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These graphs show accumulated degree days (DD) for Pukekohe, Hawke’s Bay, Manawatu, Mid-Canterbury and South Canterbury from 1 July 2014. The graph for each region also contains accumulated degree days for a hot and cold year in that region, to allow you to make comparisons with the current season. The number of psyllid (TPP) generations for the current season so far is indicated on the graph with a yellow arrow.

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Things to consider:

Psyllids are active throughout the year and therefore 1 July is an arbitrary starting point.

It is important to be aware of other plants near your crop that can sustain TPP and act as a source of infestation. These include nightshade weeds, African boxthorn, poroporo but also volunteer potatoes.

* Since the psyllid research is a work in progress, we strongly advise you to use the degree day graphs in conjunction with crop monitoring using sticky traps and plant sampling. Crop monitoring provides valuable information on TPP arrival, population build up and the life stages present in your crop (see the links below for a sticky trap monitoring brochure).

What do you need to know about insects and degree days?

Insect growth and development

Since insects are cold-blooded, temperature plays a major role in their growth and development. Insects require a certain amount of heat to develop from one stage in their life to another i.e. eggs – nymphs – adults. Insects have a lower development threshold below which insect development is negligible and an upper development threshold which is the maximum temperature before development stops. These thresholds can be used in predicting insect development.

What are degree days and what are they used for?

A degree day is a measure of the amount of heat that accumulates between a lower and upper development threshold during a 24-hr period. One degree day accumulates for each degree the average temperature remains between the lower and upper development threshold over those 24hrs, and several degree-days can accumulate during a 24-hr period.

The lower development threshold for TPP is estimated at 7.1oC and the upper development threshold at 33.6oC. These two values are used to calculate the number of degree days for a specific day. One degree-day accumulates for each degree the temperature remains above 7.1oC for 24 hours. It takes TPP 358 degree days to develop from an egg to an adult i.e. to complete 1 generation (Tran, L.T., et al., Estimating development rate and thermal requirements of Bactericera cockerelli (Hemiptera: Triozidae) reared on potato and tomato by using linear and nonlinear models. Environmental Entomology, 2012. 41(5): p. 1190-1198).

Degree days are a measure of insect growth and development in response to “heat” accumulated during each growing season and are commonly used to measure and predict insect development. Accumulated degree days are an important decision support tool in Integrated Pest Management (IPM) programmes. Weather varies hugely between years and locations, which can make it difficult to time spray applications accurately. For example, insect development can be two to three weeks faster in a year with warm spring weather than in a year with cold spring weather. Similarly, insect development may be several weeks faster in Pukekohe than Chertsey. Growers can use degree days to improve the timing of their insecticide applications rather than relying on calendar dates.

Degree days are most useful early in the season, as insecticide applications, rain and irrigation may alter psyllid populations. Additionally as the season progresses you will have all TPP life stages in your crop. Once eggs are found in your crop, 358 degree days later those eggs will potentially be adults. With the development of more targeted insecticides, timing and insecticide selection is very important. For example, as soon as TPP nymphs are found in your crop it is essential to use an insecticide which is active against nymphs. You need be aware of which life stages the insecticide you are using is targeting.

Further Information

Using Degree Days to Time Treatments for Insect Pests by Marion S. Murray, Utah State University.

Understanding Degree Days by Gabriella Zilahi-Balogh and Douglas G. Pfeiffer, Virginia Polytechnic Institute and State University.

Sticky trap monitoring brochure

* http://potatoesnz.co.nz/users/Image/Downloads/PDFs/TPP-Brochure-FINAL-WEB.pdf

Spraying: Unfortunately, the potato psyllid is a difficult pest to control. Spraying can be reasonably effective but it’s essential to cover all parts of the plant thoroughly with spray. This can be difficult because potato plants have dense foliage later in the growing season. If you do spray, remember to follow all aspects of Good Agricultural Practice, including spraying guidelines ensuring you don’t spray too close to harvesting time. Also, make sure you follow resistance management guidelines to prevent psyllid becoming resistant to the sprays. Please contact Andrea Crawford for the latest PDF version of the Spray Programme Poster.

Weeds: Another control is to keep down or eradicate other plants that are alternative hosts to psyllid. These include common weeds such as convolvulus, jimson weed (thornapple), mallow, African boxthorn (Lycium ferocissimum and black nightshade. Ornamental solanaceous plants are also potential hosts, and so is the native plant poroporo. Removing these plants from around your potatoes may reduce the likelihood of psyllid.

Contact

For further information please contact Natasha Taylor, Research Associate, Plant and Food Research, ph: (06) 975 8880