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Psyllid biology and eucalypt defoliation | AG0817 |
Nick Collett, Heidelberg
May, 2001 |  |
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This Agriculture Note provides advice on the biology of psyllids and their association with defoliation of native eucalypts in Victoria. The information provided is applicable both to plantation eucalypts and individual trees on private properties and in roadside plantings.
Among the most devastating of the insect pest groups are the sap-sucking psyllids that feed on a wide variety of eucalypts. Some of these psyllids are restricted to a single eucalypt host species or to a group of closely related species. For example, White lace lerp shows a strong preference for the closely related E. camaldulensis (river red gum) and E. blakelyi (Blakely’s red gum), while red gum basket lerp is generally found only on E. camaldulensis. While tree mortality seldom results even after severe attacks1, growth can be severely reduced, and ultimately leads to a general decline in tree vigour. Further damage may then be caused by a range of secondary insects. 1 Except where these attacks are sustained over a period of years, or where trees are stressed by abiotic factors such as drought and fire damage.
Description and life history of psyllids
The eggs are generally brown to black in colour, 1 mm long and ovoid in shape. However, colour, size and shape can vary slightly between different species.
Each female lays 45-700 eggs, either in clusters of approximately 10-150 eggs or in rows on leaf surfaces. Following a 10-20 day incubation period, the newly hatched nymphs (or ‘crawlers’) emerge and move over the leaf surface in search of food. Once settled, they commence construction of a shell (more commonly known as a ‘lerp’) from starchy material derived from the host plant. It is commonly thought that the lerp structure protects the psyllid insect from predation and desiccation during warm/hot weather.
Growth to adulthood occurs mostly within the confines of the original lerp. Adult psyllids are generally small (<5 mm length), yellow coloured with black markings and equipped with two pairs of wings. They are strong fliers and jumpers.
Figure 1. Close up of lerps, under which the nymph feeds on the foliage. (Scale: lerps are approximately 0.5mm at widest point)
Generally, the number of generations produced per year is 3-5, but can vary depending on:
- the species of psyllid
- the prevailing temperatures, and
- the environmental conditions including the availability of leaves suitable for oviposition.
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Growth and development of nymphs occurs throughout most of the year. During warm summer weather the generation turnover is approximately 8 weeks from egg to adult compared with 6 months during cool winter weather.
The optimal temperature for growth and development appears to be around 25C, but feeding and movement can still occur at below 10C and at higher than 33C. During warm winter spells, hibernating adults may resume feeding and breeding, thereby allowing additional generations to be produced.
Mode of attack and causes for outbreaks
Psyllids feed by sucking sap from the tissues of leaves and young plant shoots. A small attack will result in some leaf discolouration, wilting and distortion in the tree foliage. Severe infestations by lerp-type psyllids cause extensive foliage discolouration in tree crowns with purple to brown/rusty red spots, leading ultimately to leaf necroses and defoliation. This loss of foliage in turn reduces tree vigour and slows growth. In situations where psyllid attack has been particularly severe, tree death may ensue often in conjunction with secondary insect pests infesting the weakened trees.
Figure 2. Psyllid affected eucalypt foliage near Kyabram, Victoria.
As psyllids are most active during the summer period, attacked foliage tends to desiccate rapidly due to the warm conditions. So, by late summer, leaf fall has accelerated to the point where tree crowns often carry little foliage, and in severe situations are totally defoliated. The absence of old foliage, and the presence of dieback among the larger branches, particularly below mid-crown level, are common features in outbreak areas.
In ash-type eucalypt forests, dominant and co-dominant trees survive by producing epicormic shoots in response to persistent psyllid attacks. Whereas suppressed trees of less than 20 cm diameter at breast height over bark (DBHOB), whose proportion of living crown has declined to 18%, usually die, thereby aiding the natural thinning process.
Seed production may also decline through the early death of capsule bearing branches. Studies have linked the extent of defoliation to moisture stress in trees. They showed that the consumption of E. camaldulensis foliage by insects (including sap-sucking psyllids) is greater in non-flooded compared with the flooded areas, and that during drought periods, leaf fall occurs at a higher rate than in years of ‘normal’ rainfall.
Psyllid populations may increase suddenly to outbreak levels when several interacting environmental factors favour their rapid growth, development and generation-turnover. These factors include:
- abundant immature succulent foliage, required by ‘young leaf-feeders’ such as the free-living blue gum psyllid or ample mature foliage required by ‘mature leaf-feeders’
- host trees growing in single-species forests or plantation monocultures (mixed species forests or plantations are less susceptible to psyllid outbreaks)
- leaf nutrient levels sufficiently high to encourage psyllid growth
- warmer than average seasonal ambient temperatures boosting generation-turnover
- enough soil moisture for the host foliage to be fully turgid as this assists the sucking action of the nymphs and adults
- host trees must not be flowering, and
- low population levels of indigenous psyllid-specific predators.
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Despite substantial variations in outbreak patterns between different species of psyllid, most conform to the following trends:
- most species reach very high densities on occasion, but do not build up to devastating plague levels
- a species may occur with very high population densities interspaced with low densities in the same generation or year within a region
- an outbreak of a species may coincide with outbreaks of other species in the same locality, but in such cases usually only one species is predominant
- an outbreak of a single species may be followed by one of a different species on the same tree in the same area, providing that the host tree recovers from the first outbreak, ie. outbreak ‘succession’ may occur;
- outbreaks of most species usually last for approximately three years
- a decline of an outbreak of a species at one location is often followed by a new outbreak in an adjacent location - even within an original outbreak region, some discrete populations not in synchrony with the main outbreak, may develop into new outbreaks as the original one declines, and
- the centres of high population density, and those of subsequent outbreaks, appear to move or flow back and forth across an outbreak region in what is termed an ‘outbreak flow’.
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These factors indicate that outbreaks of psyllids tend to occur over a widespread area and appear to be cyclical in nature. Hence the research on the causes of psyllid outbreaks has emphasised factors such as weather, plant-growth-cycles and variations in leaf chemistry.
Further reading
- Conservation and Environment (1992). Detecting and controlling psyllid infestations on eucalypts. Research and Development Section Note.
- Conservation and Natural Resources (1994). Psyllids in Mountain Ash forest. Forest Research and Development Branch Note No: 27, 4pp.
- CSIRO (1996). Insect Pests of Eucalypts on Farms and in Plantations. CSIRO Identification Leaflet Series.
- Morgan, F.D. (1984). Psylloidea of South Australia. SAGPO, Adelaide.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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