) show that while soybeans can tolerate damage inflicted by moderate helicoverpa populations up to 7 larvae/m2 without yield loss, severe yield loss is inflicted by populations >7 larvae/m2 at a rate 4-5 times greater than during the pod-fill stage. The conundrum therefore is that while vegetative soybeans are far more tolerant of low to moderate helicoverpa populations (< 7/m2) than podding soybeans, they are markedly less tolerant of populations > 7/m2 than are pod-filling soybeans (see Figure 1). Read More
Control of corn earworm, Helicoverpa armigera, in maize has generally not been practised because of the high cost associated with repeated insecticide application required during silking. In most years it is a case of forsaking the top of every cob to larval damageHowever, in some years, very high pest activity results in more severe cob damage, with larvae often tunnelling into the sides of cobs. In such cases grain samples may contain fungus-affected grains and mycotoxins, causing a downgrade in the quality of harvested grain. Read More
Outbreaks of mealybugs have recently been reported from a number of cotton farms in central Queensland. While mealybugs are considered a minor pest of cotton, they have, on rare occasions, reached minor outbreak levels or ‘hotspots’ in commercial crops in central Queensland. Earlier this year mealybugs were reported causing some problems in Burdekin cotton. Reports from cotton growing areas in Pakistan and India over the last three years indicate that mealybugs have the potential to become a major pest.
C. Mares (CSIRO)
Just when you thought things were going along pretty well, something else pops out of the woodwork to bring you back down to earth. Some cotton consultants and their grower clients are concerned about the high numbers of thrips in their young cotton seedlings. Are they are problem? Do we need to control them?
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Every year we receive reports of white heads in wheat, and while there are several possible causes of this symptom, one suspect implicated in the crime is a small stem boring larva called Ephysteris silignitis (Turner) belonging to the moth Family Gelechiidae. Read More
The quick finish for winter cereals this season has resulted in the majority of crops escaping infestations of armyworm. Headers are already into some fields, but there are reports of armyworm making their presence felt in some of the later crops. Read More
How to check for parasitism in whitefly populations.
We have fielded numerous enquiries this week from consultants who would like to assess levels of whitefly parasitism. This is achievable in the field with the use of a hand lens.
Parasitism levels appear to be a little down from last year, but still good levels have been recovered. Parasitism so far has ranged from 50-90% in Emerald, 35% in St George, 20% on the Downs and 10% in lower Namoi.
Whitefly parasitoids are small wasps. There are two parasitoids that are commonly encountered in cotton, Eretmocerus and Encarsia. Eretmocerus hayati was released for biological control of whitefly in cotton and horticulture and is the most effective and abundant parasitoid in the cotton system.
The parasitoid wasps attack early instar whitefly nymphs. The female wasp is capable of laying hundreds of eggs which she deposits individually underneath the nymphs. When the parasitoid larvae hatches, it tunnels into the whitefly and eats it from the inside–out.
Identification of Parasitised Whitefly
Unparasitised whitefly nymph
• Early juvenile stages will be clear with two red eyes and two bright yellow organs (mycetomes) at the tail end (fig. 1).
• As the nymph reaches late 4th instar it will change from clear to a bright yellow with white spots appearing as the wings form (fig. 2).
• The exuviae (or spent pupal case) will be a transparent white (fig. 1).
Figure 1. (left) Early 4th instar whitefly nymph and two exuviae (spent pupal cases).
Figure 2. (right) Late 4th instar whitefly nymph.
Parastised whitefly nymph
• Nymphs are dull or dirty yellow/brown – Eretmocerus parastioid (fig. 3)
or
• Nymphs are dark brown or black – Encarsia parastioid (fig. 4)
• Sometimes the dark semi-circle of the developing parastoid larvae is visible (fig.5).
• Myecetomes appear non-symetrical or irregular.
• Exuviae is ‘dirty’ in appearance.
• Exuviae has a round exit hole where the wasp parasitoid has chewed its way free of the dead whitefly.
Figure 3. (left) Whitefly parasitised with Eretmocerus sp.
Figure 4. (centre) Whitefly parastised with Encarsia sp.
Figure 5. (right) Larvae of developing parasitoid visible inside whitefly nymph.
Correct use of a hand lens
A common complaint for why consultants do not check for parasitism or species composition in whitefly is that the whitefly are ‘too small’. Correct use of a hand lens will make it possible to identify species composition and parasitism.
The correct way to use a hand lens is to hold the hand lens right up to your eye, as close as your sunglasses would be (fig. 6). Then, bring the object into focus by moving the object, not the hand lens.
Often, users hold the hand lens away from their face which gives far less magnification making identification of parasitised whitefly impossible.
Figure 6. Correct use of a hand lens requires users to hold the hand lens very close to their eyes to get the most magnification from the hand lens.
Article and images by Zara Ludgate