Scaffolds 99 index

EYE ON THE SKIES

(Art Agnello ama4@nysaes.cornell.edu, Entomology, Geneva)

Weather Effects on Pest Activity and Control Measures

Of all the factors that can possibly have an effect on the development of a given pest population, the weather must certainly be one of the most critical. Nearly every discussion of how moderate or how severe an insect or mite problem is, was, or might be in a given season, starts with a general estimation of the temperature, wind, humidity and rainfall conditions to which that pest is subjected. We all have numerous anecdotal evidence of how the spring rains of one year prevented one insect from taking off, or how the summer heat encouraged another. The point here is not to document specific effects, which are complex and abundant, but rather to indicate the need to take weather patterns into account when planning pest management programs, both before (prevention) and after (rescue) the fact.

Developmental Rates and Thresholds

Mammals are warm-blooded, developing at a constant rate regardless of the environmental temperature because they are able to maintain an internal temperature that allows their biochemical reactions to progress normally. Insects, which are exothermic, remain at the same temperature as their environment. They do not generate body heat and therefore depend on favorable external temperature. At a certain temperature, which varies among species, an insect's biochemical reactions cannot proceed and development stops. This temperature is known as the insect's developmental threshold or developmental base. Charting the ambient temperature makes it possible to track insect development, which is directly proportional to the amount of time accumulated above the developmental threshold (up to some maximum not often reached during the season). We divide this time arbitrarily into heat units or degree-days (DD).

Degree-Day Calculation Methods

There are different ways to determine the quantity of heat units accumulated, which is equivalent to the area under a temperature versus time graph on a given day. The methods are listed below in order of precision in measuring small changes during the day or departures from idealized heating and cooling trends (see figure).

Average or Max/Min Method - This method is the simplest and least precise. It assumes that the daily temperature graph is linear and that the area beneath it is triangular.

DD = [Daily max temp + Daily min temp*]/2 - Devel. Threshold

(* If Daily min temp < Devel. Threshold, substitute Devel. Threshold)

Sine Wave (Baskerville-Emin) Method - This method is more precise and assumes that the daily temperature cycle takes the form of a sine wave. The area beneath this curve is determined by integration, which requires calculus. This method makes the same use of daily maximum and minimum temperatures and developmental threshold as does the Average Method. Using the Sine Wave Method tends to accumulate more DDs than the Average Method, particularly during the early part of the season.

Continuous Integration Method - This method is the most precise and requires multiple temperature readings hourly or more frequently throughout the day to obtain a temperature versus time graph that is truly representative of a field situation. The area beneath the curve is still calculated using integration. The data collection is most efficient if handled by a computer.

Relating Degree-Days to Life Cycle and Development

These methods are attempts to correlate a pest event or activity with another event that can be measured more precisely. Events in an insect's life cycle often occur after the same heat units have accumulated each year, but many years' observations must be collected to measure this precisely. Degree-days can be used to predict events wherever weather data are available.

Temperature - By monitoring temperature and pest activity simultaneously for many years, it is possible to build up a data base of events and the range of accumulated DDs that correspond with them (refer to NY Food & Life Sci Bull. No. 142; Communications Services Bulletins, Jordan Hall, N.Y.S. Agric. Expt. Sta., Geneva, NY 14456 Tel: 315/787-2249).

Phenology - Some events occur reliably at the same time as other, easily observed biological events in the field; for example, mites hatch from late tight cluster to pink; European apple sawflies lay eggs from late bloom to petal fall. These rules of thumb often draw on the evolved relationships between pests and their hosts.

Biofix - This is a distinct, easily monitored event in the life history of an organism, used to fine-tune our predictions of its activity; for example, first flight, first egg laid, first mine observed.

Direct Influence of Weather on Pest Activity

First of all, in NY particularly, early spring is considered to be the die-is-cast period; the growth of most prebloom arthropod populations is pretty much determined for the first half of the season by what sort of spring weather occurs. European red mite, rosy apple aphid, spotted tentiform leafminer, tarnished plant bug, San Jose scale, and mullein bugs are only the most obvious of the species that suffer from a cold, wet, rainy and windy (in other words, typical) spring. They may be slowed considerably until the summer generations, or they might fail to show up at all in some cases. Conversely, a warm, dry, quick spring can result in nearly spontaneous generation of most of these pests. After the petal fall period, the rate of heat unit (Degree Days) accumulation is a primary factor in the duration of plum curculio oviposition (hotter = shorter period) and the speed of summer mite population growth. This latter case is especially crucial, as the first summer ERM eggs are generally hatching in June so the population is already primed to expand; additionally, the trees are particularly susceptible to foliar feeding stress, so a failure to act against a threshold-level infestation early will result in a long, hard battle for the rest of the summer.

Moving into midsummer, an abundance of rainfall will obviously stimulate foliar growth, which may have some advantages to the tree's development, but can also encourage undesirable infestations of pests such as green aphids, leafhoppers and even leafrollers. Hot and dry weather can be a mixed blessing, since it's associated on the one hand with localized outbreaks of twospotted spider mites, and on the other it tends to discourage emergence of apple maggot adults and woolly apple aphid aerial colonies if the ground is hard and dried out. The objective is to keep in mind which problems the prevailing conditions might require you to watch out for (and which to de-emphasize) as you go through the year. You can prevent a lot of needless effort in some cases, and effectively respond to otherwise serious infestations in other cases, simply by being aware of these basic trends.

Weather Effects on Pesticide Activity

The effect of rainfall and humidity on pesticide behaviour is a topic that is much-debated, but about which few hard details exist. Certainly, everyone gets nervous by a long, hard rainfall immediately following a pesticide spray. How much rain does it take to wash off a residue? Does it need to be reapplied? If so, how soon? The truth is, the factors that determine the need for a re-spray are usually very specific to each case, and generalizations never give a specific enough answer. Research on this topic has shown that there are intrinsic differences between insecticides, and that advice on whether to respray if rain falls after an insecticide application is mostly dependent on the insecticide and its formulation, and not so much on the intensity of the rainfall. The guidelines we use are heuristic and anecdotal--in other words, fuzzy--but they may help you decide on the advisability of going back in with a respray. In general, we assume that a spray deposit is pretty much solidly in place on the plant surface if allowed to dry for 2 hr after being applied; anytime before this, and there may be cases where thorough drying has not taken place. After 2 hr, the potential loss in efficacy from a rain will generally vary with the duration and frequency of the rain, but not necessarily with how hard the rain falls.